STRUCTURE OF BONE. 327 



In their chemical composition, likewise, bones 

 are strikingly contrasted with the calcareous pro- 

 ducts of the Mollusca ; 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. 

 Wherever great strength and rigidity are required, 

 this is the material resorted to 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 m hich 

 the proportion of phosphate of lime is the greatest, 

 when comj^ared with that of the animal substance 

 which cements them together ; the force of mutual 

 cohesion among its own particles being much 

 greater than that imparted by the cementing in- 

 gredient. The internal bony portions of the ear, 

 where, in order perfectly to transmit the sonorous 



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

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

 animals. GeofFroy St. Hilaire contends, on the contrary, 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 framework of annulose animals, and 

 the vertebra, which enclose the spinal cord of the higher classes. 

 Professor Carus appears, in his system of organic formations, to 

 have kept in view both these analogies ; giving to the former class of 

 structures the denomination of Dermo-skeleton, and to the latter 

 that of Neuro-skeleton. (See his Tabulae Anatomiam Compara- 

 tivam illustrantes, edited by Thienemann). Analogies have also 

 been imagined to exist between the external and internal situa- 

 tions of the woody fibres of plants belonging respectively to the 

 endogenous and exogenous 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. Carolina Natur. Curios, xvi. 219.) 



