252 VERTEBRATE ZOOLOGY 



Study the ventral surface of the brain. Separate the brain 

 from its attachments and remove it from the body. In taking 

 it out note the hypophysis, a median projection of the ventral 

 surface back of the optic nerves ; it is lodged in a depression of 

 the floor of the cranial cavity and may be torn off in removing 

 the brain. 



At the anterior end of the hemispheres are the olfactory lobes, 

 from which the olfactory nerves proceed to the nasal capsules, 

 and back of them the olfactory tracts; these are continued pos- 

 teriorly to the extensive pyrif orm lobes, which occupy a large part 

 of the posteroventral surface of the hemispheres. 



These structures belong to the cerebrum, the anterior division 

 of the brain. Back of them are those which belong to the dienceph- 

 alon, the second division. These are, first, the optic nerves, and 

 the optic chiasma from which they spring ; the chiasma is formed 

 by the meeting of the optic tracts, one of which comes from each 

 side. Back of the chiasma is the tuber cinereum, a rather exten- 

 sive median swelling; from it projects the infundibulum, which 

 bears at its end the hypophysis, or pituitary body. Just back of 

 the tuber cinereum are two white projections, the mammillary 

 bodies. On each side of these structures is the large pyriform lobe 

 of the cerebrum. 



Back of the mammillary bodies are the cerebral peduncles, 

 which belong to the third division of the brain, the midbrain. 

 They are a pair of semicylindrical bodies, which proceed from 

 beneath the pyriform lobes backward, converging toward the 

 median line. The oculomotor nerves spring from this region. 



The pons, a broad belt of transverse fibers divided in two by 

 a median groove, follows posteriorly; it belongs to the fourth 

 division of the brain. At the anterior border of its lateral ends 

 the delicate trochlear nerves arise. 



Back of the pons are the structures which belong to the medulla, 

 the fifth division of the brain. They are the two trapezoid bodies, 

 one on each side, and, in the median area between them, a pair of 

 elongated pyramidal tracts, which extend far back of them. 

 Exercise 40. Draw an outline sketch of the ventral aspect of the 

 brain, showing accurately these structures. 



