Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/120

106 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 conspicuous 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 optic chiasma. It is in contact with the pinkish hypophysis or pituitary body, 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 cerebellar 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 olfactory 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