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Chapter 13: Skeleton — Movement And Locomotion

In the year 1543, a young anatomy teacher named Andreas Vesalius was working in the city of Basel. He was twenty-eight, and he had just finished a book that would change medicine.

For centuries, doctors in Europe had learned the human body from the writings of Galen, a physician who had lived more than thirteen hundred years earlier. Galen had dissected pigs, dogs and monkeys, not people, and had assumed that human bones must be arranged the same way. Vesalius decided to look for himself. In Padua he had already begun dissecting human bodies in front of his students, instead of reading aloud while an assistant did the cutting.

In Basel he went further. He took the body of a man who had been executed, cleaned the bones one by one, and wired them together in their natural positions to make a complete standing skeleton. Bone by bone, it showed where Galen had gone wrong. The human lower jaw, for instance, is a single bone, not two bones joined in the middle as in a dog.

That skeleton is still kept at the University of Basel, and is thought to be the oldest surviving anatomical preparation in the world. It began the habit this chapter follows: look at the frame first, and then ask what every part of it is for. A mounted skeleton of this kind is shown in Fig. 13.0-1.

Diagram

Fig. 13.0-1 — A Mounted Human Skeleton Line drawing of a complete articulated human skeleton standing upright on a low wooden stand, viewed from the front, in the style of a sixteenth-century anatomical preparation; label skull, lower jaw, vertebral column, rib cage, bones of the upper limb, bones of the lower limb, and the hip region.

NoteCuriosity Corner

Q1. A bone is hard enough to carry your whole body weight, yet a skeleton is surprisingly light. How can it be both? Q2. If bones are hard and cannot bend, how does your body bend at all? Q3. Why are the bones of your skull locked tight, while your shoulder swings freely in every direction? Q4. Where inside your body are new blood cells made? Q5. A muscle can only pull; it can never push. How, then, does your arm straighten again after you have bent it?

13.1 Functions Of The Human Skeleton

Almost every animal you can name has a definite shape, and can move the parts of its body. A dog turns its head, a bird folds its wing, and you can bend your fingers around a pen. When only a part of the body changes position, we call it movement.

When the whole animal shifts from one place to another — a bird flying, a fish swimming, you walking to school — the movement is called locomotion. Locomotion is simply movement of the entire body from place to place.

Neither movement nor locomotion is done by the muscles alone. Muscles can pull, but a pull is useless unless there is something firm to pull against. That firm framework of hard parts inside the body is the skeleton, and it works together with the muscles at every step.

Movement and locomotion Movement is a change of position of any part of the body — a blink, a heartbeat, a nod. Locomotion is movement of the whole animal from one place to another. Every act of locomotion involves movement, but not every movement is locomotion.

The skeleton in our body serves six main purposes.

Support And Shape

  1. Support and shape. The skeleton provides a framework on which all the soft parts of the body rest, and it gives the body its definite shape. Without it, the soft organs would have nothing to hang from and would simply collapse into a heap. Even small bones matter: the tiny bones of your fingers are what let you close your hand around a pen and hold it steady.

  2. Protection. Several delicate organs are enclosed in a casing of bone. The skull encloses the brain, the vertebral column encloses the spinal cord, and the ribs form a cage around the heart and the lungs. These organs are soft and cannot repair themselves easily, so a hard case around them is the safest arrangement. The main protective casings are shown in Fig. 13.1-1.

Diagram

Fig. 13.1-1 — Bones That Protect The Soft Organs Simplified front view of the human body outline with the skeleton drawn in, shading the protective casings; label skull enclosing the brain, vertebral column enclosing the spinal cord, and rib cage enclosing the heart and lungs.

Movement, Leverage And Storage

  1. Movement. Many bones are joined to one another in such a way that one bone can move upon another. Muscles bring these movements about. A muscle begins on one bone and ends on another, so that when it shortens, it drags the second bone towards the first.

  2. Leverage. Some bones, working with the joints between them, act as levers. A lever lets a small movement at one point produce a much larger and faster movement at another. When the muscle at the front of your upper arm shortens by only a few centimetres, your hand swings through a far longer arc. How the body’s levers work is taken up in Section 13.7.

  3. Formation of blood cells. Some long bones, such as the thigh bone, are hollow along the middle, and the hollow is filled with a soft tissue called bone marrow. Certain kinds of blood cells, including red blood cells and white blood cells, are made in this marrow.

  4. Storehouse of calcium and phosphorus. Bones hold large amounts of calcium and phosphorus. The rest of the body draws on this store when it needs these minerals, so the skeleton is a bank as well as a building.

Where the skeleton sits Zoom in from the skeleton and you reach a single bone (an organ), then bone tissue, then bone cells. Zoom out and the skeleton joins with the muscles to form the skeletal system, one of the organ systems of the whole organism.

The word ‘skeleton’ comes from the Greek word skeletos, meaning dried up. It was first used for the dried, hard parts that remain long after the soft parts of a body have decayed.

13.2 Constituents Of The Skeleton

The skeleton is not made of one material only. Three different structures build it: bones, cartilages and ligaments. Each of the three has a different degree of hardness, and that difference decides the job it does.

Bone is the hard, rigid material that forms most of the framework of the body. It is strong enough to bear weight and to keep its shape under a load.

Cartilage is firm but flexible. It supports and connects, and it bends slightly instead of snapping. The projecting part of your external ear and the tip of your nose are held in shape by cartilage, not by bone. Fig. 13.2-1 shows where cartilage is found in the body.

Diagram

Fig. 13.2-1 — Some Places Where Cartilage Is Found Outline of the human head, neck and chest with cartilage shaded in a distinct tone; label cartilage of the external ear, cartilage of the tip of the nose, cartilage rings of the wind pipe, and the cartilage joining the front ends of the ribs to the breast bone.

Ligaments are tough bands that bind one bone to another. Wherever two bones meet, ligaments hold their ends together so that the joint does not come apart when the bones move.

The three are compared in Table 13.1.

Table 13.1 — Bone, Cartilage And Ligament Compared | Feature | Bone | Cartilage | Ligament | |—|—|—|—| | Nature | Hard and rigid | Firm but flexible | Tough and slightly elastic | | Main job | Forms the framework; bears weight | Supports and connects; keeps shape while allowing some bending | Binds one bone to another | | Where found | Skull, limbs, ribs, backbone | External ear, tip of nose, ends of long bones, front ends of ribs | At every joint, across the ends of the two bones |

Hold the flap of your ear, fold it gently, and let go. It springs back to its old shape. Now feel inside it — tough, but it bends. There is no bone there at all; the shape and the support both come from cartilage.

Ligament Tough band that stretches only a little → holds the ends of two bones firmly together while still letting them move → the joint stays in place instead of slipping apart.

13.3 Bone

Bone is the chief component of our skeleton. It is the part that carries the load, and everything else in the skeleton is arranged around it. Before looking at where the bones lie in the body, it is worth asking what a bone is actually made of.

13.3.1 Composition Of Bone

A bone contains two kinds of material mixed together: organic material, which comes from living tissue, and inorganic material, which is mineral. The inorganic part makes up nearly two-thirds of the whole bone substance, and consists mainly of compounds of calcium and phosphorus.

The two parts can be separated, and separating them shows exactly what each one contributes.

If a bone is placed in weak hydrochloric acid, the mineral part dissolves away and only the organic framework is left behind. Such a bone is called decalcified bone, which simply means that its calcium has been removed. It is so soft and flexible that a thin bone can be tied into a knot.

The opposite experiment gives the opposite result. If a bone is strongly heated, its organic matter is destroyed by oxidation and only the mineral part, the ash, remains. This bone keeps its shape but turns brittle, and it snaps easily.

Q. One bone loses its hardness in acid and another loses its toughness in fire. What does each result tell you?

A. The mineral part supplies hardness and rigidity; the organic part supplies toughness and flexibility. A normal bone is strong because it has both together.

