Mammalia: Skin, Muscles, Skeleton 



597 



461), but usually some of its posterior members do not connect directly 

 with the girdle. In rodents the sacral number is commonly three or 

 four, but usually only two, or sometimes only one, join the girdle. In 

 the rhinoceros sacrum of four vertebrae, three join the girdle. In the 

 elephant all of the three sacral constituents join the girdle. Of the five 

 vertebrae in the human sacrum, usually only the anterior three join 

 the ilia. The maximum sacrum is that of the armadillo, containing 8 

 to 10 vertebrae. This great expansion of it is due to the fact that it 

 serves to support the heavy bony exoskeletal carapace. The sacrum 



Prezygapophys 

 Lateral mass 



United articular processes 

 United transverse processes 



Dorsal sacral foramen 



erse process 



Postzygapophysi 



Fig. 461. Dorsal aspect of cat's sacrum. (After Strom- 

 sten: "Davison's Mammalian Anatomy," Philadelphia, 

 The Blakiston Company.) 



not only joins the ilia, but its posterior region acquires connection with 

 broad dorsal extensions of the ventral ischia (Fig. 462). At the anterior 

 end of the sacrum each ilium bears a strong dorsal process, and at the 

 posterior region of the sacrum each ischium bears a similar process. 

 These two pairs of bony posts are attached to the inner surface of the 

 carapace. 



In the caudal region, in striking contrast to the cervical region, 

 the number of vertebrae varies with the length of the tail. Even in a 

 given species there is some variation in the number. The approximate 

 numbers of postsacral vertebrae in several mammals are as follows: in 

 some pangolins (Manidae), 50; common rat, 30; common cat, 22; 

 rabbit, 15; man, 3 to 5. In the human embryo there are usually rudi- 

 ments of eight caudal vertebrae, but several of the posterior rudiments 

 fail to complete their development. 



An important feature of the mammalian vertebral column is the 

 presence of intervertebral disks consisting of a fibrocartilage which 

 is very tough, slightly compressible, and highly elastic (Figs. 107, 452). 

 The apposed faces of the centra are usually flat (amphiplatyan centra). 



