70 LAND MAMMALS IN THE WESTERN HEMISPHERE 



there are very constantly 19 trunk- vertebrae, but the Hippo- 

 potamus has 15 dorsals and 4 lumbars, the Reindeer (Rangifer) 

 14 D., 5 L., the Ox (Bos taurus) 13 D., 6 L., the Camel iCamelus 

 dromedarius) 12 D. and 7 L. (4) Next follows the sacrum, 

 which consists of a varying number of coalesced vertebrae. 

 The number of sacral vertebrae varies from 2 to 13, but is 

 usually from 3 to 5. (5) Finally, there are the caudal vertebrae, 

 or those of the tail, which are extremely variable in number 

 and size, depending upon the length and thickness of the tail. 



We must next consider briefly some of the structural features 

 which characterize the vertebrae of the different regions. 

 (1) The length of the neck varies greatly in different mammals 

 and, up to a certain point, flexibility increases with length, but, 

 as the number of 7 cervicals is almost universally constant 

 among mammals and the lengthening of the neck is accom- 

 plished by an elongation of the individual vertebrae, a point 

 is eventually reached, where greater length is accompanied 

 by a diminution of mobility. For instance, in the Giraffe 

 the movements of the neck are rather stiff and awkward, in 

 striking contrast to the graceful flexibility of the Swan's neck, 

 which has 23 vertebrae, more than three times as many. 



The first two cervical ver- 

 tebrae are especially and pecul- 

 iarly modified, in order to 

 support the skull and give to 



Fig. 11. — Atlas of Wolf, anterior end and it the UCCCSSary degree of mO- 

 left side, cot., anterior cotyles. n.c, , ... , , ,_,, „ 



neural canal, n.a., neural arch, tr., blllty UpOU the Ueck. The first 



transverse process, v.a., posterior vertebra, or atlas, IS hardly 



opening of the canal for the vertebral 



artery. more than a ring of bone with 



a pair of oval, cuplike depres- 

 sions (anterior cotyles) upon the anterior face (superior in 

 Man) into which are fitted the occipital condyles of the skull. 

 By the rolling of the condyles upon the atlas is effected 

 the nodding movement of the head, upward and down- 

 ward, but not from side to side ; this latter movement is 



