158 VERTEBRATE ZOOLOGY 



tion of the midbrain,— the third division. Behind the optic lobes 

 are the cerebellum and the medulla oblongata,— the fourth and 

 fifth divisions. Remove the vascular posterior choroid plexus, the 

 dark-colored membrane which forms the dorsal surface of the me- 

 dulla; a triangular depression, the fourth ventricle of the brain, 

 will be exposed. 



Exercise 35. Draw the dorsal aspect of the brain on a scale of 3 or 4. 



Remove enough of the side of the cranium to expose the side 

 of the brain. Carefully tilt the brain to one side and study the 

 twelve pairs of cranial nerves. The first pair, the olfactory nerves, 

 proceeds from the olfactory lobes of the hemispheres and goes 

 to the nasal capsules. The second pair, the optic nerves, springs 

 from the ventral surface of the diencephalon and proceeds to the 

 eyes. The third and fourth nerves, the oculomotor and the troch- 

 lear, are very small nerves which pass from the posterior portion 

 of the midbrain to the muscles of the eyeballs, the oculomotor 

 arising from the ventral and the trochlear from the lateral surface 

 of the midbrain. 



The fifth nerve, the trigeminus, is a large nerve which leaves 

 the side of the anterior end of the medulla oblongata; it passes 

 forward and soon swells to form the Gasserian ganglion, which lies 

 on the inner surface of the skull and may have been removed with 

 it. From this ganglion three nerves spring, passing to the outside 

 of the skull, — the ophthalmic, which passes forward and enters the 

 orbit just back of the optic nerve ; the maxillary, which also passes 

 forward and enters the orbit, where it divides into two branches ; 

 and the mandibular, which passes out through the same foramen 

 with the superior maxillary and is distributed to the muscles of 

 the jaw. 



The sixth nerve, the abducens, is a small nerve which arises 

 from the ventral surface of the medulla near its anterior end and 

 passes to the muscles of the eye. The seventh and eighth nerves, 

 the facial and auditory, spring together from the side of the me- 

 dulla back of the trigeminus, the former passing to the muscles 

 of the upper jaw, and the latter into the ear capsule. The ninth 

 nerve, the glossopharyngeal, arises from the side of the medulla 



