44 



In some of the Anomodont Reptiles and Labyrinthodont 

 Amphibians these chevrons are attached to the intercentra or 

 imperfect disks alternating with the true centra which suggests 

 that they are primarily intercentral elements which have been trans- 

 ferred to the edges of the centra by the disappearance of the inter- 

 centra. 



Sternum. The sternum of mammals is a bone, or generally a 

 series of bones, placed longitudinally in the mesial line, on the 

 inferior or ventral aspect of the thorax, and connected on each side 



with the vertebral column by a series 

 of more or less ossified bars called 

 " ribs." It is present in all mammals, 

 but varies much in character in the 

 different groups. It usually consists 

 of a series of distinct segments placed 

 one before the other, the anterior 

 being called the presternum or " manu- 

 brium sterni " of human anatomy, and 

 the posterior the xiphisternum, or 

 xiphoid or ensiform process, while the 

 intermediate segments, whatever their 

 number, constitute the mesosternum 

 or " body." In the Whalebone Whales 

 the presternum alone is developed, and 

 but a single pair of ribs is attached 

 to it. 



Ribs. The ribs form a series of 

 long, narrow, and more or less flattened 

 bones, extending laterally from the 

 sides of the vertebral column, curving 

 downwards towards the median line 

 of the body below, and mostly joining 

 the sides of the sternum. The posterior 

 ribs, however, do not directly articulate 

 with that bone, but are either attached by their extremities to 

 the edges of each rib in front of them, and thus only indirectly 

 join the sternum, or else they are quite free below, meeting no part 

 of the skeleton. These differences have given rise to the division 

 into " true " and " false " ribs (by no means good expressions), signi- 

 fying those that join the sternum directly and those that do not ; 

 and of the latter, those that are free below are called " floating " 

 ribs. The portion of each rib nearest the vertebral column and 

 that nearest the sternum differ in their characters, the latter being 

 usually but imperfectly ossified, or remaining permanently cartila- 

 ginous. These are called "costal cartilages," or when ossified 

 " sternal ribs." 



JCJt 



Fio. 13. --Human sternum and 

 sternal ribs, ps, Presternum ; ms, 

 mesosternum ; xs, , xiphisternum ; c, 

 point of attachment of clavicle ; 1 to 

 10, the cartilaginous sternal ribs. 



