1^- 



324 THE MECHANICAL FUNCTIONS. 



however, their ossification is not only complete, but has 

 been expanded laterally, so as to form a continuous surface 

 with the extremities of the ribs and with the edges of the 

 plastron, and completely to fill up the vacancy between 

 diem; constituting a dense and solid wall, which entirely 

 closes the sides of the general bony case. So strong is the 

 tendency to ossification in all these pieces, that the sutures 

 at first formed between them are often, in process of time, 

 obliterated; and the bony fibres are continuous throughout a 

 ffreat extent of surface. 



The most remarkable metamorphosis in the osseous sys- 

 tem of this new type is that which occurs in the sternum. 

 So expanded are all its parts, that it is difficult to recognise 

 this bone under the disguised form in which it constitutes 

 the plastron, or broad plate, which, as we have seen, covers 

 the whole of the under side of the body. Yet, by a careful 

 examination of its structure, both in the young animal, and 

 also in the adult, wiien the sutures are not obliterated, we 

 may easily recognise the nine elements of the sternum; 

 namely, the one in the middle and fore part, and the four 

 pairs of lateral pieces; each having been formed from its re- 

 spective centre of ossification. In form and relative propor- 

 tion, indeed, they are widely different from the same parts 

 as they are presented in the skeletons of other animals: yet 

 in number and in relative situations they preserve that con- 

 stancy and uniformity so characteristic of the beautiful har- 

 mony 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 atmo- 

 sphere. Hence we find them covered throughout with a thin 

 horny plate, originally a production of the integument. It 

 is this substance which is commonly known by the name of 

 tortoise shell.* 



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

 externally, bear no relation to the sutures which scpai'atc the subjacent bones, 



