Mammalia: Skin, Muscles, Skeleton 595 



ments depend on tendons attached to muscles in the anterior region. 

 The nervous structures of the tail are even more restricted. Thus, in 

 the common cat there are about 22 caudal vertebrae, but there are 

 only seven or eight pairs of caudal spinal nerves. They occur in series 

 immediately behind the sacral nerves. These caudal nerves, however, 

 emerge from the spinal cord considerably anterior to their points of 

 emergence from the vertebral column, and the cord itself continues 

 into the caudal region only as a non-nervous filament, the filum ter- 

 minate (Fig. 157). 



In some monkeys (the South American Cebidae) the long tail is so 

 strongly developed as to allow the animal to swing by it from a branch 

 of a tree. Also, in the opossum (Fig. 534) and some other marsupials, 

 in some South American anteaters, and in the kinkajou, a Central and 

 South American carnivore, the tail is prehensile. The whale is an 

 exceptional mammal in having a massive tail, the trunk tapering back 

 gradually into the caudal region as in fishes. Muscles are present far 

 back in the posterior region of it. However, in the absence of hindlegs, 

 there is no recognizable sacral region of the vertebral column, and it is 

 difficult to say just where the postsacral (caudal) region begins. It is to 

 be remarked, too, that as the vertebrate tail appears in its primitive 

 functional importance in the whalelike mammals the neck vanishes — 

 all of which, of course, is a matter of streamlining the body for aquatic 

 living. How the tail "came back" in the history of whales is an as yet 

 unsolved problem in evolution. 



In the longer-tailed mammals the more anterior vertebrae have the 

 typical vertebral structure, but, passing posteriorly, the neural arches 

 and the various lateral processes of the vertebrae are progressively 

 reduced to the extent of their complete absence in the posterior region 

 of the tail, where the column consists merely of slender, elongated, 

 cylindric centra (Fig. 451). In the anterior region of a long tail com- 

 monly occur a varying number of small bones, each shaped like a 

 miniature "wishbone" or a "V" and attached by its two ends, point 

 downward, to the ventral surface of a corresponding vertebra, or else 

 attached intervertebrally. The caudal artery and vein pass through 

 the aperture between the two arms of the bone. These "chevron 

 bones," present also in many reptiles, are possibly homologous with 

 the hemal arches of anamniotes. 



The shifting of locomotion from the anamniote tail to the amiiiote 

 legs has a point of practical significance for man. Our most important 

 animal food consists of locomotor muscles. Steaks of halibut and 

 swordfish are slices cut across the tail. The oxtail may serve for soup, 

 but our heavy beefsteaks and roasts come from the upper part of the 

 hindleg and the region of the pelvic girdle. 



