106 ANATOMY OF THE RAT 



and continue on into the brain as the optic tracts. Thus 

 light stimuli received by the right eye may pass to the left 

 side of the brain, and vice versa. The two white and con- 

 spicuous optic nerves run forward a short distance from 

 the chiasma before turning outward to enter the orbits. 

 Observe that the optic tracts diverge behind the chiasma, 

 and are there connected with each other by a whitish 

 band. 



The infundibulum is the prominence just behind the 

 optio chiasma. It is in contact with the pinkish hypophy- 

 sis or pituitary hody, which is covered by a meningeal fold, 

 so that the hypophysis remains in the cranium when the 

 brain is removed. The pedunculi cerebri flank the posterior 

 part of the infundibulum, joining the cerebrum to the 

 medulla oblongata. 



The medulla is wedge-shaped, the expanded anterior end 

 running forward beneath the cerebellum. Just behind the 

 infundibulum a band of nervous tissue, the pons, crosses 

 the ventral surface of the medulla, connecting the cere- 

 bellar hemispheres. Two pyramids run back from the 

 pons along the midventral surface of the medulla. 



The distribution of many of the cranial nerves can be 

 worked out on that side of the head from which the mus- 

 cles have not been removed. There are twelve pairs of 

 these nerves, as follows. 



The first cranial nerves {olfactory) consist of a number 

 of filaments proceeding from the anterior ends of the ol- 

 factory lobes, through the cribriform plate of the ethmoid 

 bone, to the mucous membrane of the nasal cavity. They 

 will probably not be seen. 



The second cranial nerves {optic) have already been 

 described. 



The third nerves {oculomotor) arise from the cerebral 

 peduncles and extend anteriorly beneath the dura mater 