This also explains a change that comes with age. In older people the organic part of the bone is reduced, so the bones become more fragile and a fracture takes much longer to rejoin.

Put a clean, thin chicken bone in a glass of vinegar and leave it for three or four days. Vinegar is a weak acid, and it dissolves the mineral part slowly. Rinse the bone and try to bend it. It will bend like rubber, because only the organic framework is left.

13.3.2 Classification Of Bones On The Basis Of Shape

Bones are not all the same shape, because they do not all do the same work. Shapewise, they are classified into four kinds: long bones, short bones, flat bones and irregular bones. The scheme is set out in FC 13.1 and drawn in Fig. 13.3-1.

Flowchart

FC 13.1 — Classification Of Bones By Shape Bone → classified by shape → branches into four kinds: Branch 1: Long bones → shaft with a knob at each end → e.g. bones of the arm, bones of the leg, ribs Branch 2: Short bones → box-like → e.g. bones of the wrist, bones of the ankle Branch 3: Flat bones → two parallel plates → e.g. shoulder blade, bones of the skull, breast bone Branch 4: Irregular bones → complex shape → e.g. bones of the face, bones of the backbone

Diagram

Fig. 13.3-1 — The Four Shapes Of Bones Four separate outline drawings side by side, each with its heading; label long bone (a limb bone showing shaft and two knobbed ends), short bone (a box-like wrist bone), flat bone (the shoulder blade), and irregular bone (a bone of the backbone).

A long bone consists of a shaft with a knob at each end. The shaft is thick-walled and tubular, and is filled with yellow marrow; the two ends are spongy. The bones of the arms and legs are long bones, and so are the ribs.

Short bones are box-like and spongy, and they allow only a little movement. The bones of the ankle and the wrist are short bones.

Flat bones are made of two or more parallel plates of hard bone enclosing spongy bone between them. The shoulder blade, the bones of the skull and the breast bone are flat bones, and each of them is a shield over something soft.

Irregular bones have complex shapes, and the amount of bone tissue in them varies, with both hard and spongy parts. The bones of the face and the bones of the backbone are irregular bones.

13.3.3 Structure Of A Long Bone

The thigh bone is a good example of a typical long bone, and its structure is shown in Fig. 13.3-2. It is a highly calcified — that is, heavily mineralised — hard and rigid connective tissue, and it is very strong. It can withstand severe stresses without giving way.

Inside it, the bone cells, called osteocytes, are arranged in concentric rings. These rings sit in a ground matrix in which collagen fibres and mineral salts of calcium and phosphorus have been deposited. The fibres give the matrix its toughness and the salts give it its hardness.

Diagram

Fig. 13.3-2 — Longitudinal Section Of A Long Bone Longitudinal section of a long bone such as the thigh bone; label periosteum, compact dense bone of the shaft, marrow cavity containing yellow marrow, spongy bone at each end containing red marrow, hyaline cartilage covering the ends, and ligament attached near the end.

The outer surface of a bone is covered by a membrane called the periosteum. It has an outer fibrous layer and an inner cellular layer, and it is richly supplied with blood vessels. Bone is living tissue, and those vessels are how it is fed.

Not all the bone tissue is equally solid. The wall of the shaft is compact bone, which is dense and solid, while the ends are spongy bone, which has a mesh of tiny bars with spaces between them.

A long bone is hollow along the middle, and this cavity is filled with marrow. Marrow is of two types. Yellow marrow is made up of adipose tissue and blood vessels, and it stores fat. Red marrow is present at the ends of the bone, and it produces red blood cells and white blood cells.

Q1. A bone is hard enough to carry your whole body weight, yet a skeleton is surprisingly light. How can it be both?

A. Because a long bone is not a solid rod. Its shaft is a tube of compact bone around a hollow marrow cavity, and its ends are spongy rather than solid. A tube resists bending almost as well as a solid rod of the same width, using far less material.

Q4. Where inside your body are new blood cells made?

A. In the red marrow, which lies in the spongy bone at the ends of long bones such as the thigh bone. Red blood cells and white blood cells are produced there and pass into the blood vessels that supply the bone.

Shaft of a long bone Hollow tube of compact bone rather than a solid rod → keeps almost all its strength against bending while using much less bone → the limb stays strong yet light enough for a muscle to swing quickly.

Yellow marrow and red marrow Yellow marrow lies in the cavity of the shaft and stores fat. Red marrow lies in the spongy bone at the ends and makes blood cells. Same bone, two different tissues, two different jobs.

Why Calcium In The Diet Matters The body draws calcium out of the bones whenever the blood runs short of it, so a diet poor in calcium slowly weakens the skeleton. Milk, curd, ragi, sesame seeds and green leafy vegetables are useful sources. In later life, some people develop a condition in which the bones lose mineral faster than it is replaced and become fragile, so that a small fall can cause a fracture. Building good bone strength while the skeleton is still growing, and staying physically active, both help.

13.4 The Human Skeleton

The human skeleton altogether consists of 206 bones. This count includes three very tiny bones inside each ear, which are the smallest bones in the body.

These 206 bones fall into two main divisions. The axial skeleton is the basic central framework that runs along the mid-line of the body. The appendicular skeleton is made of the bones of the limbs and of the girdles that attach the limbs to that central framework. The division is set out in FC 13.2, and the whole skeleton is shown in Fig. 13.4-1.

Flowchart

FC 13.2 — The Two Divisions Of The Human Skeleton Human skeleton → divides into two: Branch 1: Axial skeleton → skull → vertebral column → ribs → sternum Branch 2: Appendicular skeleton → bones of the limbs (forelimbs and hind limbs) → girdles (pectoral girdle and pelvic girdle)

Diagram

Fig. 13.4-1 — The Human Skeleton Side-facing view of the complete human skeleton in a seated-forward posture; label skull, mandible, cervical vertebrae, clavicle, sternum, ribs, humerus, radius, ulna, carpals, metacarpals, phalanges of the hand, lumbar vertebrae, pelvis, acetabulum, femur, patella, tibia, fibula, tarsals, metatarsals and phalanges of the foot. Shade the axial skeleton in one tone and the appendicular skeleton in another, with a key.

13.4.1 The Axial Skeleton

The axial skeleton consists of the skull, the vertebral column, the ribs and the sternum. Notice what these four have in common: each of them lies along the mid-line of the body, and each of them encloses something soft.

13.4.1.1 The Skull

The skull is the skeleton of the head, and it contains two parts. The upper part is the cranium, or brain-box, and it is made of eight bones. These bones are so joined to one another that they are permanently fixed and cannot move.

The other part of the skull forms the face, and contains a total of fourteen bones. The upper and lower jaws are formed by some of these bones.

At the back of the cranium there is a large hole called the foramen magnum. The spinal cord emerges from the brain and passes through this hole to continue behind into the backbone. Without such an opening the brain and the spinal cord could not be joined at all.

One more bone belongs here, though it is not part of the skull proper. The hyoid bone is situated in the neck, and it provides support to the tongue during speech and swallowing. The skull is shown in Fig. 13.4-2.

Diagram

Fig. 13.4-2 — The Human Skull Two drawings: (a) side view of the skull, labelling cranium, bones of the face, upper jaw, lower jaw; (b) view of the base of the skull from below, labelling foramen magnum. Below these, a small drawing of the neck region labelling the hyoid bone with the tongue outlined above it.

13.4.1.2 The Vertebral Column

The vertebral or spinal column is popularly called the backbone. It is composed of a total of 33 individual ring-like bones called vertebrae, and when the fused ones are counted as single bones the number comes to 26.

The column is not straight. It is curved, and the curves keep the body balanced in an erect position. They also absorb pressure and shock while walking and running, so that the column is not jarred straight up into the skull and does not break.

