Fig. 142.—Ventral Surface of the Midbrain and ’Tween-brain, with the Pons.
a, pedunculi cerebri; b, tractus transversus peduncularis; c, corpus geniculatum mediale; d, optic tract; e, optic chiasma; f, optic nerve; g, mammillary bodies; h, tuber cinereum; i, opening for infundibulum (which has been removed); j, posterior perforated area; k, pons. III, V, VII, VIII, third, fifth, seventh, and eighth cranial nerves.
C. Prosencephalon.—The prosencephalon or primitive forebrain includes the diencephalon or ’tween-brain and the telencephalon or cerebral hemispheres.
4. Diencephalon.
—The diencephalon or ’tween-brain includes the thalami and the other parts bounding the third ventricle. The diencephalon is seen in entire brains only in ventral view ([Fig. 138], c, d, e, etc.)
The diencephalon may be considered as forming almost or quite the most cranial portion of the median nervous tube,—the cranial wall of the third ventricle (the lamina terminalis) ([Fig. 143], d), ending in the median line in the deep fissure between the hemispheres of the cerebrum. Parts of the brain which extend farther craniad than this are lateral portions, due to the forward growth of the lateral hemispheres. The two hemispheres of the cerebrum may be considered as lateral outgrowths of the central ’tween-brain; these outgrowths have extended dorsad, laterad, craniad, and caudad, so as to cover almost completely the ’tween-brain.
In early stages the cerebral hemispheres are projections from the cranial end of the ’tween-brain, so that the plane of junction was nearly transverse, the cranial end of the ’tween-brain joining the caudal end of the hemispheres. With the increasing size and backward growth of the latter, the attachment to the ’tween-brain has been shifted from a cranial to nearly a lateral position, and at the same time the originally lateral surface of the ’tween-brain has become nearly caudal. This is shown in [Fig. 141], the deep fissure at 1 marking the line of attachment between the ’tween-brain and the hemispheres. The dorsoventral plane of junction of ’tween-brain and hemispheres is (as [Fig. 141] shows) not wholly lateral, but oblique, passing from its cranial end near the middle line caudolaterad.
A second peculiarity of the ’tween-brain lies in the thinness of its roof. The roof is exceedingly thin and is so intimately connected with the pia mater that they cannot be removed separately. The ventral thick floor of the ’tween-brain is directly continuous with the similar floor of the cerebrum; but where the roof of the ’tween-brain joins the roof of the cerebrum along the oblique plane already indicated, the roof is very thin and is intimately connected to the pia mater, and is at the same time folded into the lateral ventricles to form, together with the pia mater, the choroid plexuses of the lateral ventricles ([Fig. 148], e). When the pia mater is removed the thin roof of the brain along the line of junction of the ’tween-brain and the cerebrum is brought away and there appears to be a direct communication between lateral ventricles and the exterior.
We may now take up the parts of the diencephalon in detail.