THE SKULL AND VERTEBRAE. 89 



The membral bones have been shown from the dorsal aspect in 

 the normal position of the parts in Fig. 6, but in Fig. 30 the animal 

 is represented from the left side, in one of its natural attitudes while 

 standing or walking. 



In this attitude the limbs are directed ventrad instead of laterad, 

 and are thus nearly parallel to each other ; while their segments 

 and the general divisions of the somatic skeleton, the head, neck, 

 trunk and tail, form angles with each other, imparting to the whole 

 skeleton a graceful and spirited appearance. 



207. Skull (Fig. 56-62). The cephalic division of the so- 

 matic skeleton is the skull, consisting of the cranium or brain- 

 case, and the face, to which appertains the mandible or lower jaw. 

 The transverse ridge at the junction of the dorsal and caudal as- 

 pects of the skull is the Crista lamdoidalis (Fig. 56, Cst. Imd.); 

 the darkly shaded area between the cranium and the face repre- 

 sents the left orbit, and the longitudinal, irregular bar just ventrad 

 of it is the zygoma, or arcus zygomaticus (Fig. 56). 



The Os Tiyoides will be described in 224. 



208. Vertebrae (Fig. 51-55). Caudad from the skull extends 

 a series of bony segments the Vertebra constituting the Columna 

 vertebralis or spine or spinal column. As seen from the side, the 

 Columna presents two curvatures in opposite directions, like an 

 elongated letter s. 



For convenience and more or less naturally, the vertebrae may 

 be grouped in five divisions, cervical, thoracic, lumbar, pelvic or 

 sacral and caudal. The numbers following the names of these 

 groups upon Fig. 30 are those which are most commonly observed, 

 but variations sometimes occur, as will be mentioned in the descrip- 

 tion of the Columna vertebralis. 



The vertebrae of each group have certain features in common, 

 and also individual peculiarities by which, excepting most of the 

 caudales, they may be distinguished when completely isolated. 

 On the entire skeleton, the groups are most readily recognized 

 on account of the connections of three of them with other parts, as 

 follows : 



The thirteen thoracic vertebrae are costiferous, that is, connected 

 with the ribs ; the cervical series begins at the skull, and the three 

 sacral vertebrae are united so as to form a single bone the sa- 

 crum to which is attached the Os innominatum on either side. 

 The lumbar vertebrae intervene between the thoracic and the sa- 