Diagram

Fig. 13.4-3 — Side View Of The Vertebral Column Side view of the whole vertebral column showing its curves; bracket and label the five regions with their numbers — cervical (7), thoracic (12), lumbar (5), sacrum, coccyx.

The regions of the column are shown in Fig. 13.4-3.

Structure Of A Vertebra

Each vertebra is a somewhat ring-like structure, drawn in Fig. 13.4-4. Its lower part in the drawing — which lies towards the front in the actual body — is a solid cylinder of bone called the centrum, or the body of the vertebra. The two opposite ends of the centrum are flat, so that one vertebra can rest squarely on the next.

On the dorsal, or back, side of the centrum there is a canal called the neural canal. It is formed by the union of two neural arches that arise from the sides of the centrum. In its natural position the spinal cord runs through this canal, protected on every side by bone.

From the point where the two neural arches meet, a flat longitudinal ridge projects upward. This is the neural spine. Thick sideward projections from the neural arches are the transverse processes. Both are places where muscles and ribs can attach.

The neural arches also bear articular facets, one in front and one behind on either side. These help in joining one vertebra to the vertebra behind it, so that the whole column holds together as a chain.

Between any two vertebrae there is a pad of cartilage called the intervertebral disc, or gristle. It forms a cushion, and it keeps the hard bony surfaces from grinding against each other.

Diagram

Fig. 13.4-4 — A Thoracic Vertebra Anterior view of a single thoracic vertebra; label centrum (body), neural arch, neural canal, neural spine, transverse process, articular facet and rib facet.

Intervertebral disc A pad of cartilage between two vertebrae → it squashes slightly and springs back → each jolt from walking or jumping is absorbed instead of being passed bone-to-bone up to the skull.

The Five Groups Of Vertebrae

The vertebrae are divided into five groups according to the region they occupy. The neck region has seven cervical vertebrae. Below them are twelve thoracic vertebrae, five lumbar vertebrae, five sacral vertebrae fused into one sacrum, and four fused vertebrae forming the coccyx.

The first cervical vertebra is called the atlas, and the second is the axis. The remaining five neck vertebrae do not have any special names. The atlas and the axis are shown in Fig. 13.4-5.

Diagram

Fig. 13.4-5 — The Atlas And The Axis Two drawings: (a) front view of the first (atlas) vertebra, labelling neural arch, neural canal, transverse process and the anterior facets for articulation with the condyles of the skull; (b) side view of the atlas and axis in position together, labelling atlas, axis and the peg of the axis on which the atlas turns.

Thoracic vertebrae have long neural spines that are directed backward. Each of their transverse processes bears a facet at its tip, and a rib joins the vertebra at this facet.

Lumbar vertebrae have well-developed neural spines and transverse processes. These give attachment to the powerful muscles of the back, which is why the lower back can hold the trunk upright.

The sacrum is a large bone formed by the fusion of five vertebrae. The hip bones are articulated to it on either side, so the weight of the trunk passes into the legs through the sacrum.

The coccyx is the last part of the backbone. It is made up of four fused vertebrae, and it represents the rudimentary tail of the human body. Very rarely, a baby is born with a small tail-like projection at this point, which doctors usually remove.

Reading down the backbone, the numbers run 7, 12, 5, 5, 4 — cervical, thoracic, lumbar, sacral, coccygeal. They add up to 33. Remember them as breakfast at 7, lunch at 12, and tea at 5.

13.4.1.3 The Sternum

The sternum, or breast-bone, is a long flat bone lying in the middle of the front part of the chest. It is the anchor at the front to which the ribs are fastened.

13.4.1.4 The Ribs

There are twelve pairs of ribs. Along with the thoracic vertebrae behind and the sternum in front, they constitute the bony cage or rib cage, shown in Fig. 13.4-6.

All twelve pairs are joined to the vertebral column at the back, but they differ in how they are joined at the front. The first seven pairs are the true ribs, and each is attached in front to the sternum with the help of hyaline cartilage.

The 8th, 9th and 10th pairs do not articulate directly with the sternum. Each joins the seventh rib with the help of hyaline cartilage, so it reaches the sternum only indirectly.

The last two pairs, the 11th and the 12th, are not attached to the sternum at all. Their front ends lie free in the muscle of the body wall, and they are therefore known as floating ribs.

Diagram

Fig. 13.4-6 — The Bony Thorax, Seen From The Front Front view of the rib cage; number the ribs 1 to 12 on one side, and label first rib, sternum, costal cartilage joining the ribs to the sternum, vertebral column, and the floating ribs (11th and 12th).

13.4.2 The Appendicular Skeleton

The appendicular skeleton is made of the bones of the limbs, together with the girdles that fasten those limbs to the axial skeleton.

13.4.2.1 Bones Of The Limbs

The bones of the forelimbs and of the hind limbs largely correspond to each other. Once you have learnt the pattern in the arm, the leg repeats it with different names.

The forelimb consists of a single long bone, the humerus, in the upper arm. The lower arm has two long bones, the radius on the side of the thumb and the ulna on the other side.

Beyond these come the smaller bones of the hand. There are eight carpals, or wrist bones, in the wrist, five metacarpals in the palm, and fourteen phalanges in the fingers — 2 in the thumb and 3 each in the remaining four fingers.

The hind limb consists of a long bone, the femur, in the thigh. It is the longest and strongest bone of the body, which it must be, since it carries the weight of the whole trunk.

The shank has two long bones, the inner tibia and the outer fibula. Then come seven tarsal bones in the ankle, five long metatarsal bones in the middle of the foot, and fourteen phalanges in the toes, 2 in the great toe and 3 each in the other four toes.

The hind limb has one additional bone that the forelimb does not have. The patella, or knee-cap, is joined to the lower end of the femur, and it is a bone developed from a tendon.

Radius and ulna Both are bones of the lower arm. The radius lies on the same side as the thumb; the ulna lies on the side of the little finger. Turn your palm over and it is the radius that rolls across the ulna.

13.4.2.2 The Girdles

Girdles are the parts of the skeleton that help to articulate the limb bones to the main axial skeleton. A limb cannot simply be stuck on to the backbone; it needs a broad piece of bone to hold its upper end. There are two girdles, the shoulder girdle and the hip girdle.

The shoulder or pectoral girdle consists of two flat triangular shoulder blades, each called a scapula, drawn in Fig. 13.4-7. They lie dorsally on the upper ribs on either side of the vertebral column, in the thorax region.

The outer apex of the scapula bears a large, somewhat cup-shaped glenoid cavity, into which the rounded upper head of the humerus fits. Close to this joint the shoulder blade has a small raised part to which the long, curved collar-bone or clavicle is attached. The other end of the clavicle is joined to the uppermost part of the sternum, and this is the only bony link between the arm and the axial skeleton.

Diagram

Fig. 13.4-7 — Left Pectoral Girdle, Viewed From The Back Back view of the left shoulder blade; label scapula, spine of the scapula, glenoid cavity, outer border, and the raised part for attachment of the collar-bone (clavicle).

The hip or pelvic girdle is a large trough-shaped part formed by two hip bones that are joined medially to the sacrum, as shown in Fig. 13.4-8. Each hip bone is itself made up of three fused bones — the ilium, the ischium and the pubis.

On each side the hip bone bears a large cup-shaped articular cavity, the acetabulum, into which the large round head of the thigh-bone fits. Besides giving support to the skeleton of the hind limbs, the hip girdle protects and supports the organs of the abdomen.

Diagram

Fig. 13.4-8 — The Pelvis, Seen From The Front Front view of the pelvis; label ilium, ischium, pubis, acetabulum (cavity for the head of the thigh bone), sacrum and coccyx.

