xvi THE NERVOUS SYSTEM 291 



the plexus ventral to the posterior portion of the ileum ; 

 it is distributed to the muscles of the abdomen and skin of 

 the anterior part of the thigh. The largest nerve coming 

 from the plexus is the sciatic, which emerges from the 

 body cavity just behind the posterior end of the crest of the 

 ileum, passing between the pyriformis and posterior head of 

 the triceps extensor muscle and extending down the back 

 of the thigh ; a short distance proximal to the knee it 

 divides into the tibialis nerve and the perotieus. The 

 former extends along the posterior or flexor side of the 

 leg, and enervates the gastrocnemius, tibialis posticus, and 

 numerous muscles of the plantar surface of the foot. The 

 peroneus runs under the tendon of the triceps and extends 

 along the extensor surface of the cms, giving branches to 

 the peroneus muscle, the tibialis anticus, and the muscles on 

 the extensor surface of the foot. 



The tenth nerve with a branch from the ninth forms the 

 ischio-coccygeal plexus, from which branches are given off to 

 the bladder, cloaca, oviducts, and posterior lymph hearts. 

 The tenth nerves are of small size and emerge from small 

 foramina in the sides of the urostyle near the anterior end. 

 An eleventh spinal nerve sometimes occurs. When present 

 it emerges from the urostyle behind the opening for the 

 tenth and joins the ileo -coccygeal plexus. All of the 

 plexuses are subject to considerable variation in different 

 individuals. 



The Brain. — The brain is composed of the following 

 parts taken in order from behind forward : the medulla 

 oblongata, or hind-brain ; the cerebellum ; the ?nid-brain ; 

 the thalamejicephalon ; and the fore-brain, which consists of 

 the cerebral hemispheres and olfactory lobes. 



The medulla oblongata is formed by a widening of the an- 

 terior end of the spinal cord. On its dorsal side is situated 



