Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/119

Rh sheet which will be observed in front of the corpora quadrigemina when the hemispheres are pressed apart.

The paraflocculus projects laterally from the lateral lobe of the cerebellum. Cautiously remove the petrotympanic bone, a bit at a time, and note that the paraflocculus occupies the floccular fossa (see the petrotympanic bone). The paraflocculus resembles a toadstool, the enlarged distal end lying in the fossa. The flattened floccus proper is anteroventral to, and in contact with, the paraflocculus.

Remove the brain from the skull, being careful to preserve the roots of the cranial nerves. Care should be taken to keep the paraflocculus intact. The connections between the cranial nerves and the brain are easily broken. If considerable care is used in dissecting away the bone around the foramina of the nerves, many of them may be removed almost entire on that side of the head where the muscles were taken away. Separate the brain from the spinal cord and immerse it in a weak formalin solution, where it may be studied.

Each olfactory lobe will now be seen to contain two parts — the olfactory bulb, and the olfactory tract. Each bulb is a swelling at the anterior end of the tract, and in a preserved specimen the former is considerably darker than the latter. The tract rests in a depression on the oblique anterior surface of the cerebral hemisphere. By its distinct whiteness each tract may be followed backward and outward to the level of the optic chiasma, where it dis- appears. Two pear-shaped areas darker than the olfactory tracts extend from the olfactory lobes back to the optic chiasma. They meet in the median plane, and each is limited laterally by the olfactory tract.

The optic nerves of mammals cross at the optic chiasma,