44 



GENERAL ANATOMICAL CHARACTERS 



ms' 



In some of the Anomodont Keptiles and Labyrintkodont 

 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 tke mesial line, on tke 

 inferior or ventral aspect of tke tkorax, and connected on eack side 



Avitk tke vertebral column by a series 

 of more or less ossified bars called 

 " ribs." It is present in all mammals, 

 but varies muck in ckaracter in tke 

 different groups. It usually consists 

 of a series of distinct segments placed 

 one before tke otker, tke anterior 

 being called tke presternum or " manu- 

 brium sterni " of kuman anatomy, and 

 tke posterior tke xipkisternum, or 

 xipkoid or ensiform process, wkile tke 

 intermediate segments, Avkatever tkeir 

 number, constitute tke mesosternum 

 or " body." In tke Wkalebone Wkales 

 tke presternum alone is developed, and 

 but a single pair of ribs is attacked 

 to it. 



Bibs. — Tke ribs form a series of 

 long, narrow, and more or less flattened 

 bones, extending laterally from tke 

 sides of tke vertebral column, curving 

 downwards towards tke median line 

 of tke body below, and mostly joining 

 tke sides of tke sternum. Tke posterior 

 ribs, kowever, do not directly articulate 

 witk tkat bone, but are eitker attacked by tkeir extremities to 

 tke edges of eack rib in front of tkem, and tkus only indirectly 

 join tke sternum, or else tkey are quite free below, meeting no part 

 of tke skeleton. Tkese differences kave given rise to tke division 

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

 fying tkose tkat join tke sternum directly and tkose tkat do not ; 

 and of tke latter, tkose tkat are free below are called " floating " 

 ribs. Tke portion of eack rib nearest tke vertebral column and 

 tkat nearest tke sternum differ in tkeir ckaracters, tke latter being 

 usually but imperfectly ossified, or remaining permanently cartila- 



Fig. 13. -Human sternum and 

 sternal ribs, ps, Presternum ; ms, 

 mesosternum ; xs, xiphisterimm ; c, 

 point of attachment of clavicle ; 1 to 

 10, the cartilaginous sternal ribs. 



Tkese are called " costal cartilages," or wken ossified 



" sternal ribs." 



