188 VERTEBRATE ZOOLOGY 



of the brain between the ventral portions of the cerebrum and the 

 cerebellum. This latter division is the fourth; it is marked by 

 transverse grooves ; posteriorly it overlaps the medulla oblongata, 

 the fifth division, which is continuous with the spinal cord. At 

 about the point where the spinal cord passes into the brain, the 

 former makes a sharp turn ventrally. 



Exercise 29. Draw a view of the dorsal aspect of the brain. 



Study the lateral aspect of the brain and the twelve pairs of 

 cranial nerves, so far as these can be dissected. Entirely remove 

 one side of the cranium, but preserve the ventral portion of the 

 orbit, with the large nerves which cross it. The first pair of cranial 

 nerves, the olfactories, are slender prolongations of the olfactory 

 lobes ; they pass to the nasal capsules. The second pair, the optic 

 nerves, emerge from the optic chiasma on the ventral side of the 

 second division of the brain and pass to the orbits. In order to 

 see them, separate the floor of the cranial cavity from the cere- 

 brum, beginning at its anterior end. The whole anterior end of 

 the skull can be pressed down, away from the brain, so as to expose 

 the ventral surface of the cerebrum and the optic nerves. The 

 optic tracts will also be laid bare. They will be seen immediately 

 back of the optic chiasma on each side and abutting on the optic 

 lobes. The third pair of cranial nerves, the oculomotors, are small 

 nerves which arise close to the median line, just back of the optic 

 tracts, and go to the orbits. 



The fourth nerve, the trochlear or pathetic, will be seen arising 

 between the optic lobe and the cerebellum on the side of the brain 

 and passing forward to the orbit. The fifth nerve, the trigeminal, 

 is a large nerve which arises on the side of the medulla just beneath 

 the optic lobe and passes forward to the large Gasserian ganglion. 

 This structure lies in a depression in the wall of the skull and gives 

 off three branches : ( i ) the ophthalmic nerve, which runs through 

 the floor of the skull into the orbit, passing along the upper 

 portion of the orbit to a foramen in its anterior wall, where it 

 enters the nasal capsule, through the entire length of which it 

 runs; (2) the maxillary nerve, which enters the orbit and passes 

 along its floor to the upper mandible; (3) the mandibular nerve, 