How Your Body Weight Reaches The Ground 1. The weight of the head and trunk presses down along the vertebral column, disc by disc. 2. At the lower end, the load reaches the sacrum. 3. From the sacrum it passes sideways into the two hip bones of the pelvic girdle. 4. Each hip bone hands the load to the head of the femur, sitting in the acetabulum. 5. The femur carries it down to the tibia and fibula of the shank. 6. The tarsals and metatarsals spread it across the arch of the foot, and the foot presses on the ground.

13.4.3 The Total Count Of Bones

Table 13.2 gives a regionwise break-up of the 206 bones of the human skeleton. Reading down it, you can see how the axial skeleton accounts for 80 bones and the appendicular skeleton for the remaining 126.

Table 13.2 — Regionwise Count Of Bones In The Human Skeleton | Region of skeleton | Number of bones | |—|—| | 1. Axial skeleton | | | Skull — cranium | 08 | | Skull — face | 14 | | Skull — ear ossicles (\(3 \times 2\)) | 06 | | Skull — hyoid | 01 | | Vertebral column | 26 | | Ribs (\(12 \times 2\)) | 24 | | Sternum | 01 | | Total, axial | 80 | | 2. Appendicular skeleton | | | Forelimbs — humerus, radius, ulna (\(1 \times 2\) each) | 06 | | Forelimbs — carpals (\(8 \times 2\)) | 16 | | Forelimbs — metacarpals (\(5 \times 2\)) | 10 | | Forelimbs — phalanges (\(14 \times 2\)) | 28 | | Total, forelimbs | 60 | | Hind limbs — femur, tibia, fibula, patella (\(1 \times 2\) each) | 08 | | Hind limbs — tarsals (\(7 \times 2\)) | 14 | | Hind limbs — metatarsals (\(5 \times 2\)) | 10 | | Hind limbs — phalanges (\(14 \times 2\)) | 28 | | Total, hind limbs | 60 | | Shoulder girdles — scapula and clavicle (\(1 \times 2\) each) | 04 | | Hip girdles — hip bone (\(1 \times 2\)) | 02 | | Total, girdles | 06 | | Total number of bones in the body | 206 |

The 26 counted for the vertebral column already treat the sacrum as one bone and the coccyx as one bone. If instead the vertebrae are counted as 33 separate bones, the total for the whole skeleton comes to 213.

Two small points are worth fixing here. The three tiny ear ossicles named in the table are the malleus, the incus and the stapes, joined in a chain on the inner side of the ear drum; they are the smallest bones in the body. And although the nails and the teeth are hard, neither of them is a bone, so neither appears in the count.

13.4.4 Differences Between The Male And Female Skeleton

The two skeletons are built on exactly the same plan, and every bone named in this chapter is present in both. They differ only in a few proportions, and the differences are set out in Table 13.3.

Table 13.3 — The Male And Female Skeleton Compared | Feature | Male skeleton | Female skeleton | |—|—|—| | General build | Generally larger and heavier | Generally smaller and lighter | | Pelvis | Narrower and deeper | Wider and trough-shaped | | Reason for the difference in the pelvis | Not adapted for carrying a foetus | Adapted for accommodating the foetus in the uterus during pregnancy |

Lifting A Heavy Load Safely When you bend forward at the waist to lift a heavy bag, the discs of the lower back are squeezed unevenly at the very moment they are carrying the most weight. Over years, this can damage a disc, and a damaged disc may press on the nerves leaving the spinal cord, causing pain in the back or down the leg. Bending the knees instead, and keeping the load close to the body, keeps the vertebral column upright and lets the strong bones of the legs do the work.

13.5 Joints

A bone cannot bend. Every bend your body makes happens at a place where two bones meet, and that meeting point is called a joint.

NoteDefinition

Joint The point at which two separate bones of the skeleton meet.

Joints are not all alike, because the body does not need the same freedom everywhere. Some places must not move at all, some must give a little, and some must swing freely. There are accordingly three major categories of joints in our body: immovable, partially movable and freely movable. FC 13.3 sets out the whole scheme.

Flowchart

FC 13.3 — Classification Of Joints Joint → divides into three categories: Branch A: Immovable joints → bones joined very tightly by fibrous connective tissue → e.g. bones of the skull Branch B: Partially movable joints → bones joined with the help of cartilage → e.g. rib with the sternum, one vertebra with the next Branch C: Freely movable joints → synovial sac filled with synovial fluid between the two bone ends → divides into four types → gliding joint / pivot joint / hinge joint / ball and socket joint

Q2. If bones are hard and cannot bend, how does your body bend at all?

A. It does not bend anywhere along a bone. It bends only at the joints, where two separate bones meet and one is free to move upon the other. The hardness of the bone is never lost; the movement is handed over to the joints.

13.5.1 Immovable Joints

In this type of joint, no movement at all is possible between the two bones. The two bones are held very tightly by fibrous connective tissue, and their edges are locked together.

The structures between the bones of the cranium are the clearest example, and they are shown in Fig. 13.5-1. Eight separate bones meet there, and none of them can shift. That is exactly what the brain needs: a case that does not move at all.

Diagram

Fig. 13.5-1 — Immovable Joints In The Skull Side view of the cranium showing the wavy, interlocking lines where the skull bones meet; arrows pointing to two of these lines, labelled immovable joints (in skull).

13.5.2 Partially Movable Joints

Here only very little, or partial, movement occurs between the two bones. The bones are joined with the help of cartilage, which yields slightly under a pull without letting the bones separate.

The joint between a rib and the breast-bone is one example, and the joint between one vertebra and the next is another. Each such joint moves only a fraction, but the many joints of the rib cage and the backbone add up. That is how your chest can expand for a breath, and how your back can curve.

13.5.3 Freely Movable Joints

In this type, varying degrees of movement are possible between the two bones forming the joint. Between the two bone ends there is a fluid-filled sac, which is why these joints move so easily. The four major types of freely movable joints are described below.

13.5.3.1 Gliding Joint

In a gliding joint the bones simply glide over each other to some extent, with no rotation and no swing. Gliding joints occur between the bones of the wrist, between the bones of the ankle, and also between the vertebrae. One is drawn in Fig. 13.5-2.

13.5.3.2 Pivot Joint

Here one bone is rotated over a pivot-like end of another bone, turning about an axis like a key in a lock. The joint between the atlas and the axis is the standard example: the atlas, carrying the skull with it, turns on the peg-like upper end of the axis vertebra. This is the joint you use when you turn your head to say no.

Diagram

Fig. 13.5-2 — Gliding And Pivot Joints Two drawings: (a) gliding type — the wrist bones of the hand with arrows showing them sliding over one another, labelled wrist bones; (b) pivot type — superior view of the atlas facing the skull with the peg of the axis in place, labelled atlas, axis, and an arrow showing partial rotation. Beside them, a small drawing of two vertebrae with the intervertebral disc and hyaline cartilage labelled.

Shake your head slowly to mean no, then nod it to mean yes. The turning movement happens at the pivot joint between the atlas and the axis. The nodding movement happens between the skull and the atlas. Two different joints, two different movements, both in your neck.

13.5.3.3 Hinge Joint

This joint moves in one plane only, just like the hinge of a door. It cannot twist, and that limitation is a strength: a hinge joint is capable of bearing a heavy load, and hinge joints usually give sufficient power.

The elbow joint between the humerus and the ulna is a hinge joint, and so are the joints between the bones of the fingers and the toes. The knee joint is a hinge joint too, though less perfectly than the others. Fig. 13.5-3 shows a hinge joint.

13.5.3.4 Ball And Socket Joint

Here one end of a bone, which is rounded and ball-like, fits into a cup-like depression of another bone. Because the ball can turn every way inside the cup, this joint provides movement in any direction, and it is the most movable joint in the body.

