THE CENTRAL NERVOUS SYSTEM. 353 
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 out- 
growths 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 junc- 
tion was nearly transverse, the cranial end of the ‘tween-brain 
joining the caudal end of the hemispheres. With the increas- 
ing 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 sur- 
face 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 hemi- 
spheres 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, ¢). 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 bea direct 
communication between lateral ventricles and the exterior. 
We may now take up the parts of the diencephalon in detail. 
a. Thalamus (Fig. 141, ¢).—The two thalami are large 
oblique rounded ridges, forming the sides of the ’tween-brain. 
They lie just craniad of the cranial corpora quadrigemina (¢), 
but separated from them by a broad groove, and are completely 
