28 ANATOMY OF THE RAT 



cranially. The articular surface of the prezygapophysis 

 faces mediodorsad. The facet of the prominent postzyga- 

 pophysis faces laterally. The transverse process projects 

 cranioventrally from the base of the pedicle. 



The sacrum consists of four vertebrae more or less com- 

 pletely fused together. The transverse processes project 

 horizontally from the centra. They are fused at their 

 distal ends. The transverse processes of the first two sacral 

 vertebrae are considerably thicker than the others. They 

 are expanded laterally, forming the auricular surface, 

 which articulates with the pelvic girdle. Since this girdle 

 is attached to the hind legs, the weight of the posterior part 

 of the body falls on these two sacral vertebrae — hence their 

 stout construction. The intervertebral formina are located, 

 as usual, between the vertebrae. The prezygapophyses of 

 the first sacral vertebra are well developed, for they are 

 part of a movable joint. The remaining functionless 

 zygapophyses of the sacrum are rudimentary. They lie 

 close to the bases of the high spinous processes. 



The caudal, or tail, vertebrae show gradual and progres- 

 sive stages of simplification from the sacrum to the pos- 

 terior end of the tail. The neural canal is small in the 

 anterior part of the tail, and soon completely disappears. 

 The anterior caudal vertebrae have ventral pairs of spines 

 between which run the caudal blood vessels. There is a 

 chevron hone in some of the vertebrae which partly in- 

 closes these vessels on the under side. 



Exercise V. Make a labeled sketch of each vertebra which has 

 been described. 



The sternum, or breastbone, comprises seven bony seg- 

 ments {sternehrae) united end to end. The first, or manu- 

 brium, is keeled ventrally, and has a broad anterior region, 

 concave above. The first rib is attached to it. The sue- 



