CHAP, in.] SKELETON OF THE HEAD AND TRUNK. 47 



towards the middle or more distal part of the tail are distorted, 

 atrophied, and more or less fused together at the place where the 

 tail is so suddenly contorted. 



With age, the first caudal may anchylose with the third sacral 

 vertebra, which ^it resembles much both in size and shape, though 

 its neural spine is smaller and its transverse process (which projects 

 strongly backwards and slightly outwards) is narrower from behind 

 forwards than is the lateral mass of the third sacral vertebra. The 

 two next caudal vertebra closely resemble the first, though they are 

 slightly longer. The fourth caudal vertebra is again longer, its 

 neural spine is hardly to be detected, and its shorter transverse 

 processes project outwards and backwards from the hinder part of 

 the side of its centrum. Two slight hypapophysial prominences are 

 also to be detected side by side and but little separated, at the 

 anterior end of the ventral surface of the centrum. The neural 

 canal is also much reduced in size. The fifth caudal vertebra 

 exaggerates the same characters, as also does the sixth, in which 

 the neural canal is very small. Here also a minute transverse 

 process begins to show itself projecting outwards from the anterior 

 end of each side of the centrum, while that projecting from its 

 posterior end is so reduced as to be scarcely, if at all, larger than is 

 the transverse process thus newly appearing at the anterior end of 

 this vertebra ; in which, moreover, the prezygapophyses no longer 

 articulate with the postzygapophyses in front of them. 



In the seventh caudal vertebra the minute neural canal is hardly 

 enclosed by bone, and is only so near the median part of the bone. 

 The prezygapophyses are the longest processes, and the anterior 

 transverse processes are rather longer than the posterior ones. The 

 transverse processes in the eighth vertebra are hardly more prominent 

 than are the hypapophysial ones, but the whole bone continues to 

 increase in length. In the ninth vertebra there is an open groove 

 instead of a neural canal. The tenth is about the absolutely longest 

 vertebra. Thence onwards the processes become less and less marked, 

 and the vertebras, from the eleventh or twelfth, begin manifestly to 

 decrease in length and all other dimensions the last vertebrae being 

 little more than small cylindrical ossicles, each formed of a centrum 

 only, with faint indications at each end of processes corresponding 

 with those described as existing in the more anterior vertebra?. 

 Thus the last vertebra is the very opposite to the first (or atlas), 

 being all centrum, while the atlas has no proper centrum at all. 



Certain very small Y-shaped bones called chevron bones are arti- 

 culated beneath the interspaces and adjacent ends of the caudal 

 vertebrae, from the second to the tenth or eleventh vertebra. 



13. The WHOLE SERIES of vertebrae thus form a jointed rod 

 the spinal or vertebral column. Its component vertebrae, moreover, 

 are so disposed that the backbone, when seen in profile, presents, 

 between the atlas and the tail, two curvatures (see ante, 

 Fig. 2), directed alternately upwards and downwards. Thus the 

 cervical vertebrae form a curve which is convex downwards, while 



