194 CHARACTERS OF VERTEBRATE ANIMALS 



vertebrae end and the lumbar ones begin, and the numbers are as 

 follows : — Cervical, 8 ; thoracic and lumbar together, 2 2 ; sacral, 2 ; 

 caudal, very numerous, but of no constant number. The ends 

 of the centra or bodies of the vertebrae are neither flat as in 

 Mammals nor saddle-shaped as in Birds, but concave in front and 

 convex behind, or, technically speaking, procoelous. This is the 

 commonest arrangement among recent reptiles. The first two 

 joints of the neck are constituted by atlas and axis as in a 

 Mammal (see p. 26) or Bird ; but the odontoid peg which 

 projects forwards from the front end of the latter is never 

 fused with, though firmly fixed to, it. The peg is in reality a bit 

 of the atlas centrum which has been appropriated by the axis 

 for the purpose of making a pivot. There are two interesting 

 peculiarities about the tail, one of which consists in the presence 

 of small Y-shaped bones united to the under sides of many of the 

 caudal vertebrae, the stem of the Y being directed downwards. 

 In this way a canal is formed in which the great blood-vessels 

 of the tail are sheltered, an arrangement which is found in 

 various groups of backboned animals. Another and more inter- 

 esting peculiarity is found in the fact that the body of almost 

 every vertebra in this region remains unossified, i.e. is not of 

 bony nature, across a narrow central strip. The tail of a lizard 

 is extremely brittle, partly as a result of the arrangement 

 described, and this would appear to be a defensive provision, 

 whereby, supposing the animal to have been attacked in the rear 

 by an enemy, a part is sacrificed to save the rest. 



Slender curved ribs are attached from the fourth neck-vertebra 

 right back to the sacrum, and, as elsewhere, the first thoracic 

 vertebra is taken to be the one bearing the first rib that is 

 connected with the sternum. Only five pairs are so connected 

 in the Sand Lizard, the means of union being slender gristly 

 sternal ribs, by which the bony vertebral ribs are continued in 

 the downward direction {see p. 29). Each of the two sacral 

 vertebrae possesses a strong transverse process on each side, by 

 which attachment to the hip-girdle is effected, and it has been 

 shown that these processes ossify or become bone independently 

 of the bodies of these vertebrae, fusing with them subsequently. 

 On this account the processes are looked upon as ribs which have 

 united with the vertebrae to which they belong — a phenomenon 

 which is paralleled elsewhere. 



