636 COLLEGE ZOOLOGY 



are widely separated from the grinding teeth (15). There are 

 two pairs of incisors (16) lodged in sockets (alveoli) in the pre- 

 maxillae of the upper jaw, and one pair (14) projecting forward 

 from the anterior end of the lower jaw. Only the outer, curved 

 surface of the incisors is covered with enamel, and since the 

 inner dentine wears away more rapidly than the enamel, a chisel- 

 shaped form results that is admirably fitted for gnawing. The 

 grinding teeth are called premolars and molars. The premolars 

 develop after a preceding set of " milk " teeth have fallen out; 

 the molars have no deciduous predecessors. The upper jaw 

 contains three pairs of anterior premolars and three pairs of 

 posterior molars. The last molar is smaller than the others. 

 The lower jaw is provided with two pairs of premolars and three 

 pairs of molars; the last molar is small. 



The vertebral column, as in other vertebrates, supports the 

 body, and protects the spinal cord. The vertebra move upon one 

 another; are separated by intervertebral disks of fibrocartilage, 

 except in the sacrum; and are connected by intervertebral liga- 

 ments. The vertebrae of the neck, or cervical vertebra, are al- 

 most always seven in number; those of the chest, the thoracic 

 vertebra, bear movably articulated ribs; those of the trunk 

 region are called lumbar vertebra; the three or more sacral ver- 

 tebra are fused together and support the pelvis; and the caudal 

 vertebra, about sixteen in number, form the skeletal axis of the 

 tail. 



The ribs and sternum constitute the framework of the thorax, 

 and not only protect the vital organs in that region, but also 

 play an important role in respiration. There are twelve, or 

 sometimes thirteen, pairs of ribs (Fig. 510). The first seven 

 pairs articulate with the sternum; the others do not reach the 

 sternum. The sternum is a long, laterally compressed structure 

 consisting mostly of bone. It is situated in the ventral wall of 

 the thorax, and is transversely divided into six segments, or 

 sternebrae. 



The pectoral girdle consists of two scapulae, two imperfect 



