CHAP, viii.] GENERAL CHARACTERS 107 



part of the skeleton. These differences have given rise to 

 the division into true ribs and false ribs (by no means good 

 expressions), signifying those that join the sternum directly 

 and those that do not ; and of the latter, those that are 

 free below are called floating ribs. 



Each primary piece of cartilage, out of which one of the 

 half hoops or ribs is developed, is, moreover, divided trans- 

 versely into two portions, which assume different characters, 

 as they usually undergo a different mode of ossification, and 

 remain more or less distinguishable from each other during 

 life. The portion nearest the vertebral column is called 

 the vertebral rib. This is the larger segment, and becomes 

 firmly ossified at an early period by ectostosis ; l it is the 

 bone commonly spoken of as a "rib." 



The portion towards the sternal extremity or sternal rib 

 is usually imperfectly ossified, and always at first (except 

 in Monotremes) by cndostosis? Sometimes it remains per- 

 manently in a cartilaginous state ; but, on the other hand, in 

 some cases (as in Armadillos and Sloths) it becomes as firmly 

 ossified as the vertebral ribs. 



The vertebral ribs are variously connected with the sternal : 

 by continuous cartilage, by intercalation of fibrous tracts, 

 or by synovial joints. 



Occasionally, in the Mammalia, an " intermediate " 

 portion of the rib is segmented off, as in Reptiles ; this is 

 best developed in the Monotremata (Fig. 44, />), where, 

 however, it is only partially ossified by cndostosis. In all 

 other instances in which it occurs (e.g., Odontoccti} it is 

 quite rudimentary. 



1 The bony deposit commencing at the surface, and advancing in- 

 wards, as in ordinary long bones. 



' J The bony deposit being irregularly scattered throughout the carti- 

 lage, often beginning near its central part. 



