62 ANATOMY OF A CIIELONIAN'. 



a branch (157) from the first dorsal nerve. Trace the 

 trunks springing from the plexus to their distribution 

 in the fore limbs. 



160. Clear away the muscles still attached to the 

 spinal column in the carapace region and remove what 

 remains of the pelvis, taking care to leave the sacral 

 ribs. Note that the sacrum has a considerable amount 

 of mobility from side to side, the enlarged outer end of 

 the first sacral rib gliding over a smooth elevation on 

 the carapace (15). Such movements, when the parts 

 are in their natural connections, would carry the pelvis 

 with them, so that the pelvic arch is less rigidly fixed 

 than in most vertebrates. Next, remove the pectoral 

 girdle, noting the stout ligament which fixes the dorsal 

 end of each scapula to the carapace. 



161. Cut away the centra of all the vertebrae sufficiently 

 to open the neural canal and expose the ventral surface 

 of the dura mater. Then slit open this membrane along 

 the middle line, and reflect it to each side, so as to ex- 

 pose the spinal cord ; take great care not to injure the 

 roots of the spinal nerves. Note the considerable space 

 between the cord and the dura mater. 



162. The spinal cord, in its course, will be seen to 

 present two considerable etilargeinents. The anterior^ or 

 cervical^ extends from the front of the sixth cervical to 

 the back of the first dorsal vertebra, and gives origin to 

 the roots which enter the brachial plexus. The pos- 

 terior^ or lumbar enlargement^ extends from the ninth 

 dorsal to the first caudal vertebra inclusive, and gives 

 origin to the trunks of the sciatic plexus. 



