146 



HIGH SCHOOL ZOOLOGY. 



5. In all Mammals (with the exception of some aquatic forms) 

 it is possible to distinguish all the five i-egions of the vertebral 

 column — cervical, dorsal, lumbar, saci'al, caudal. In some 

 aquatic forms (Fig. 101) the hinder extremities have so nearly 



Fig. 101. Skeleton of Porpoise. (Phoccenn). vc, cervical vertebra) anchylose<l ; 

 vth, dorsal ; vl, lumbar; v.x, cauflal re'.rioiis; jkI, dorsai , pc, caudal fins ; pi, pelvic rudi- 

 ments ; c' false, c true ribs ; st, sternum ; 9c, scapula ; h, humerus ; r, radius ; ca, car- 

 pus ; po, pollex ; mc, metacarpus ; ph, phalanges. 



disappeared that it becomes impossible to distinguish a sacral 

 region, but the ribs are always present and connected to the 

 sternum in such a way as to mark off the cervical, dorsal and 

 lumbar regions. As we advance through the Vertebrate series 

 we see a tendency towards the reduction in number, and 

 towai'ds a constancy in the reduced number of certain parts. 

 The teeth, for example, will furnish us with a good illustmtion 

 of this rule, but it is also well exemplified in the cervical region 

 of the vertebi-al column, for not only is it much shorter than 

 usual in the Reptiles, but (with the exception of a few genera 

 which also in other respects present primitive characteristics) 

 it always contains seven vertebrae, and that in spite of the ex- 

 tremely long and short necks which we meet with in diflferent 

 members of the class. The other regions vary in diffei-ent 

 ordera as to the contained number of vertebra?, within naiTower 

 or wider limits, the caudal most of all ; the Cat, e.g.y has thir- 

 teen doi-sal, seven lumbar, tlirce sacral, and twenty caudal verte- 

 bra. (Fig- 102). Instead of the sternal ends of the ribs being 

 ossified as they are in birds, they are always cartilaginous, and 