The two best examples are the shoulder joint and the hip joint. In the shoulder joint the head of the humerus fits into the glenoid cavity of the shoulder girdle. In the hip joint the large ball-like head of the femur fits into the deep socket, the acetabulum, of the hip girdle.

Diagram

Fig. 13.5-3 — Hinge And Ball And Socket Joints Two drawings: (a) hinge type — the elbow, labelling humerus and ulna, with a curved arrow showing movement in one plane only; (b) ball and socket type — the hip, labelling femur and acetabulum, with arrows showing rotation in all directions.

Q3. Why are the bones of your skull locked tight, while your shoulder swings freely in every direction?

A. Because the two places have opposite jobs. The skull must protect the brain, so its bones are held by immovable joints and cannot shift at all. The shoulder must reach in every direction, so it is a ball and socket joint, in which the rounded head of the humerus turns freely inside the cup of the glenoid cavity.

Pivot joint and ball and socket joint Both allow turning. In a pivot joint the bone turns about one axis only, like a key. In a ball and socket joint the rounded head turns inside its cup in every plane, so the limb can also swing forward, back and sideways.

13.5.4 Synovial Joints

The freely movable joints described above, such as the shoulder joint and the knee joint, are also called synovial joints. They allow a considerable degree of movement, and this brings two special requirements with it.

Firstly, the bones must be held firmly in position, or a large movement would pull them apart. Secondly, the surfaces coming into contact must be well lubricated, so that friction is removed and the surfaces are not worn away.

The lubricating fluid is called synovial fluid, and it is from this fluid that the joint takes its name. A typical synovial joint is shown in Fig. 13.5-4.

Diagram

Fig. 13.5-4 — Section Of A Typical Movable Synovial Joint Diagrammatic section through a synovial joint between the ends of two bones; label bone, cartilage covering each bone end, ligament, joint capsule, synovial membrane and synovial fluid.

The knee joint, drawn in Fig. 13.5-5, is a good example of a synovial joint. It is held in position by strong ligaments and is well protected by an outer fibrous capsule. The ligaments prevent dislocation during normal movement.

The synovial fluid is contained in a sac formed of a very thin synovial membrane, and this sac serves as a cushion between the bones. So the joint is at once tied down, cushioned and oiled.

Diagram

Fig. 13.5-5 — The Knee Joint Diagrammatic section of the knee joint; label femur, tibia, patella (knee cap), articular cartilage, ligament, fibrous capsule, synovial membrane, synovial fluid, muscle and tendon.

Synovial fluid A slippery fluid held in a thin-walled sac between the bone ends → the cartilage surfaces slide instead of rubbing → friction and wear are reduced, and the joint can be used tens of thousands of times a day.

When A Joint Is Injured Or Worn A sudden twist can stretch or tear the ligaments of a joint, an injury called a sprain; the joint swells and is painful because the tie-down has been damaged. Over many years the smooth cartilage that caps the bone ends can also wear thin, most often at the knee and the hip, and a person with this condition finds the joint stiff and painful to move. Keeping body weight moderate and the surrounding muscles strong reduces the load that the joint surfaces have to carry.

13.6 Muscles

Bones and joints by themselves are a framework and nothing more. Nothing in that framework moves until something pulls it, and the pulling is done by the muscles.

Muscles do three things for the body. They provide the means of all its movements. They cover the skeletal framework and give the body its contour, or shape. And they hold the body in position, keeping your posture steady while you sit, stand or walk.

13.6.1 How A Muscle Moves A Bone

Most muscles are long bundles of contractile tissue — tissue that can shorten. Each muscle usually has two ends, and the two ends are not alike. One is the fixed end, where the muscle originates and stays anchored. The other is the movable end, which pulls some other part.

The movable end is drawn out into a tough structure called a tendon, which is attached to the bone. Tendons are made of tough collagen fibres, so they withstand the strains involved without stretching or tearing. A tendon is shown in Fig. 13.6-1.

Diagram

Fig. 13.6-1 — A Muscle Attached To A Bone By Its Tendon Drawing of a muscle narrowing at one end into a tendon that is fixed to a bone; label muscle, tendon and bone, with a cut end of the muscle showing its bundles of fibres.

When the muscle is stimulated by a nerve, it contracts. In contracting it becomes shorter and thicker, so it pulls on the tendon, and the tendon pulls the bone at the movable end. The steps are set out in FC 13.4.

Flowchart

FC 13.4 — How A Nerve Signal Produces A Movement Nerve carries the stimulus to the muscle → muscle contracts → muscle becomes shorter and thicker → tendon at the movable end is pulled → bone attached to the tendon is pulled → bone swings at its joint → movement of the body part

There is one limit here that decides much of what follows. A muscle can only contract and relax; it cannot lengthen itself. It can pull, but it can never push.

Not every muscle is attached to a bone. The muscles that move your eyelids, and the muscles that push food along the food canal, are not attached to bones at all, and they work on soft structures instead.

Tendon and ligament A tendon joins a muscle to a bone and carries the pull of that muscle. A ligament joins one bone to another and holds the joint together. Muscle-to-bone is a tendon; bone-to-bone is a ligament.

13.6.2 Antagonistic Muscles

Because a muscle can only pull, a structure that has been moved by one muscle cannot return to its original position by itself. Some other muscle must pull it back. Muscles that cause opposing movements in this way are called antagonistic muscles.

The arm is the standard example, drawn in Fig. 13.6-2. The biceps muscle of the upper arm is the flexor: when it contracts it flexes the arm, that is, it bends the lower arm over the upper arm.

Straightening, or extension, of the lower arm is brought about by the triceps muscle, which is the extensor. While one of the pair contracts, the other relaxes. The biceps and the triceps are therefore antagonistic to each other.

Diagram

Fig. 13.6-2 — Antagonistic Muscles Of The Arm Two drawings of the same arm, one above the other: (a) arm bent at the elbow, with the biceps shown short and thick and the triceps relaxed; (b) arm straightened, with the triceps shown short and thick and the biceps relaxed. Label biceps, triceps, humerus, radius, ulna and tendon in both.

Q5. A muscle can only pull; it can never push. How, then, does your arm straighten again after you have bent it?

A. A second muscle pulls it back. The biceps at the front of the upper arm pulls the lower arm up and bends it. To straighten the arm again, the triceps behind the upper arm contracts and pulls it down, while the biceps relaxes. Every bend the body makes is undone by such an antagonistic partner.

Bend your right arm at the elbow and feel the front of your upper arm with your left hand. The biceps has become short and hard. Now straighten the arm against a light resistance and feel the back of the upper arm. This time it is the triceps that is hard.

13.6.3 Kinds Of Muscles

There are about 650 muscles in our body, and together they make up nearly one-half of the body weight. They fall into two categories, according to whether or not you can control them.

Voluntary muscles are under the control of our will. You decide to lift your hand, and it lifts. Involuntary muscles are not controlled by our will; they work whether you attend to them or not, which is fortunate, since the food in the food canal must keep moving while you sleep.

A few muscles belong to both categories. The muscles of the eyelids and of the diaphragm are voluntary as well as involuntary: you can blink or hold a breath on purpose, but both also continue without any decision from you.

Cardiac muscles make up the walls of the heart. They are capable of rapid movements and, unlike the muscles of the limbs, they do not tire easily — a property the heart cannot do without, since it never gets a rest. The three kinds are compared in Table 13.4.

Table 13.4 — Voluntary, Involuntary And Cardiac Muscles Compared | Feature | Voluntary muscle | Involuntary muscle | Cardiac muscle | |—|—|—|—| | Under control of the will | Yes | No | No | | Where found | Muscles attached to the skeleton, such as those of the limbs | Walls of the food canal and other internal structures | Walls of the heart only | | Tiring | Tires with continued use | Does not tire easily | Does not tire easily | | Example of its work | Lifting the arm | Pushing food along the food canal | Pumping blood |

A few muscles in our body are vestigial, that is, non-functional. The muscles of the projecting ears are an example. Other mammals such as the cow and the horse still turn their ears towards a sound using these muscles. Some people, with some effort, can learn to move their ears slightly.

