ANATOMY OF THE NERVOUS SYSTEM 129 



plies the lower part of the trunk, the buttocks, the front of the 

 thigh, and inner side of the leg. 



The sacral plexus is formed by the anterior primary branches 

 of the fifth lumbar and the first four sacral nerves, which unite 

 in one great cord and so form the sciatic nerve, which is the largest 

 in the Body and, running down the back of the thigh, ends in 

 branches for the lower limb. The roots of the trunks which form 

 the sacral plexus arise from the lumbar enlargement of the cord. 



Cranial Nerves. Twelve pairs of nerves leave the skull by 

 apertures in its base, and are known as the cranial nerves. Most 

 of them spring from the under side of the brain, and so they are 

 best studied in connection with the base of that organ, which is 

 represented in Fig. 64. The first pair, or olfactory nerves, spring 

 from the under sides of the olfactory lobes, /, and pass out through 

 the roof of the nose. They are the nerves of smell. The second 

 pair, or optic nerves, II, spring from the optic thalami and corpora 

 quadrigemina, and, under the name of optic tracts, run down to 

 the base of the brain, where they appear passing around the crura 

 cerebri, as represented in the figure. In the middle line the two 

 optic tracts unite to form the optic chiasma, from which an optic 

 nerve proceeds to each eyeball. 



All the remaining cranial nerves arise from the hind-brain. 

 The third pair (motores oculi) arise from the front of the pons 

 Varolii, and are distributed to most of the muscles which move 

 the eyeball and also to that which lifts the upper eyelid. 



The fourth pair of nerves, (pathetici) IV, arise from behind the 

 crura cerebri. From there, each curls around a crus cerebri (the 

 cylindrical mass seen beneath it in the figure, running from the 

 pons Varolii to enter the under surface of the cerebral hemispheres) 

 and appears on the base of the brain. Each goes to one muscle of 

 the eyeball. 



The fifth pair of nerves (t rigeminales) , V, resemble the spinal 

 nerves in having two roots; one of these is much larger than the 

 other and possesses a ganglion (the Gasserian or semilunar gan- 

 glion) like the dorsal root of a spinal nerve. Beyond the ganglion 

 the two roots form a common trunk which divides into three 

 main branches. Of these, the ophthalmic is the smallest and is 

 mainly distributed to the muscles and skin over the forehead and 

 upper eyelid; but also gives branches to the mucous membrane 



