490 THE NERVOUS SYSTEM. 
of the cerebellum (Fig. 362, EK, v.). This is placed much nearer the lower than the upper 
border of the organ, and it represents at this stage the angular peak of the tent-like roof 
of the fourth ventricle in the adult brain. As growth goes on, the portions of the cere- 
bellum in front and behind this groove approach each other, so as to deepen the groove 
and bring about the backward prolongation of the ventricular cavity towards the cer ebellum. 
THE MESENCEPHALON. 
The mesencephalon or mid-brain is the short, narrow part of the brain-stem 
which occupies the aperture of the tentorium cerebelli (incisura tentorii), and 
connects the cerebrum which hes above with the parts which occupy the posterior 
cranial fossa. It is about three-quarters of an inch in length, and it consists of a 
dorsal part, composed of the corpora quadrigemina, and a much larger ventral part, 
which is formed by the two crura cerebri. 
In the undissected brain the corpora quadrigemina are completely hidden from 
view by the splenium of the corpus callosum, which projects backwards over them, 
and also by the superimposed cerebral hemispheres. The hinder end of each optic 
thalamus likewise, to some extent, overhangs the upper part of the mesencephalon 
on its dorsal and lateral aspect (Fig. 533, p. 447). On this portion of the optic 
thalamus are seen two projections, which are specially related to the mesencephalon. 
These are the eushion-lke pulvinar, which forms the inner and hinder part of the 
thalamus, and the corpus geniculatum externum, an ill-defined oval swelling on the 
outer and under aspect of the posterior end of the thalamus. 
The crura cerebri can to some extent be seen on the base of the brain, where 
they bound the posterior part of the interpeduncular space. Encircling the upper 
end of each crus cerebri, where it plunges into the cerebrum, is the optic tract 
(Fig. 325, p. 439). 
The mesencephalon is tunnelled from below upwards by a narrow passage, 
called the aqueduct of Sylvius, which connects the fourth ventricle with the third 
ventricle (Fig. 364, p. 492). This channel hes much nearer the dorsal than the 
ventral aspect of the mesencephalon. 
Corpora Quadrigemina.—This name is applied to four rounded eminences on 
the posterior aspect of the mesencephalon (Fig. 333, p. 447). The superior pair are 
larger and broader than the inferior pair, but they are not so well defined nor are 
they so prominent. A longitudinal and a transverse groove separate the quad- 
rigeminal bodies from each other. The longitudinal groove occupies the mesial plane 
and extends upwards to the posterior comnussure of the brain. The upper end of 
this groove widens out into a shallow depression, in which the pineal body, a small 
conical structure which belongs to the diencephalon, rests. From the lower end of 
the same groove a short but well-defined and projecting band of white fibres, the 
frenulum veli, passes to the valve of Vieussens, which hes immediately below the 
inferior pair of quadrigeminal bodies. The transverse groove curves round behind 
vach of the superior pair of quadrigeminal bodies and separates them from the 
inferior pair. It is also continued in an upward and forward direction on the 
lateral aspect of the mesencephalon. 
The quadrigeminal bodies are not marked off laterally from the sides of the 
mesencephalon, but each has in connexion with it, on this aspect, a prominent 
strand of white matter, which is prolonged upwards and forwards under the 
projecting pulvinar and corpus geniculatum externum. These strands are called 
the brachia of the corpora quadrigemina, and they are separated from each other by 
a continuation on the side of the mesencephalon of the transverse groove, which 
intervenes between the two pairs of bodies. 
The corpus geniculatum internum (corpus geniculatum mediale) is closely 
associated with the brachia. It is a small, sharply-defined oval eminence, which 
lies on the side of the upper part of the mesencephalon under shelter of the pulvinar 
of the optic thalamus. 
The inferior brachium (brachium quadrigeminum inferius), proceeding upwards 
from the lower quadrigeminal body, advances towards the corpus geniculatum 
internum and disappears from view under cover of this prominence. Upon the 
