Unusual Struclwe of Cervical Fertehrce. 515 



riorly. It is from the shape of those which stick out behind 

 that the whole column derives its name of the spine. The 

 Jong series of these vertebrae which extends all along the back 

 forms a tube or canal, nearly round, but rather flattened in 

 front, for the reception of the spinal marrow, which is a pro- 

 longation of the brain ; and which, from the important offices 

 it is destined to fulfil, and its extreme delicacy (the least pres- 

 sure causing paralysis or death), requires to be as strongly 

 defended as it is. On the top of this column is placed the 

 head. Now, when we consider how necessary it is, for the 

 preservation and comfort of the animal, that the head should 

 have free play up and down, to the right and to the left, we 

 shall be led to examine with wonder and delight the beautiful 

 yet simple mechanism by which this is effected. The up-and- 

 down (supination and pronation) is produced by a peculiar 

 formation of the first cervical vertebra {^fig. 103. a). This is 

 termed the atlas, because it gives support to the globe of the 

 head. It has, on its upper surface, cavities to receive the con- 

 dyles (or knobs) of the occipital bone of the head : thus a 

 hinge is formed, and the head acts freely in that direction. 



But now we want horizontal movement ; and for this end 

 the second vertebra (b) presents a peculiar mechanism. From 

 the body of this bone a process, called, from its resemblance 

 to a tooth, the processus dentatus (^), arises, and sticks 

 103 ^^ up perpendicularly ; and, fitting 



into a hollow in the interior sur- 

 face of the body of the atlas, allows 

 of the horizontal action of the 

 skull. Thus we have the com- 

 pound action required, amply and 

 beautifully provided for. Now, 

 though in the structure of the 

 spine of all vertebrated animals 

 there exist some slight modifica- 

 tions suited to the habits of the dif- 

 ferent animals, the general type is uniform ; but, in the instance 

 of the two vertebrae now the subject of this paper, there ap- 

 pears a very essential departure from the general mechanism. 

 Upon examining the first vertebra {Jig. 102. a), I found that the 

 transverse processes {c c), as well as the upper surface of the 

 body of that vertebra, were hollowed out, in order to receive the 

 condyles of the occipital bone : hence it is evident that this is 

 the adas, and that the animal had the power of moving the 

 head up and down. I next examined the second vertebra {b). 

 The tooth-like process is totally wanting ; there are no traces 

 of its ever having existed ; there is no cavity in the atlas for 



LL 2 