13.6.4 Coordination Of Muscles

Almost no action of the body is the work of a single muscle. Standing, walking, running and playing tennis all require the combined action of several muscles at once.

For this to produce a smooth movement rather than a struggle, the muscles have to be co-ordinated. Each must contract by the right amount at the right moment, while its antagonist relaxes by exactly as much. To a great extent, it is this co-ordination that makes one kind of movement different from another.

13.7 Lever Mechanisms

Movements of the different parts of the body are usually made easier by lever actions in the skeleton. A lever is a rigid bar that turns about a fixed point, and in the body the rigid bar is a bone.

Three things are needed before any lever can work. The fulcrum (F) is the fixed point about which the bar turns; in the body it is a joint. The power (P) is the effort applied; in the body it is the pull of a muscle. The weight (W) is the load that has to be moved.

What decides the kind of lever is simply the order in which these three lie along the bar. Because the muscle usually attaches close to the joint, a short contraction swings the far end of the bone through a long arc, as Fig. 13.7-1 shows. Speed and distance are gained, which is why leverage was listed as one of the six functions of the skeleton.

Diagram

Fig. 13.7-1 — Mechanics Of Flexion And Extension At The Elbow Side view of the arm holding a glass, with the elbow bent and the same arm shown in outline in the straightened position; label humerus bone, biceps muscle flexing the arm, triceps muscle straightening the arm, the short arc through which the muscle end moves, and the long arc through which the hand moves.

13.7.1 Levers Of The First Order

In a lever of the first order, the power and the weight act at the two ends, with the fulcrum in between them.

In the body this arrangement is represented by extension of the arm at the elbow by the action of the triceps muscle. The elbow joint is the fulcrum, the pull of the triceps behind it is the power, and the hand end of the forearm carries the weight.

The same order is seen at the top of the vertebral column, where the skull rests on the first and second neck vertebrae. The joint is the fulcrum, the weight of the face lies in front of it, and the pull of the muscles at the back of the neck is the power that keeps the head from nodding forward.

13.7.2 Levers Of The Second Order

In a lever of the second order, the fulcrum is at one end of the lever and the power at the other end, with the weight in between.

This is represented by the gastrocnemius muscle of the calf raising the weight of the body on the toes. When you rise on tiptoe, the toes pressing on the ground are the fulcrum, the weight of the body passes down through the ankle in the middle, and the power is the pull of the calf muscle on the heel.

Stand up and rise slowly on to your toes, with one hand resting on the back of your calf. You will feel the calf muscle harden as it pulls the heel upward. The whole weight of your body is being lifted at the middle of a second-order lever.

13.7.3 Levers Of The Third Order

In a lever of the third order, the fulcrum is at one end and the weight at the other, with the power acting in between.

This is represented by the biceps muscle flexing the arm. The elbow is the fulcrum at one end, the load held in the hand is the weight at the other end, and the biceps pulls at a point on the forearm between the two.

This arrangement demands a large pull from the muscle, because the muscle acts on a very short part of the bar while the load sits at the far end. What the body gets in return is speed and range: a small shortening of the biceps sweeps the hand rapidly through a wide arc.

The three orders are drawn together in Fig. 13.7-2 and compared in Table 13.5. The human skeleton provides examples of all three.

Diagram

Fig. 13.7-2 — Simple Levers In The Human Skeleton Three pairs of drawings side by side, each pair showing a body part above and its simple lever bar below with P, F and W marked. A — elbow, extension of the arm by the triceps muscle, first order. B — toes and heel, lifting of the heel by the calf muscle, second order. C — elbow, flexion of the arm by the biceps, third order. Give the key P = power, F = fulcrum, W = weight.

Table 13.5 — The Three Orders Of Levers In The Body | Feature | First order | Second order | Third order | |—|—|—|—| | Position along the bar | Fulcrum lies between power and weight | Weight lies between fulcrum and power | Power lies between fulcrum and weight | | Example in the body | Extension of the arm at the elbow by the triceps; the skull resting on the first two neck vertebrae | Raising the body on the toes by the calf muscle | Flexion of the arm at the elbow by the biceps |

Read the middle position in order — 1, 2, 3 gives F, W, P. In a first-order lever the Fulcrum is in the middle, in a second-order lever the Weight is in the middle, and in a third-order lever the Power is in the middle.

Levers are not a biological invention. The same three orders are studied in Physics, where a see-saw is a first-order lever, a wheelbarrow a second-order lever and a pair of tongs a third-order lever. The body simply uses bones as the bars and joints as the fulcrums.

13.8 Summary

  1. Movement is a change of position of a part of the body; locomotion is movement of the whole animal from place to place. Both are carried out by the skeleton and the muscles working together.

  2. The skeleton serves six main purposes: support and shape, protection of delicate organs, movement, leverage, formation of blood cells, and storage of calcium and phosphorus.

  3. The skeleton is built of three materials — bone, which is hard and rigid; cartilage, which is firm but flexible; and ligaments, which bind one bone to another.

  4. Bone contains organic and inorganic material. The inorganic part is nearly two-thirds of the bone substance and consists mainly of compounds of calcium and phosphorus.

  5. A bone placed in weak hydrochloric acid loses its mineral part and becomes decalcified — soft and flexible. A bone that is strongly heated loses its organic part and becomes brittle. The mineral part gives hardness; the organic part gives toughness.

  6. Shapewise, bones are classified as long, short, flat and irregular.

  7. A long bone has a shaft of compact bone with a hollow marrow cavity, spongy ends, and an outer covering called the periosteum. Its bone cells, the osteocytes, lie in concentric rings in a matrix of collagen fibres and mineral salts.

  8. Marrow is of two types: yellow marrow in the cavity of the shaft stores fat, and red marrow at the ends produces red and white blood cells.

  9. The human skeleton has 206 bones, divided into the axial skeleton (skull, vertebral column, ribs, sternum) with 80 bones, and the appendicular skeleton (limbs and girdles) with 126 bones.

  10. The skull has a cranium of eight bones and a face of fourteen bones. The foramen magnum at the back of the cranium lets the spinal cord pass into the backbone, and the hyoid bone in the neck supports the tongue.

  11. The vertebral column has 33 vertebrae, counted as 26 when the fused ones are taken as single bones: 7 cervical, 12 thoracic, 5 lumbar, 5 fused as the sacrum and 4 fused as the coccyx. Its curves keep the body balanced and absorb shock.

  12. A typical vertebra has a solid centrum, two neural arches enclosing the neural canal through which the spinal cord runs, a neural spine, transverse processes and articular facets. An intervertebral disc of cartilage cushions each pair of vertebrae.

  13. The first cervical vertebra is the atlas and the second is the axis.

  14. There are twelve pairs of ribs. The first seven pairs are true ribs joined to the sternum by hyaline cartilage; the 8th, 9th and 10th pairs join the seventh rib; the 11th and 12th pairs are floating ribs, not attached to the sternum.

  15. Each forelimb has the humerus, radius, ulna, 8 carpals, 5 metacarpals and 14 phalanges. Each hind limb has the femur — the longest and strongest bone — the tibia, fibula, 7 tarsals, 5 metatarsals, 14 phalanges and the patella.

  16. The pectoral girdle consists of the scapula, bearing the glenoid cavity, and the clavicle. The pelvic girdle consists of two hip bones, each formed of the fused ilium, ischium and pubis, and bearing the acetabulum.

  17. A joint is the point at which two separate bones meet. Joints are immovable, partially movable or freely movable.

  18. Freely movable joints are of four kinds — gliding, pivot, hinge, and ball and socket.

  19. Freely movable joints are also called synovial joints. They are held by ligaments and a fibrous capsule, and lubricated by synovial fluid held in a sac of synovial membrane.

