410 THE ME€HA\I( AL FUNCTIONS. 



be formed; namely, the one in the middle and fore 

 part, and the four pair of lateral pieces ; each 

 having been formed from its respective centre of 

 ossification. In form and relative proportion, in- 

 deed, they are widely different from the same parts 

 as they are presented in the skeletons of other ani- 

 mals ; yet in number and in relative situations they 

 preserve that constancy and uniformity so charac- 

 teristic of the harmony which pervades all animal 

 structures. 



It is to be noticed, also, that as the plates, which 

 form this investing case, are bony structures, they 

 could not with any safety have been exposed to the 

 action of the atmosphere. Hence we find them 

 covered throughout with a thin horny plate, origin- 

 ally a production of the integument. It is this 

 substance which is commonly known by the name 

 of tortoise shell* 



The immobility of the trunk is compensated, as 

 far as regards the safety of the head, by the great 

 flexibility of the neck ; which is composed of seven 

 vertebrae, unencumbered by processes, and capable 

 of taking a double curvature like the letter S, when 

 the head is to be retracted within the carapace. 

 These vertebrae are joined by the ball and socket 

 articulation common to all the existing species of 

 reptiles. t The articulation of the head with the 

 neck is effected in the same manner ; but it is 



* It should be observed, that the divisions of these plates, which 

 appear externally, bear no relation to the sutures which separate the 

 subjacent bones, so that it is not possible to draw inferences respect- 

 ing the form of the latter from the mere inspection of the external 

 shell. 



f The expression of this fact is thus qualified, because it does not 

 apply to many fossil or extinct species, such as the Ichthyosaurus. 



