FORE-BRAIN. 501 
median fold or ridge. Both of these conditions are transitory. As the corpora quadri- 
gemina take shape, the median ridge disappears and is replaced by the median longitudinal 
groove, which separates the quadrigeminal bodies. Only its lower part is retained, and this 
is represented by the frenulum veli of the adult brain. The diverticulum of the cavity 
eradually becomes reduced, and finally disappears as the aqueduct assumes form. 
FORE-BRAIN. 
PARTS DERIVED FROM THE DIENCEPHALON. 
Under this heading we have to consider: (1) the thalamus ; (2) the epithalamus, 
which comprises the pineal body and the habenular region; (3) the metathalamus, 
or the corpora geniculata ; and (4) the hypothalamus. 
The hypothalamus consists of two portions, viz. the pars mammillaris hypothalami, 
which comprises the corpus mammillare and the portion of the central gray matter 
which forms the floor of the third ventricle in its immediate vicinity ; and the pars 
optica hypothalami, which embraces the tuber cinereum, the infundibulum, the 
pituitary body, and the lamina cinerea. Strictly speaking, the optic part of the 
hypothalamus does not belong to the diencephalon, but it is convenient to study 
the parts which it represents at this stage. It is also convenient to examine, at 
the same time, the subthalamic tegmental region, although a very considerable part of 
this is apparently developed in connexion with the mesencephalon. 
The original cavity of that part of the brain-tube which forms the diencephalon 
is represented by the greater part of the third ventricle of the brain. 
Optic Thalamus (thalamus).—The optic thalamus is the principal object in 
this section of the brain. It is a large ovoid mass of gray matter, which hes 
obliquely across the path of the crus cerebri as it ascends into the cerebrum. The 
smaller anterior end of the thalamus lies close to the mesial plane, and is only 
separated from the corresponding part of the opposite side by a very narrow 
interval. The enlarged posterior ends of the two thalami are placed more widely 
apart, and in the interval between them the corpora quadrigemina are situated. 
As previously stated, the crusta of the crus cerebri, composed of corticifugal fibres, 
gradually inclines outwards as it is traced upwards, and thus it assumes a place on 
the outer aspect of the optic thalamus and passes into the internal capsule of the 
brain. The tegmental part of the crus, on the other hand, comes into relation 
with the under surface of the thalamus, and forms in this situation the sub- 
thalamic tegmental region. To a very large extent the longitudinal fibres of the 
tegmentum are corticipetal. For the most part they enter the thalamus, and 
either end within it in fine arborisations around the thalamic cells or are carried 
upwards through the thalamus into the internal capsule. 
The two optic thalami, in their anterior two-thirds, le close together on either 
side of a deep mesial cleft, which receives the name of the third ventricle of the 
brain. Each thalamus presents an anterior and a posterior extremity and four 
surfaces. The inferior and external surfaces are in apposition, and, indeed, directly 
connected with adjacent parts of the brain, and on this account it is only possible 
to study them by means of sections through the brain. The superior and internal 
surfaces are free. 
The external or lateral surface of the thalamus is applied to a thick layer of 
white matter interposed between it and the lenticular nucleus, called the internal 
capsule, and composed of fibres passing both upwards towards, and downwards from, 
the cerebral cortex. A large proportion of these fibres descend to form the 
crusta or ventral part of the crus cerebri. From the entire extent of the external 
surface of the thalamus large numbers of fibres stream out and enter the internal 
capsule, to reach the cerebral cortex. These fibres constitute what is termed the 
thalamic radiation, and by this the thalamus is brought into connexion with the 
entire extent of the cerebral cortex. As the fibres leave the thalamus they inter- 
sect each other at acute angles, and over the whole of the external surface of the 
ganglionic mass they form a very distinct reticulated zone or stratum, which is 
termed the external medullary lamina. 
The inferior or ventral surface of the thalamus rests chiefly on the subthalamic 
32 b 
