CHEMICAL COMPOSITION OF BONE. 257 



rate system of the vcrtcbrata, the skeleton is composed of 

 true bones; that is, of solid pieces, which, althouirh they arc 

 dense calcareous structures, yet continue organized during 

 the whole period of development, and form as much a part 

 of the living system as any other organ of the body. We 

 have formerly seen that the membrane in which the calca- 

 reous matter of the shell is deposited, should properly be 

 classed among the integuments; being analogous to them not 

 only in being situated externally, but also in their structure 

 and in their function. It is not so with bone, which is 

 essentially an internal structure.* 



In their chemical composition, likewise,bones are striking- 

 ly contrasted with the calcareous products of the INIolIusca: 

 for in the former, the earthy portion consists almost wholly 

 of phosphate of lime: a material which appears to have been 

 selected for this purpose from its forming much harder com- 

 pounds with animal membrane than the carbonate. Where- 

 ver great strength and rigidity are required, this is the ma- 

 terial depended on for imparting these qualities; and it has, 

 accordingly, been employed for the osseous structures, which 

 are among the most elaborate results of organization. The 

 densest and hardest of these structures are those in which 



* De Blainville rcg-ards the hard covering's of insects, tog-ether with the 

 shells of the Crustacea, as structures derived aUog-cther from the intcg-uments, 

 and as perfectly anaiog-ous, in this respect, to the scales, hoofs or other horny 

 productions of the skin in vertebrated animals. Geoffrey St. Hilaire con- 

 tends, on the contrary, that the former constitute the true skeleton of the 

 lower classes, and that a perfect analogy may be traced between the ring-s, 

 which arc the essential constituents of the frame-work of annulose animals, 

 and the vertebrae, which enclose the spinal cord of the higher classes. Pro- 

 fessor Carus appears in his system of org-anic formations, to have kept in view 

 both these analogies; g-iving- to the former class of structures the denomina- 

 tion of Uermo-skeleioJi, and to the latter that of Netiro-skeklon (See his Ta- 

 bulx Anatomiam Comparativam illustrantes, edited by Thienemann.) Ana- 

 log'ies have also been imag-ined to exist between the external and internal 

 situations of the woody fibres of plants belong-ing respectively to the endoge- 

 nous and exogenous classes, and that of the corres[)onding relative situations 

 of the skeletons of invertebrated and vertebrated animals. Sec a Memoir by 

 Dumortier, in the Nova Acta Physico-Medica Acad. Ciesar. Leopold. Caro- 

 lina Natur. Curios. XVL, 219.) 



Vol. I. 33 



