60 ANATOMY OF A CHELONIAN. 



there are four on each side. Posteriorly they arise 

 without distinct tendons, from the bodies of the dorsal 

 vertebrae from the fourth to the eighth, the one which 

 extends farthest forward having its origin farthest back. 

 This, the longest of the muscles, ends in front by a 

 tendon which is inserted on the posterior part of the 

 skull ; the next is inserted on the ventral surface of the 

 anterior outer angle of the centrum of the fifth cervical 

 vertebra ; the remaining muscles are similarly fixed to 

 the sixth and seventh cervical vertebrae respectively. 



The Protractors of the Head and Neck, which run from 

 the carapace and pectoral arch to the neck, may now 

 also be noted. 



153. Cut away the retractor muscles ; also the ventral 

 three-fourths of the ilium on the side where the sciatic 

 nerve has already been seen (113, b\ and trace the 

 nerve down the limb, and also up to its formation by 

 the union of four trunks (sciatic plexus) which appear 

 throusfh the inter-vertebral foramen between the last 

 dorsal and first sacral vertebrae and the three succeed- 

 ing foramina. The second of these trunks is the largest ; 

 the fourth the smallest. This plexus also communi- 

 cates with the lumbar plexus (154) by a small branch, 

 and gives some small twigs to the pelvis. 



154. The nerve-trunks which appear through the two 

 foramina on each side which are next in front of those 

 through which the roots of the sciatic plexus pass out, 

 unite to form the lumbar plexus, oft'shoots of which may 

 be traced to the inner aspect of the thigh. 



155. Clear away the muscles, etc., on the anterior 