  20. A muscle has a fixed end and a movable end, and the movable end is drawn out into a tendon attached to a bone. On being stimulated by a nerve, the muscle contracts, becoming shorter and thicker, and pulls the bone.

  21. Muscles can only contract and relax; they cannot lengthen themselves. Opposite movements at a joint therefore require antagonistic muscles, such as the biceps and the triceps of the arm.

  22. Muscles are voluntary or involuntary; a few, such as those of the eyelids and the diaphragm, are both. Cardiac muscle forms the walls of the heart and does not tire easily.

  23. Bones and joints act as levers, with the joint as the fulcrum, the muscle pull as the power and the load as the weight. The human skeleton provides examples of all three orders of lever.

Flowchart

FC 13.5 — The Chapter In One Chain Skeleton made of bone, cartilage and ligament → arranged as axial skeleton and appendicular skeleton → bones meet at joints (immovable, partially movable, freely movable) → muscles attached across the joints by tendons contract → bones act as levers turning about the joints as fulcrums → movement of a part, and locomotion of the whole body

13.9 Key Terms

  1. Acetabulum — the cup-shaped cavity on each hip bone into which the head of the femur fits.
  2. Antagonistic muscles — a pair of muscles that produce opposite movements at the same joint, such as the biceps and the triceps.
  3. Appendicular skeleton — the part of the skeleton made of the bones of the limbs and the girdles.
  4. Articular facets — small surfaces on the neural arches by which one vertebra joins the vertebra in front of and behind it.
  5. Atlas — the first cervical vertebra, on which the skull rests.
  6. Axial skeleton — the central framework of the skeleton: the skull, vertebral column, ribs and sternum.
  7. Axis — the second cervical vertebra, on whose peg-like upper end the atlas turns.
  8. Biceps — the flexor muscle of the upper arm, which bends the lower arm at the elbow.
  9. Bone — the hard, rigid material that forms the main framework of the skeleton.
  10. Bone marrow — the soft tissue filling the hollow spaces inside bones.
  11. Cardiac muscles — the muscles forming the walls of the heart, which act rapidly and do not tire easily.
  12. Carpals — the eight small bones of the wrist.
  13. Cartilage — a firm but flexible skeletal material that supports and connects, as in the external ear and the tip of the nose.
  14. Centrum — the solid cylinder of bone forming the body of a vertebra.
  15. Cervical vertebrae — the seven vertebrae of the neck region.
  16. Clavicle — the collar-bone, joining the shoulder blade to the upper end of the sternum.
  17. Coccyx — the last part of the backbone, made of four fused vertebrae.
  18. Compact bone — the dense, solid bone forming the wall of the shaft of a long bone.
  19. Cranium — the brain-box of the skull, made of eight permanently fixed bones.
  20. Decalcified bone — bone from which the mineral part has been removed, leaving a soft and flexible organic framework.
  21. Femur — the thigh bone, the longest and strongest bone of the body.
  22. Fibrous capsule — the tough outer covering that protects and encloses a synovial joint.
  23. Fibula — the outer of the two long bones of the shank.
  24. Fixed end — the end of a muscle that stays in place, where the muscle originates.
  25. Flat bones — bones made of parallel plates of compact bone enclosing spongy bone, such as the shoulder blade.
  26. Floating ribs — the 11th and 12th pairs of ribs, which are not attached to the sternum.
  27. Foramen magnum — the large hole at the back of the cranium through which the spinal cord passes.
  28. Fulcrum — the fixed point about which a lever turns; in the body, a joint.
  29. Gastrocnemius — the calf muscle, which raises the body on the toes.
  30. Glenoid cavity — the cup-shaped cavity on the scapula that receives the head of the humerus.
  31. Humerus — the single long bone of the upper arm.
  32. Hyoid bone — a bone in the neck that supports the tongue during speech and swallowing.
  33. Ilium — one of the three fused bones forming each hip bone.
  34. Intervertebral disc — the pad of cartilage that cushions the space between two vertebrae.
  35. Involuntary muscles — muscles that are not under the control of our will.
  36. Irregular bones — bones of complex shape, such as the bones of the face and the backbone.
  37. Ischium — one of the three fused bones forming each hip bone.
  38. Joint — the point at which two separate bones meet.
  39. Lever — a rigid bar turning about a fixed point; in the body, a bone turning about a joint.
  40. Ligament — a tough band that binds one bone to another at a joint.
  41. Locomotion — movement of the whole body from one place to another.
  42. Long bones — bones consisting of a tubular shaft with a knob at each end, such as the bones of the limbs.
  43. Lumbar vertebrae — the five vertebrae of the lower back, bearing well-developed spines and processes.
  44. Metacarpals — the five bones of the palm.
  45. Metatarsals — the five long bones of the middle of the foot.
  46. Movable end — the end of a muscle that pulls the part being moved.
  47. Neural arch — either of the two arches arising from the sides of the centrum and enclosing the neural canal.
  48. Neural canal — the canal in a vertebra through which the spinal cord runs.
  49. Neural spine — the flat ridge projecting upward where the two neural arches meet.
  50. Osteocytes — the bone cells, arranged in concentric rings within the bone matrix.
  51. Patella — the knee-cap, a bone developed from a tendon at the lower end of the femur.
  52. Pectoral girdle — the shoulder girdle, formed of the scapula and the clavicle.
  53. Pelvic girdle — the hip girdle, formed of two hip bones joined to the sacrum.
  54. Periosteum — the membrane covering the outer surface of a bone, richly supplied with blood vessels.
  55. Phalanges — the bones of the fingers and the toes.
  56. Power — the effort applied to a lever; in the body, the pull of a muscle.
  57. Pubis — one of the three fused bones forming each hip bone.
  58. Radius — the bone of the lower arm lying on the side of the thumb.
  59. Red marrow — the marrow at the ends of long bones, which produces red and white blood cells.
  60. Rib cage — the bony cage formed by the ribs, the thoracic vertebrae and the sternum.
  61. Sacrum — the large bone formed by the fusion of five vertebrae, to which the hip bones are joined.
  62. Scapula — the flat triangular shoulder blade.
  63. Short bones — box-like spongy bones allowing little movement, such as those of the wrist and ankle.
  64. Skeleton — the framework of hard parts inside the body that supports it and works with the muscles.
  65. Spongy bone — bone made of a mesh of tiny bars with spaces between them, found at the ends of long bones.
  66. Sternum — the breast-bone, a long flat bone in the middle of the front of the chest.
  67. Synovial fluid — the fluid that lubricates a freely movable joint.
  68. Synovial joint — a freely movable joint containing synovial fluid between the bone ends.
  69. Synovial membrane — the very thin membrane forming the sac that holds the synovial fluid.
  70. Tarsals — the seven bones of the ankle.
  71. Tendon — the tough structure of collagen fibres by which the movable end of a muscle is attached to a bone.
  72. Thoracic vertebrae — the twelve vertebrae of the chest region, to which the ribs are attached.
  73. Tibia — the inner of the two long bones of the shank.
  74. Transverse processes — the thick sideward projections from the neural arches of a vertebra.
  75. Triceps — the extensor muscle of the upper arm, which straightens the lower arm at the elbow.
  76. True ribs — the first seven pairs of ribs, attached in front to the sternum by hyaline cartilage.
  77. Ulna — the bone of the lower arm lying on the side away from the thumb.
  78. Vertebrae — the ring-like bones that make up the vertebral column.
  79. Voluntary muscles — muscles that are under the control of our will.
  80. Weight — the load that a lever has to move.
  81. Yellow marrow — the marrow in the cavity of the shaft of a long bone, which stores fat.

