RAT, A REPRESENTATIVE MAMMAL 113 



typical complete dentition for mammals. This formula for the sheep 

 is 0/3, 0/1, 3/3, 3/3; for the horse: 3/3, 1/1, 3 or 4/3, 3/3; for 

 rabbit: 2/1, 0/0, 3/2, 3/3, and for man 2/2, 1/1, 2/2, 3/3. The 

 teeth of mammals are set in sockets or alveoli in the jaws, a condition 

 known as thecodont, and since there are different kinds in the same 

 mouth they are also heterodont. 



The skull serves primarily as a protection for the brain and the 

 special sense organs of the head and furnishes a means of securing 

 and masticating food material. Numerous muscles are attached to 

 the external surfaces of its bones. 



The vertebral coliiynn or backbone is a series of bones, the vertebrae, 

 extending from the skull through the axis of the trunk and tail. 

 This column is divided into five regions or groups of vertebrae : 

 seven cervical in the neck, thirteen thoracic in the chest ; six lumbar 

 in the small of the back or loin region ; four in the sacral or pelvic 

 region ; and twenty-seven to thirty in the caudal or tail region. Each 

 vertebra articulates with the adjacent one but is separated from it 

 by intervertebral discs of cartilage. Intervertebral ligaments serve 

 to connect the adjacent vertebrae. The vertebral column supports 

 the ribs and pelvic girdle as well as serves for attachment of numer- 

 ous muscles. 



The typical parts of the typical vertebra are the solid, rather 

 cylindrical, ventral body or centrum; the neural arch at the dorsum 

 over the central canal; the neural spine or spinous process projecting 

 dorsally from the arch ; the transverse processes, one extending later- 

 ally on each side ; zygapophyses or articular processes, a pair project- 

 ing anteriorly and a pair posteriorly from the arch to articulate 

 with adjacent vertebrae. The two most anterior cervical vertebrae, 

 the atlas and axis, are highly modified. The atlas is a heavy ring 

 of bone with a distinct ventral process; broad, flat transverse proc- 

 esses; a large spinous process; and anterior articular processes, artic- 

 ulating with the occipital condyles of the skull, and posterior 

 zygapophyses articulating with the axis just behind. The axis has 

 a broad, flat centrum, reduced transverse and ventral processes, and 

 a large neural spine. The caudal vertebrae are modified by presence 

 of ventral chevron bones on the anterior ones and almost complete 

 loss of processes as well as reduction of size in the posterior ones. 



There are thirteen pairs of ribs which articulate with the thirteen 

 thoracic vertebrae. The anterior seven pairs are the true ribs, for 



