410 THE MECHANICAL FUNCTIONS. 



hydrostatic pressure, which supports the body 

 on all sides, supersedes the necessity of that co- 

 hesive rigidity of frame, which is essential to the 

 safety of terrestrial animals. Hence we find 

 that in one whole tribe of fishes, the skeleton is 

 composed merely of cartilage ; and, in all, it ex- 

 hibits much less of the osseous character than in 

 the higher classes. The frame-work of the ske- 

 leton, even of osseous fishes, has not the com- 

 pactness possessed by that of quadrupeds or 

 reptiles: the pieces which compose it are joined 

 together less firmly ; many of them, indeed, 

 remain in an imperfectly ossified condition, their 

 elementary pieces being detached from one ano- 

 ther, as if the usual process of consolidation had 

 been arrested at an early stage. The texture of 

 the bones of cartilaginous fishes corresponds to 

 this primeval condition ; for it is composed merely 

 of granules of calcareous phosphate, interspersed 

 amidst the cartilaginous substance in detached 

 masses, or presenting the appearance of coarse 

 fibres, thinly scattered through the semitrans- 

 parent bone. Compared with the quantity of 

 gelatin which enters into their composition, the 

 bones of fishes contain but a small proportion of 

 earthy ingredient ; a circumstance which ex- 

 plains the pellucidity of the mass, and the readi- 

 ness with which the osseous fibres it contains 

 can be distinguished. Another consequence of 

 the want of density in the bones of fishes is, that 



