550 



CHORDATA 



the pterygoid forniing tlie lamina interna). TIius the hinder sphenoid, 

 Hke the temporal, contains cranial and visceral elements. 



The vertebra" are conrpletely ossilled and are connected by interver- 

 tebral ligaments, discs of filn'ous cartilage. The cer\ical and rib-bearing 

 thoracic vertebnc are ahvays distinct, and except in the cetacea, lumbar, 

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

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

 seven except in Bradvpns triihulyliis (q), Cliola-piis liofmanni and Manatns 

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

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

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

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

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



the amphibia, two (carnivores, pri- 

 mates) or rarely more (EehiJna, many 

 edentates). The pscudosacral verte- 

 bra are rarely lacking entirely; usually 

 their number varies between cme 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 ecraeoid 

 process of the scapula. More rarely 

 the cla\icle is lacking (rapid runners); 

 hi the monotremes it extends to the 

 episternum (fig. 5Q9, CI, Ep)\ else- 

 where it appears to articulate with the 

 sternum, in reality by the intervenlion 

 of interarticular cartilages (once regarded as a rudimentarv episternum, 

 now called preclaviic). In the pelvis all three elemenls are fused to a 

 single OS innoininatiim; puliis and ischium unite ventrallv with each other, 

 enclosing between ihenr the ol'lnralor foramen (lig. ()0()). 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 

 of the mammals. In the climl)ing species the lliumb and great toe become oppos- 

 able, rcsultmg in a grasping hand or foot; but usually they are used to supjiort 

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

 ground (plaiiligradr) but usually the basal joints of the feet contribute to the 

 length of the leg, the phalanges alone forming the 'sole' {dlgili,i^rade) or {inf^uli- 

 grade) the weight being supiunted on the tips of the toes. 



Fig. 5QQ. — Sternum and shouKlcr 

 girdle of Oniilliivliyirhiis puraitoxiis 

 (from Wiedershcini). C!, cla\-icle; c'l', 

 C~()', coracoid; lip, episternum; t/, 

 glenoid fossa for humerus; 5, sc;ipula; 

 SI, manubrium sterni (.anterior element 

 of sternumV 



