See the thick middle commissure joining the two thalami, just as the corpus callosum and fornix join the hemispheres. These are all embryological aftergrowths. Seek also the anterior commissure crossing just in front of the anterior pillars of the fornix, as well as the posterior commissure with its lateral prolongations along the thalami, just below the pineal gland.
On a median section, note the thinnish anterior wall of the third ventricle and its prolongation downwards into the infundibulum.
Turn up or cut off the rear end of one hemisphere so as to see clearly the optic tracts turning upwards towards the rear corner of the thalamus. The corpora geniculata to which they also go, distinct in man, are less so in the sheep. The lower ones are visible between the optic-tract band and the 'testes,' however.
The brain's principal parts are thus passed in review. A longitudinal section of the whole organ through the median line will be found most instructive ([Fig. 37]). The student should also (on a fresh brain, or one hardened in bichromate of potash or ammonia to save the contrast of color between white and gray matter) make transverse sections through the nates and crura, and through the
Fig. 37.—Median section of human brain below the hemispheres. Th, thalamus; Cg, corpora quadrigemina; VIII, third ventricle; Com, middle commissure; F, columns of fornix; Inf, infundibulum; Op.n, optic nerve; Pit, pituitary body; Av, arbor vitæ. (After Obersteiner).
hemispheres just in front of the corpus albicans. The latter section shows on each side the nucleus lenticularis of the corpus striatum, and also the inner capsule (see [Fig. 38], Nl, and Ic).