SKELETAL tissues are hard substances formed by living cells. Frequently they contain non-living mineral matter such as calcium salts. The structures made of such non-living material can nevertheless grow and change as a result of the activities of living cells which dissolve away and replace the hard materials.
Exoskeletons. Where the hard material is formed mainly on the outside of the body it is often called an exoskeleton. Insects and crustaceans such as crabs have exoskeletons or cuticles, though there are projections from the exoskeleton into the body cavity for muscle attachment. Animals with exoskeletons increase their size periodically by dissolving and absorbing most of the cuticle, splitting and shedding the outermost layers, and forming a new cuticle on the exposed surface. This is called ecdysis.
Endoskeletons. Vertebrate animals have bony skeletons within their bodies. These animals can grow by a continuous increase in size and not by a series of ecdyses.
Functions of the skeleton The functions can be grouped conveniently under the headings, support, protection, movement and locomotion, and muscle attachment.
Support. There are many invertebrate animals which have no skeleton. Those living in water may become fairly large because the water supports them and buoys them up toa certain extent. In others, e.g. the earthworm, they are supported by the pressure of fluid in their body cavities acting outwards against a muscular body wall. In larger, land-dwelling animals, a rigid skeletal support raises the body from the ground and allows rapid movement, it suspends some of the vital organs and prevents them from crushing each other, and maintains the shape of the body despite vigorous muscular activity. Examination of the skeleton of the rat will show the backbone as a bridge-like arch or span from which the organs of the body are suspended.
Protection. Certain delicate and important organs of the body are protected by a casing of bone. The brain is enclosed in the skull, the spinal cord in the "backbone", while the heart and lungs are surrounded by a cage of ribs between the sternum and spine. The organs are thus protected from distortion resulting from pressure, or injury resulting from impact. The rib cage plays a positive part in the breathing mechanism in addition to protecting the organs of the thorax. In animals with exoskeletons the entire body is protected by the cuticle.
Movement. Many bones of the skeleton act as levers. When muscles pull on these levers they produce movements such as the chewing action of the jaws, the breathing movements of the ribs and the flexing of the arms. Locomotion is the result of the co-ordinated action of muscles on the limb bones and is dis cussed more fully, Movements of the skeleton require a system of joints and muscle attachments.
JOINTS. Where two bones meet, a joint is formed. Sometimes, as in the sutures between the bones of the skull, no movement is permitted; in others, e.g. the vertebrae of the spine, only a very limited movement can occur, while the most lamiliar joints, synovial joints, allow a considerable degree of movement. The ball and socket joints of the humerus and Scapula or femur and pelvis allow movement in two planes. The hinge joints of the elbow and knee allow movement in only one plane.
The surfaces at the heads of the bones which move Over each other are covered with a tough cartilage which is slippery and smooth. This, together with a liquid called synovial fluid which is formed in the joint, allows friction-free movement The relevant bones of the joint are held together by strong ligaments which prevent dislocation during normal movement. Surrounding the joint is a capsule of fibrous material whose inner lining, the synovial membrane, secretes the synovial fluid.
The vertebrae of the spine can also move slightly so that the backbone as a whole is flexible. The vertebrae are separated by discs of cartilage.
MUSCLE ATTACHMENT. The muscles must be attached to the limb bones at one end in order to produce movement but, in addition, they must have a rigid attachment at the other end so that only one part of the limb moves when the muscle contracts.
Sometimes the "stationary" end is attached to the upper half of the limb. The extensor muscle which extends the foot is attached to the femur. The muscles which move the femur, however, are attached to the pelvic girdle. Bones frequently have projections or ridges where muscles are attached.
GIRDLES. To produce movement of the body as a whole the backward thrust of the limbs against the ground or the water must be imparted to the body. The force is usually transmitted through a skeletal structure called a girdle, which is attached to the spinal column. The pelvic girdle is rigidly joined to the base of the spine, so that in walking or jumping the force of the leg-thrust is transmitted to the spine, which is the central support of the whole body. By this means also, the weight of the body is supported when at rest. The shoulder blades, which form part of the pectoral girdle are not fused to the spine in mammals, but bound by muscles to the back of the thorax. The fusion of the pelvic girdle to the spine is very effective in transmitting force from the legs to the body.
The muscular attachment of the shoulder blades to the spine is less effective in transmitting force from the arms to the body, but this function is not so necessary in the foreas in the hind-limbs; and the free movement of the shoulders allows greater mobility of the arms. The shoulder in man is more mobile than in most mammals, and the clavicle acts as a radius, limiting the movement of the scapula.
THE SPINE Consists of a number of vertebrae separated from each other by discs of cartilage. Each vertebra can move slightly with respect to its neighbour, though the movement is restricted by ligaments and the processes, zygapophyses, which interlock with each other. The sum effect of this limited movement is to make the spine a flexible support for the body.
