94 



THE STERNUM. 



[CHAP. 



Parker, ectosteally. 



first pair of sternal ribs, and terminating anteriorly in a 

 conical rounded projection. 



The segments of the mesosternum are elongated, and 

 more or less four-sided, contracted at the middle, and 

 widening at each extremity. They ossify, according to 

 or from without inwards, the bony 

 deposit commencing in the inner layer 

 of the perichondrium, as in the shafts 

 of long bones ; and they remain per- 

 manently distinct from each other. 



The xiphisternum is long, narrow, 

 and flat, and generally ends in an ex- 

 panded flattened cartilage. 



In the Pinnipedia, the presternum 

 is produced considerably in front of 

 the attachment of the first pair of 

 ribs. 



In the INSECTIVORA, the sternum 

 is variable in form, but always more 

 or less elongated and segmented. 

 The presternum is always more or 

 less expanded laterally in this clavi- 

 culated group of aninials. It is bilo- 

 FIG. 33. sternum and sternal bate in front in the Hedgehog 



ribs of Dog (Ca.nisfa.tni- , . \ ^ -i i ., ^i 01 



Han's), if. /.? presternum ; (fcrinaceits), trilobate in the bhrews 



jus mesosternum ; .rs xiphi- /r , \ T /-> , , v i j 



sternum. (Sorex). In Rliynchocyon it is broad 



in front, narrow posteriorly, strongly 



keeled below, and with two horn-like processes projecting 

 outwards and forwards between the attachment of the 

 clavicles and the first pair of ribs. 



The mesosternum is usually narrow, as in the Carnivora, 

 but in the Hedgehog, where it consists of three segments, 

 it is broad and flat posteriorly, and to the last segment three 



