162 



ANATOMY OF THE RABBIT 



the thoracic region, being directed forward and downward, as well 

 as outward. The tip of each is formed by a thin triangular plate 

 (processus triangularis), which represents a vestigial rib fused with 

 the original process. At the posterior side of the base of each is a 

 short, flattened projection, the accessory process (processus 

 accessorius). The spinous process is notably broad and is directed 

 forward. The articular processes are rotated upward, so that their 

 surfaces are directed more nearly toward, or away from, the median 



& 



^^w>X-^M 



pas. p-i 



Fig. 83. The os sacrum: A, ventral (pelvic) surface; B, dorsal surface. 

 C.V., bodies of coalesced vertebrae; f.a., auricular surface; f.s.a., anterior 

 sacral foramina; f.s.m., median sacral foramina; f.s.p., posterior sacral 

 foramina; p.a.s., superior articular process of first vertebra; p.m., mamillary 

 process of first vertebra; pr., promontory; p.s., spinous processes. 



plane, instead of to the dorsal or ventral surface, so that the move- 

 ment provided for is chiefly a bending of the body dorso-ventrally. 

 The anterior articular surfaces are borne on the bases of stout, 

 upwardly-directed mamillary processes (processus mamillares) 

 upon which the powerful muscles of the back originate (m. sacro- 

 spinalis, p. 339). The latter processes are most characteristic of 

 the lumbar vertebrae, but, as mentioned above, appear in the 

 posterior thoracic region as small elevations of the transverse 



