550 



CHORDATA 



G 



the pterygoid forming the lamina interna). Thus the hinder sphenoid, 

 like the temporal, contains cranial and visceral elements. 



The vertebrae are completely ossified and are connected by interver- 

 Idn-al ligaments, discs of fibrous cartilage. The cervical and rib-bearing 

 thoracic vertebrae are always distinct, and except in the cetacea, lumbar, 

 sacral and caudal vertebrae as well. These regions are little variable in 

 extent. The cervicals, including atlas and axis (epistropheus) number 

 seven except in Brady pus tridactylus (9), Clwhvpus hofmanni and Manafus 

 ((>). In the formation of the sacral region two steps are distinguishable. 

 There is the union of the true sacral vertebrae with the os ilium and then 

 the fusion of these and with them certain of the caudal vertebrae (pseudo- 

 sacral vertebra 1 ) to form the os sacrum. The number of true sacral 

 vertebrae is frequently (Marsupials, ungulates, rodents, lemurs) one, as in 



the amphibia, two (carnivores, pri- 

 mates) or rarely more (Echidna, many 

 edentates). The pseudosacral verte- 

 brae are rarely lacking entirely; usually 

 their number varies between one and 

 three. 



Of the appendicular skeleton the 

 girdles are most interesting. The 

 coracoid, which in monotremes 

 reaches the sternum, is reduced in 

 all other mammals to a small coracoid 

 process of the scapula. More rarely 

 the clavicle is lacking (rapid runners) ; 

 in the monotremes it extends to the 

 episternum (fig. 599, Cl, Ep); else- 

 where it appears to articulate with the 

 sternum, in reality by the intervention 

 of interarticular cartilages (once regarded as a rudimentary episternum, 

 now called predavia). In the pelvis all three elements are fused to a 

 single os innominatmn; pubis and ischium unite ventrally with each other, 

 enclosing between them the obturator foramen (fig. 606). The pubes of 

 the two sides unite by a symphysis which can extend back to the ischia. 



The great differentiation of the appendages is also highly characteristic 



he mammals. In the climbing species the thumb and great toe become oppos- 



, resulting m a grasping hand or foot; but usually they are used to support 



body on the ground. Occasionally the whole hand or foot rests on the 



ound (plantigrade) but usually the basal joints of the feet contribute to the 



v;th ot the leg, the phalanges alone forming the 'sole' (di^iti^rade) or (unguli- 



ide) the weight being supported on the tips of the toes. 



599. Sternum and shoulder 

 girlie of Ornithorhyn-hus paradoxus 

 (from Wiedersheim). Cl, clavicle; Co, 

 Co', coracoid; Ep, episternum; G, 

 glenoid fossa forhumerus; 5, scapula; 

 St, manubrium sterni (anterior element 

 of sternum). 



