286 



THE MECHANICAL FUNCTIONS. 



joints of terrestrial animals; for it is evident that where the, 

 parts are soft and flexible, joints are not required. 



In the osseous fishes, the bony structures are more finished; 

 and they even arrive at a degree of hardness, equal to that 

 of the higher classes. But this development is not uniform in 

 all the bones; in the head of the pike, for instance, while 

 some of the bones have acquired a great hardness, others re- 

 main wholly and permanently in a cartilaginous condition. 

 The bones of fishes, however advanced in their ossification, 

 never reach that stage of the process in wdiich cavities are 

 formed ; thus there is no space for marrow, nor even for the 

 cellular or cancellated structure which we have noticed in 

 the more perfect bones.* The general disposition of the 



bones which compose the entire skeleton will be understood 

 from Fig. 184, w'hich represents that of the Cijpriniis carpio, 

 or carp. The muscular flesh of fishes is likewise softer than 

 that of the higher classes; and the cellular substance more 

 attenuated and more gelatinous; so that the membranes 

 which it forms are of a looser and m.ore pulpy texture. 



Progressive motion in fishes is effected by the simplest 

 means, the principal instrument employed for this purpose 

 beina; the tail ; for the fins, as we shall presently find, are 

 merely auxiliary organs, serving chiefly to balance the body 

 while it receives its propulsion from the tail. A fish moves 

 in the water upon the same principle as a boat is impelled 



Cuvier, sur les Polssons. Tom. i. p. 218. 



