408 THE MECHANICAL FUNCTIONS. 



bony fibres are continuous throughout a great 

 extent of surface. 



The most remarkable metamorphosis in the 

 osseous system 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, when 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 pair of lateral pieces ; 

 each having been formed from its respective 

 centre of ossification. In form and relative 

 proportion, indeed, they are widely difierent 

 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 harmony which pervades all animal 

 structures. 



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

 which form this investing case, are bony struc- 

 tures, 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, originally a production of the inte- 