13.10 Points To Ponder

  1. The skull is made of eight separate bones locked so tightly that none of them can move. If they never move, why is the cranium built from eight bones at all, instead of being cast as a single piece?

  2. A decalcified bone bends without breaking, and a strongly heated bone keeps its shape but shatters. Which of the two failures would be more dangerous in a living skeleton, and why?

  3. The shoulder joint allows movement in every direction, while the elbow allows movement in one plane only. What does the body gain from the elbow’s restriction, and what would be lost if the elbow were a ball and socket joint?

  4. Every muscle can only pull, never push. Trace out what this single limitation forces upon the design of the body — how many muscles are needed, where they must be placed, and why so many of them come in pairs.

  5. The last two pairs of ribs are not attached to the sternum in front. Suggest what advantage a cage might gain by being left open at its lower edge.

  6. Bones store calcium, and the rest of the body withdraws calcium from them whenever the blood runs short. What does this tell you about whether a bone should be thought of as a finished structure or as a tissue still being built and unbuilt?

13.11 Objective Questions

  1. Choose the correct option. Which of the following does NOT belong to the axial skeleton? (1 mark)
    1. Sternum
    2. Ribs
    3. Scapula
    4. Vertebral column
  2. Choose the correct option. The number of bones forming the cranium is: (1 mark)
    1. six
    2. eight
    3. fourteen
    4. twenty-six
  3. Choose the correct option. The joint between the atlas and the axis is a: (1 mark)
    1. hinge joint
    2. gliding joint
    3. pivot joint
    4. ball and socket joint
  4. Choose the correct option. Red blood cells and white blood cells are produced in the: (1 mark)
    1. yellow marrow
    2. periosteum
    3. red marrow
    4. compact bone of the shaft
  5. Choose the correct option. A bone kept in weak hydrochloric acid for some days becomes: (1 mark)
    1. brittle and easily broken
    2. soft and flexible
    3. heavier and harder
    4. reduced to ash
  6. For the assertion-reason item below, choose the correct option. (1 mark) Assertion (A): The eleventh and twelfth pairs of ribs are called floating ribs. Reason (R): They are attached in front to the sternum with the help of hyaline cartilage.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true.
  7. For the assertion-reason item below, choose the correct option. (1 mark) Assertion (A): The muscles of the arm work in antagonistic pairs. Reason (R): A muscle can contract and relax, but it cannot lengthen itself.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true.
  8. Match each item in Column I with the correct item in Column II. (5 marks)
Column I Column II
a. Glenoid cavity i. Body of a vertebra
b. Acetabulum ii. Covering membrane of a bone
c. Centrum iii. Scapula
d. Periosteum iv. Knee
e. Patella v. Hip bone
  1. Fill in the blanks. (5 marks)
    1. The large hole at the back of the cranium through which the spinal cord passes is the …………… .
    2. The bone of the lower arm on the side of the thumb is the …………… .
    3. The pad of cartilage lying between two vertebrae is called the …………… .
    4. The fluid that lubricates a freely movable joint is called …………… .
    5. The muscle that straightens the lower arm at the elbow is the …………… .
  2. State whether each statement is true or false. (5 marks)
    1. Ligaments attach a muscle to a bone.
    2. The sacrum is formed by the fusion of five vertebrae.
    3. Cardiac muscle is under the control of our will.
    4. Yellow marrow is present at the ends of a long bone.
    5. Raising the body on the toes is an example of a second order lever.
  3. Name the following. (5 marks)
    1. The longest and strongest bone of the human body.
    2. The bone of the neck that supports the tongue.
    3. The three bones that fuse to form each hip bone.
    4. The type of joint found between the bones of the wrist.
    5. The bone cells arranged in concentric rings inside a bone.
  4. Pick the odd one out in each group and give a reason for your choice. (3 marks)
    1. Humerus, radius, femur, ulna
    2. Scapula, clavicle, hip bone, sternum
    3. Hinge, pivot, gliding, immovable

13.12 Check Your Concepts

  1. Define a joint, and state the three major categories into which joints are grouped. (2 marks)

  2. Name the two divisions of the human skeleton, and state which bones belong to each. (2 marks)

  3. What is meant by a decalcified bone? State one property of such a bone. (2 marks)

  4. State two functions of the skeleton other than support and protection. (2 marks)

  5. Give a reason for each of the following. (3 marks)

    1. The bones of the cranium are joined by immovable joints.
    2. The shaft of a long bone is hollow rather than solid.
    3. A person’s bones become more fragile in old age.

  6. Differentiate between yellow marrow and red marrow, stating the basis of difference. (3 marks)

  7. Why is the vertebral column curved rather than straight? Explain how this helps the body. (3 marks)

  8. Differentiate between a tendon and a ligament, stating the basis of difference, and give one example of the position of each in the body. (4 marks)

  9. Describe the structure of a typical vertebra, naming the parts through which the spinal cord passes and by which one vertebra joins the next. (4 marks)

  10. Explain, with one example each, the difference between a hinge joint and a ball and socket joint. State which of the two can bear a heavier load, and why. (4 marks)

  11. Describe the structure of a typical long bone, mentioning the periosteum, the marrow cavity and the two kinds of bone tissue in it. (5 marks)

  12. What is meant by antagonistic muscles? Using the muscles of the arm, explain why the body cannot manage with a single muscle at a joint. (5 marks)

  13. What would happen in each of the following cases? (5 marks)

    1. The synovial fluid of the knee joint dried up.
    2. The intervertebral discs were absent from the backbone.
    3. The ribs were joined to the sternum by immovable joints.
    4. The head of the femur did not fit into the acetabulum.
    5. The red marrow of the long bones stopped working.

13.13 Figure-Based Questions

  1. Study Fig. 13.3-2, which shows a longitudinal section of a long bone. (4 marks)
    1. Name the membrane covering the outer surface of the bone, and state one function of it.
    2. Name the tissue filling the cavity of the shaft, and state its function.
    3. Name the kind of bone tissue found at the two ends.
    4. Give one example of a long bone in the human body.
  2. Study Fig. 13.4-4, which shows a thoracic vertebra. (4 marks)
    1. Name the solid cylindrical part that forms the body of the vertebra.
    2. Name the opening through which the spinal cord runs.
    3. Name the thick sideward projections arising from the neural arches.
    4. State one way in which a thoracic vertebra differs from a lumbar vertebra.
  3. Study Fig. 13.4-6, which shows the bony thorax seen from the front. (3 marks)
    1. Name the three parts of the skeleton that together form this cage.
    2. Which pairs of ribs shown in the figure are the floating ribs?
    3. State one function of this cage.
  4. Study Fig. 13.4-8, which shows the pelvis seen from the front. (3 marks)
    1. Name the three fused bones that make up each hip bone.
    2. Name the cavity marked for the head of the thigh bone, and name the joint formed there.
    3. Name the bone of the vertebral column to which the two hip bones are joined.
  5. Study Fig. 13.5-4, which shows a section of a typical movable joint. (3 marks)
    1. Name this type of joint.
    2. Name the fluid present between the two bone ends, and state its function.
    3. State the function of the ligament shown in the figure.
  6. Study Fig. 13.6-2, which shows the muscles of the arm in two positions. (3 marks)
    1. Name the muscle that is contracted when the arm is bent at the elbow.
    2. Name the muscle that brings about straightening of the arm.
    3. What term is used for such a pair of muscles?
  7. Study Fig. 13.7-2, which shows simple levers in the human skeleton. (4 marks)
    1. State the order of lever represented by the raising of the body on the toes.
    2. In the flexion of the arm by the biceps, which of the three — power, weight or fulcrum — lies in the middle?
    3. Redraw the simple lever bar for a first order lever and mark P, F and W on it.
    4. Name the joint that acts as the fulcrum in the bending of the arm.