COMPOSITION OF BONE. 177 



outermost cylindrical lamella of bony matter ; and each of these lamellae 

 derives its nourishment from the vessels of the central canal, through 

 the lamellae which intervene between itself and the vascular membrane 

 lining that tube. 



296. Thus every one of the. Haversian canals is the centre of a 

 cylindrical ossicle, which is complete in itself, as far as its elementary 

 structure is concerned, and which has no dependence on, or connexion 

 with, other similar ossicles. These are arranged, however, side by side., 

 like sticks in a faggot ; they are bound together by a thin cylinder of 

 bone, on the exterior of all, which derives its nourishment from the 

 periosteum, or enveloping membrane : in like manner, the hollow bundle 

 is lined by a similar cylinder, which surrounds the great medullary 

 cavity, and is nourished by its vascular membranes; and the spaces 

 that here and there intervene between the ossicles are filled up with 

 laminae, which are parallel to those of the external and internal cylinders, 

 and which seem to derive their nutriment from them (Fig. 49, 4). In 

 this manner, the whole structure acquires great density and solidity. 

 The structure of the outer and inner tables of the skull, and of other 

 thick solid layers of bone, is precisely similar ; except that the Haver- 

 sian canals have no such definite directions, and form an irregular net- 

 work. 



297. Thus we see that each of the lamellae of bone surrounding an 

 Haversian canal, or bounding the cancelli, may be regarded as a repe- 

 tition of the simple bony plate, which draws its nourishment direct from 

 the vascular membrane covering its surface, by means of its system of 

 lacunae and canaliculi. The membrane lining the Haversian canals, 

 cancelli, and central medullary cavity, is an internal prolongation of 

 that which clothes the exterior ; just as the mucous membranes, with 

 their extensions into glandular structures, are internal prolongations of 

 the true skin. Every Haversian canal and every cancellus are repe- 

 titions of each other in all essential particulars, their form alone being 

 different. The central medullary canal is but an enlarged Haversian 

 canal or cancellus. And the whole cylindrical shaft is a collection of 

 ossicles, each of which is a miniature representation of itself, being a 

 hollow cylinder, with a central vascular cavity. 



298. The principal features of the Chemical constitution of Bone are 

 easily made evident. After all the accessory parts have been removed, 

 and nothing remains but the real osseous texture, tjiis may be separated, 

 by simple processes, into its two grand constituents, the animal basis, 

 and the calcareous matter. The latter may be entirely removed by 

 maceration of the bone in dilute Muriatic or Nitric acid ; and a substance 

 of cartilaginous appearance is then left, which, when submitted to the 

 action of boiling water for a short time, is almost entirely dissolved 

 away, and the solution forms a dense jelly on cooling. The same sub- 

 stance, Gelatine, may be obtained by long boiling under pressure, from 

 previously unaltered bone ; and the calcareous matter is then left in a 

 friable condition. By submitting a bone to a heat sufficient to decom- 

 pose the animal matter, without dissipating any of the earthy particles, 

 we may obtain the whole calcareous matter in situ; but the slightest 

 violence is sufficient to disintegrate it. The bones of persons long 



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