NERVOUS SYSTEM AND SENSE ORGANS 29 



154. the third spinal nerve which curves cephalad to 

 meet the second and forms the brachial plexus. 



155. The fourth spinal nerve runs caudad at an angle 

 of forty-five degrees to the vertebral column and 

 supplies the body wall. 



156. The fifth spinal nerve is parallel to the fourth. 



157. The sixth spinal nerve is similar to the fourth and 

 fifth. 



158. The seventh spinal nerve runs caudad, parallel to 

 the vertebral column, and joins the eighth to take 

 part in the formation of the lumbro-sacral (sciatic) 

 plexus. Shortly before the point of junction, it 

 gives off the ilio-hypogastric nerve to the body wall 

 of the abdomen. 



159. The eighth ^ spinal nerve is parallel to the seventh 

 but mesiad to it. After its junction with the seventh 

 it unites with the ninth spinal nerve. At this point 

 a crural nerve is given off to the thigh. 



160. The ninth spinal nerve, parallel and mesiad to the 

 the eighth, unites with the other elements of the 

 sciatic plexus to give rise to the great sciatic nerve, 

 the largest single nerve in the body. 



161. The tenth spinal (coccygeal) nerve is very small. 

 It emerges from small openings in the urostyle (231) 

 and after receiving a branch from the ninth to form 

 the ischio-coccygeal plexus, supplies the cloaca and 

 bladder. 



(If time permits, trace the brachial and sciatic nerves to 

 the toes.) 



162. The sympathetic trunks are longitudinal nerves of 

 the autonomic system lying along the systemic arches 

 and dorsal aorta. Each is connected by 



* Largest element of the sciatic plexus of R. cateshiana. 



