Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/42

28 cranially. The articular surface of the prezygapophysis faces mediodorsad. The facet of the prominent postzygapophysis faces laterally. The transverse process projects cranioventrally from the base of the pedicle.

The sacrum consists of four vertebrae more or less completely 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 progressive stages of simplification from the sacrum to the posterior 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 bone in some of the vertebrae which partly incloses 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 segments (sternebrae) united end to end. The first, or manubrium, is keeled ventrally, and has a broad anterior region, concave above. The first rib is attached to it. The suc-