The Role of the Skeleton
In Ezekiel’s famous vision of bones as described in the Bible, “there were very many in the open valley and, lo, they were very dry,” but it should be recalled that the prophet bravely addressed himself to the bones as follows: “I will lay sinews upon you, and will bring up flesh upon you, and cover you with skin, and put breath in you, and ye shall live.”
To make dry bones, wherever found, live again is exactly what biologists have been trying to do ever since, with the gratifying and somewhat surprising result that the study of bones, with their many transformations and homologies, turns out to be one of the most animated chapters in all comparative anatomy.
The student of anatomy usually begins with bones, since they are easily available, can be used over and over again, and are not unpleasant to handle. If he succeeds in passing undismayed through this valley of dry bones, he feels himself initiated and ready to go on with what are generally regarded as the livelier aspects of the subject.
The vertebrate skeleton is considerably more than dry bones that furnish a scaffolding for the softer parts of the body. There are at least five specific uses to which the skeleton is put, namely, (1) to give protection to other parts; (2) to make a framework for the support of the body; (3) to furnish a firm as well as an adequate surface for the attachment of muscles; (4) to provide suitable leverage for locomotor purposes; and (5) to keep up a continuous manufacture of red blood corpuscles in the marrow tissue within the cavities of the bones.
Protection is one of the earliest of skeletal functions to develop phvlogenetically, being found wherever life teems, even in plants. Among invertebrate animals it reaches its most efficient elaboration in the limy shells of mollusks and the chitinous armor of arthropods. Traces of the external skeleton of invertebrates in the form of scales, nails, and various hard comeal structures still persist among vertebrates, although here the function of protection is principally taken over by the living endoskeleton as represented by the boxlike cranium around the brain, by the neural arches of the spinal column enclosing the nerve cord, and by the thoracic basket which provides sanctuary for various important soft organs (Fig. 421).
The skeleton as a support, that is, as an internal living scaffolding on the outside of which muscles are arranged, instead of a lifeless external armor, is a brand new idea peculiar to vertebrates and is of far-reaching evolutionary significance in the animal kingdom. Obviously, a living growing endoskeleton avoids the limitations of size imposed by invertebrate exoskeletons. Its capacity for continuous growth and adaptation keeps easy pace with the increasing demands of the enlarging organism as no non-living outside skeleton could do. During the Mesozoic “Golden Age of Reptiles,” while arthropods and mollusks were still imprisoned with clumsy unyielding armor, Mother Nature was reveling in the possibilities furnished by a living internal skeleton among her emancipated vertebrates. Gigantic dinosaurs, plesiosaurs, iguanodons, and theromorphs, together with all their monstrous and bizarre kith and kin that have long since vanished, literally lifted tons of flesh into the air upon majestic bony scaffoldings, which today are a source of never-ceasing wonder to the visitor of museums where they are exhibited. No arthropod or mollusk of such astonishing dimensions as these extinct reptiles displayed is conceivable. There still remain colossal elephants on land and enormous whales in the oceans as living examples of how far it is possible to go in the matter of body size, when an adequate internal support is provided.
Since sufficient solid surface must be conveniently placed so as to give proper foothold for the muscles, the skeleton arises to meet this necessity. Not all muscles require the aid of bones. Those responsible for peristaltic movement in the intestinal wall, for pulsating arteries and the throbbing heart, as well as various sphincters, such as those that close the anus or pucker the whistling lips, are not directly attached to hard skeletal parts. Most muscles, however, through the mediation of tendons do have bony anchorage both at their origin and at their insertion.
The skeleton of the flying bird furnishes an excellent illustration of what is meant by having a relatively large solid expanse for muscle attachment. In spite of the necessity in every flying machine for economizing weight, it will be seen (Fig. 422) that in the bird’s skeleton there is a relatively enormous breastbone, with a thin “keel” at right angles to it, which practically doubles the expanse of surface available for the attachment of flying muscles without adding very much to the total weight. The ribs of the bird are much flattened also to make the same amount of bone furnish the largest possible surface, while the swollen ends of the leg bones have a considerably larger relative expanse available than do corresponding mammalian bones.
In man conspicuous skeletal surfaces are to be found on the innominate bones of the pelvis, the muscles of which have to do with maintaining an upright posture, as well as on the broad shoulder blades that furnish anchorage for the muscles operating the swinging arms.
In the evolutionary emergence of vertebrates from water to land and air, levers in the form of legs and wings appeared as organs of locomotion. Such specialized levers are unnecessary for a fish which moves about by wagging the entire body sidewise in a watery medium sufficiently resistant to make lateral strokes effective. The difference in density between water and air is so great, however, that a mechanism moving successfully by sculling in water would prove entirely ineffective when operated in the thin and comparatively non-resistant medium of air.
One of the first steps in the evolution of locomotion upon land, therefore, has been the elevation of the elongated body from the ground by means of proplike legs to minimize the amount of frictional surface and at the same time to provide a system of skeletal levers upon which the muscles of locomotion may act in propelling the animal forward.
In birds the body is poised upon the hind legs alone, while the fore legs are released from terrestrial locomotion, becoming transformed into wings for flight through air. In both cases a system of bony locomotor levers is emphasized.
The last of the five general uses of the skeleton, that of the manufacture of red blood corpuscles, is accomplished, in mammals at least, by means of vigorous marrow tissue within the hollow bones.
Four general kinds of skeletal tissues may be recognized in vertebrates, namely, notochordal, connective, cartilaginous, and bony. These tissues have been briefly described in Chapter VII.