366 THE MECHANICAL FUNCTIONS. 



membrane, in which the calcareous 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 strikingly contrasted with the calcareous 

 products of the MoUusca : for in the former, the 

 earthy portion consists almost wholly of phos- 

 phate of lime : a material which appears to have 



* De Blainville regards the hard coverings of insects, together 

 with the shells of the Crustacea, as structures derived altogether 

 from the integuments, and as perfectly analogous, in this respect, 

 to the scales, hoofs, or other horny productions of the skin in 

 vertebrated animals. Geoffrey St. Hilaire contends, on the con- 

 trary, that the former constitute the true skeleton of the lower 

 classes, and that a perfect analogy may be traced between the 

 rings, which are the essential constituents of the frame-work of 

 annulose animals, and the vertebrae, which enclose the spinal 

 cord of the higher classes. Professor Cams appears, in his 

 system of organic formations, to have kept in view both these 

 analogies ; giving to the former class of structures the denomina- 

 tion of Dermo-skeleton, and to the latter that of Neuro-skeleton 

 (See his Tabulae Anatomiam Comparativam illustrantes, edited 

 by Thienemann). Analogies have also been imagined to exist 

 between the external and internal situations of the woody fibres 

 of plants belonging respectively to the endogenous and exoge- 

 nous classes, and that of the corresponding relative situations of 

 the skeletons of invertebrated and vertebrated animals. (See a 

 Memoir by Dumortier, in the Nova Acta Physico-Medica Acad. 

 Caesar. Leopold. Carolinae Natur. Curios. XVI., 219). 



I 



