Bones are not subservient to locomotion only ; they have, in some parts of the body, the important office of enclosing and defending from external injury the more delicate organs of the system. We shall find, therefore, that in the young animal, according as they fulfil the one office or the other, their development will differ. But whatever be the functions of the bones, they require, for the perfection of that function, three mechanical properties,—firm ness, lightness, and tenacity. They must not admit of flexion, and, at the same time, the density of their substance must not render them cumbrous by weight, or brittle in texture. To present these three conditions, the organs in question consist of two principal ingredients, an animal matter and an earthy matter, most intimately interwoven ; the one preventing such vibrations as would occasion risks of fracture, the other affording the necessary strength in supporting weights, and in resisting the divellent tendencies of antagonist muscles. The pro portion which these parts bear to each other varies with the ages of the human subject. Viewed as a part of the system devoted to the life of relations, bones are used as pillars of support, as levers in various attitudes and mo tions, and as points d'appui to the muscles and tendons. On examining the constitution of these portions of the osseous system in the new-born infant, we find the quantity of cal careous salts comparatively small, and even the animal substance softer than in later pe riods, in consequence of the greater ratio of gelatine. In growth these proportions undergo a gradual alteration ; the gelatine is diminished, the cartilage becomes firmer, and both give way to the deposition of earthy particles : in the increase of density produced by this de position consists the process of ossification. To particularize the incompleteness of the osseous system would require us to enter upon the anatomy of almost every bone in the body, an investigation incompatible with the limits of this article. Some idea of it may be obtained from the fact that all the epiphyses of the long bones, and the greater number of the apophyses are as yet but cartilaginous ; they derive their ossification, not from an extension of the pro cess in the bones to which they are attached, but from ossific centres within their own spheres. In the tarsus the only bones in which ossification has commenced are the as tragalus and os calcis. The carpus is entirely cartilaginous. The os innominatum of the pel vis consists of three separate bones ; ossifica tion has but just commenced in the descending ramus of the pubis, and the ascending part of the ischium ; and the consolidation of the pel vis is not complete till after the thirteenth year. The long bones have no central medullary cavity in the early periods of intrauterine life ; but in the foetus at its full term, the animal matter which occupied that space has begun to be absorbed, and the deposition of osseous matter takes place in the form of a cylindrical sheath, so that the canal exists at this period, though in an incomplete state. The medullary canal is analogous to the cells of the short and flat bones, and of the extremities of the long bones, which are also incomplete in infancy. The shape of the cylindrical bones is mani festly different from that which they afterwards assume ; thus there is a much smaller dispro portion between the diameters of the extremi ties and that of the shaft ; the surface is less furrowed by sinuses or roughened by ridges; differences exactly corresponding to the imper fect development of the muscles, which, when more bulky in their middle portions, require a larger space for their accommodation about the body of the bone, and when stronger in contraction, require attachments that will match them in firmness. The osseous system is not complete till after the age of twenty.
There is no part of the skeleton in which we have a more striking illustration of its gradual development than in the bones of the face and in the cranium. It is not till the seventh year that
a separation begins to take place between the tables of the skull, that the frontal sinus begins to open, that the nasal bones lengthen, that the cells of the malar and upper maxillary bones are enlarged, that in consequence of this ex pansion of their cavities the outer lamina pro jects, and that the lower jaw is elongated. The stationary condition of the tabula vitrea is conformable to the arrest in the increment of the brain ; the extension of the outer table to the increasing power and action of the muscles attached to it ; the development of the sinuses and cells to that of the voice and certain of the senses ; and the projection of the jaws to the increased number of the teeth. But although these changes commence as early as the seventh year, they are not complete till the twenty-first, or even later. At this time the countenance becomes settled, not merely by the full deve lopment of the muscles, which express the predominant emotions of the individual, but also by the complete adjustment of the bony arrangements just enumerated.
Those portions of the osseous system which are employed in protecting the organs enclosed by them from external compression or injury, have attained a degree of growth far surpass ing that of the bones devoted to locomotion and to the mechanism of sensation. The ribs, for instance, defending the lungs and the heart, and playing so important a part in respiration, are farther advanced in the ossific process than the bones of the extremities. But the most striking fact of this kind is presented in the spinal column. The annular portions of the vertebrae which form the canal of the medulla spinalis, are found strongly ossified at birth, but the bodies of these bones, which are to be used hereafter in supporting the weight of the head and trunk, are very slightly expanded, and all but devoid of earthy particles, while the processes to which the muscles employed in the flexion and extension of the column after wards contract attachments, are either only shaped in cartilage, or may be said to have no existence.
Passing from the bones to the muscles, we observe the latter no less incomplete in infancy as it regards their physical characters ; they are pale, flabby, and easily torn ; they contain less fibrine than in after years; their contractility is weak though easily excited ; and the fasciculi and fibres are but loosely connected from the want of the fasciae and aponeuroses which brace them in later periods. As life advances, the fibres become redder, more distinct, and stronger. A readiness to contract is manifested very early, but it is not till maturity that these organs are able to maintain contraction for any length of time. They suffice well for the quick and buoyant motions of the lively child, but fail in those violent and prolonged exertions required by the labours of manhood. The form of the muscles changes materially in the progress of years ; thus, they swell out in the middle, and occasion a great difference in the proportions of the limbs. Those portions of the locomotive apparatus attached to the muscles and articulations, viz. the tendons and liga ments, undergo corresponding changes. In infancy they are soft and gelatinous ; gradually they firmer, their gelatine acquires a more glutinous character, and the membrane which envelopes them is more condensed. Every one knows the different products obtained by boiling the tendinous parts of young and adult animals ; in the one they have the qualities of jelly, in the other of glue. The readiness with which the joints of a child are strained or dis located is likewise well known. The imma ture condition of the infant is strongly marked in the ankles, which are turned inwards, and would never suggest the use to which the feet are to be applied, but for our familiarity with the change that afterwards occurs. The car tilages and fibro-cartilages are subjected to a development corresponding to that of the fibrous tissue.