60 



INTRODUCTION. 



vertebra, is slightly backward and forward ; whilst the mechanism of 

 the vertebra dentata limits the motion of the atlas upon it to one which 

 is rotatory. The first movement is exhibited by slightly nodding, the 

 neck being held firmly upright ; the second, by turning the head from 

 side to side, the neck being still retained in the same rigid position. 



From the vertebrae of the neck we pass to the dorsal vertebrae, or those 

 of the back : in these the spinous processes are long, and more or less 

 inclined backward, their shape being flat, or compressed laterally. In 

 all animals, which have to support either a ponderous head, a head placed 

 at the extremity of a long neck, or, more especially, a head produced into 

 a snout destined for ploughing up the ground in quest of food, as in the 

 Hog, these spinous processes are of extraordinary length and stoutness, and 

 indicate the volume of the muscles acting upon the back of the skull, 

 the transverse occipital ridge of which is, accordingly, more or less pro- 

 minent. To the top of each of these processes the ligamentum nuchae at- 

 taches itself, as it passes along the spine. It is to the dorsal vertebrae 

 that the ribs are articulated : each rib has its head fitted into a depression 

 formed by the bodies of two vertebrae, so that every dorsal vertebra 

 (except the last) has on each side two ribs, partially united to its body ; 

 while each rib is farther united, by a tubercle, at the point where it 

 forms an acute arch, to the transverse process of the lower vertebra, or 



Figs. 53 to 57.- Dorsal vertebrae, in different aspects. 



Fi. 53, Anterior. 54, Posterior 55, Superior. 56, Inferior. 67, Lateral. a, body; b, spinous process; c, c, oblique 

 processes ; d, <Z, transverse processes. (The same letters refer to the same part in all the above figTires.) 



that which is the lowermost of the two, to which its head is affixed. As 

 exceptions to this mode of union, it may be observed, that the last ribs 

 are, in many instances, affixed to single vertebrae, and to the bodies of 

 them alone. In the Cetacea, the ribs, at least the anterior ribs, are at- 