Each vertebra consists of a solid cylinder of bone, the centrum. On the dorsal (upper) side of the centrum runs the spinal cord enclosed in an arch of bone, the neural arch. Spinal nerves leave the spinal cord through gaps between the neural arches of adjacent vertebrae. The neural spine and transverse processes serve for attachment of ligaments and muscles, while the zygapophyses articulate with the next vertebra.
The spine in man acts as a vertical rod, balancing the skull and holding the body erect. In other mammals, the spine forms a kind of girder bridge and the vertebrae down its length are submitted to different stresses and have structures differing quite markedly from each other in certain sections.
Cervical (neck) vertebrae There are 7 of these of which the first is called the atlas. Its foremost end articulates with the skull in such a way as to permit a nodding movement. The atlas vertebra is articulated behind to the axis vertebra which permits a rotary movement between the two. These two and the remaining five neck vertebrae form a column which supports the head. They are recognized by the relatively short neural spine and transverse processes, and the vertebrarterial canal.
Thoracic vertebrae. There are 12 or 13 of these in most mammals and their long neural spines form an anchorage for the muscles and ligaments that support the head and neck.
The ribs articulate with facets on the short transverse processes and on the central between adjacent vertebrae.
Lumbar vertebrae.. The well developed neural spine and transverse processes of the lumbar vertebrae serve for the attachment of the powerful back muscles that maintain posture, and flex the spine in movement. Sheep and rats have 6 lumbar vertebrae, man has 5.
Sacrum. This consists usually of 4 or 5 vertebrae fused to each other. The first two are closely articulated to the pelvic girdle making an almost rigid structure which transmits the thrust of the legs directly to the spine.
MUSCLES are bundles of elongate cells enclosed in sheaths of connective tissue. Each end of a muscle is drawn out to form an inextensible tendon, which is attached to the tough membrane, periosteum, surrounding the bones of the skeleton.
Muscle cells, if stimulated by a nervous impulse, will contract to about two-thirds or one-half their resting length. This makes the muscle as a whole shorter and thicker and, according to its attachments at each end, it can pull on a bone and so produce movement.
Muscles usually act across joints in such a way that the bones are worked as levers with a low mechanical advantage, may help to make this clear. The muscles can Contract only a short distance, but because they are attached near to the joint the movement at the end of the limb is greatly magnified. The biceps muscle of the arm may contract only about 10 cm, but the hand will move about 60 cm
Muscles can only contract and relax, they cannot lengthen of their own accord. They have to be pulled back to their original length. Consequently most muscles are in pairs: one produces movement in one direction, the other in the opposite direction.
Where such antagonistic pairs act across a hinge joint they are called extensor and flexor muscles. The extensor tends to extend or straighten the limb while the flexor bends or flexes it. Of such pairs of muscles one is usually much stronger than the other. The biceps for flexing the arm is better developed than is the triceps for extending it. The frog's hind-leg extensor muscles which make it leap are stronger than the flexors which return the limb to rest. Locomotion is brought about by the co-ordinated movement of limbs by sets of antagonistic. When the body is at rest, both antagonistic muscles remain in a state of tension or tone and so hold the body in position. The skeletal muscles and others such as those in the tongue are called *voluntary" muscles because they can be contracted at will. The muscle of the alimentary canal and arterioles is "involuntary because it cannot be consciously controlled.
Locomotion
The limbs are moved in a co-ordinated sequence, each one the thrusting backwards on the ground and so propelling the animal forwards. While the limb is recovering its forward position it must be removed from contact with the ground.
In some of the muscles of the hind-limb of the rabbit are shown in diagram form. If muscle A contracts it wil pull the femur backwards. Friction between the ground and the toes prevents the foot sliding back so that a forward thrust is transmitted through the pelvic girdle to the spine and thence to the whole animal. Such a contraction would contribute to walking or crawling if the tone of muscles B and C was maintained or slightly adjusted. Relaxation of A followed by conyraction of a and c would bring the hind-limb forward with the foot raised clear of the ground.
Contraction of B will straighten the leg at the knee. Contraction of C will extend the foot. Thus if muscles A, B and C contract simultaneously, the leg will extend and straighten at the same time as it is swinging backwards. The downwards and backwards thrust of both hind-limbs, with corresponding movements of the fore-limbs and spine will produce a leaping action. Muscles antagonistic to A, B and C, of which only a and c are shown, will contract when their opposite numbers relax, thus flexing the limb and returning it to its original position, before it touches the ground again.
There are, of course, many more muscles involved than are shown in and their coordinated action is more subtle and complex than can be described here.
To produce effective movement it is essential that the contraction of the many sets of muscles is co-ordinated so that, for example, the legs are moved in a logical sequence and Conantagonistic muscles do not contract simultaneously. Contributing to this co-ordination there is a system of stretchteceptors in the muscles. These fire nervous impulses to the spinal cord when the muscle is being stretched. Such internal sensory organs or proprioceptors, linked to the nervous system, feed back information to the brain about the position of the limbs and enable a pattern of muscular activity to be computed by the brain, so producing effective movement.