The structure, disposition, and connection of the individual bones accomplish in the most perfect manner the following mechanical uses :-1. By their hardness and firmness they afford a support to the euft parts, fdi-ming pillars to which the more delicate and flexible organs are attached, and kept in their relative positions. 2. By the same properties of hardness and firmness they defend the soft and tender organs, by forming solid and strong cases in which such organs are lodged and protected ; as the case formed by the bones of the cranium for the lodgment and protection of the brain ; by the bones of the vertebral column for the lodgment and protection of the spinal cord ; and by the bones of the thorax for the lodgment and protection of the lungs, the heart, and the great vessels connected with it. 3. By affording fixed points for the action of the muscles, and by assisting in the formation of joints, they aid and are indeed indispensable adjuncts to the muscles in accomplishing the function of locomotion.
Bone is a complex organ, and the arrangement and combination of its constituent parts are highly curious. It is composed essentially of two distinct substances, an animal and an earthy matter. The animal matter is composed of gelatine ; the earthy matter consists principally of phosphoric acid combined with lime, forming phosphate of lime.
This structure of bone is rendered manifest by subjecting it to certain chemical processes. If a bone be placed in a charcoal fire, and the heat be gradually raised to whiteness, it appears on cooling as white as chalk ; it is extremely brittle ; it has lost very much of its weight, yet its bulk and shape are little changed. In this case the membranous matter is wholly consumed by the fire, while the earth is left unaltered. Over the surface of a bone so treated are visible a number of minute crevices, the spaces which were filled in the natural state of the bone with the animal matter ; and on breaking the bone across, the size and shape of the cavities which contained the marrow become manifest. If on the other hand the same bone be placed in an acid sufficiently diluted to prevent its injuring the animal mem brane, and yet strong enough to dissolve the phosphate of lime—if for this purpose it be macerated in diluted nitric or hydrochloric acid —every particle of the phosphate of lime may be removed, and the animal matter alone will remain perfectly uninjured and unaltered. Accordingly the remaining substance retains the exact figure and dimensions of the original bone, but it has lost all its other mechanical properties. It is so soft and flexible that if either of the long bones of the human arm—that, for example, called the radius—be treated in this manner, it can with the utmost ease be tied in a knot. By the first process the earth is obtained, deprived of its animal constituent ; by the second, the membranous matter free from the earth. In the
bone both are combined ; in every constituent atom of it there is an earthy in intimate combination with an animal matter. The first gives it hardness, the second tenacity ; and thus by the intimate combination of these elements two qualities which in unorganised matter are scarcely compatible are combined. By increasing the pro portion of phosphate of lime any degree of hardness can be obtained : the bony portions of the ear, the bony portions of the teeth, for example, are as hard as marble, or even flint ; but substances so hard would not do for the ordinary purposes of bone, because they would be brittle in proportion to their hardness, and would be productive of fatal mischief whenever they were subject to any sudden and violent concussion.
In certain diseased states of the human system the earthy matter preponderates in the whole osseous system, and in this condition per sons are liable to fracture their bones by the slightest accident. On the other hand, the earthy matter is sometimes deficient ; then the bones give way and become bent, and ultimately the body becomes an immoveable mass.
Bones not only differ so much from one another in their comparative hardness according to the office which each has to serve that no two bones possess the same degree of rigidity, but no bone is equally hard in its entire substance. When a section of a bone is made in such a manner as to show its structure throughout, it is seen to consist of two varieties, a hard or compact and an alveolar or spongy substance. In general the compact forms the external and the spongy the internal portion of the bone ; the compactest part of the bone forms a com pletely solid body, exhibiting scarcely any visible arrangement, without apparent fibres and laminae; but towards the inner part of the bone the substance becomes less and less dense: until at length it presents the appearance of minute and delicate fibres, which intersect each other in every direction, forming the cells termed cancelli (lattice work). The transition from the compact to the spongy or cancellated part is not marked by any distinct boundary ; the one passes into the other by insensible degrees, showing that there is no essential difference between them; and indeed the evidence is complete that, although in the densest part of the bone there is scarcely any trace of specific organisation, it is made up of fibres and plates perfectly similar to those of the spongy or cancellated part, differing from it principally in its greater degree of condensation. Often in the centre of the bone there is scarcely any even of the spongy matter, but a hollow space is left, which is filled up with a series of membranous cells in which the substance called marrow is lodged.