t16 ANTHROPOLOGY. 
_ The pineal gland. This body, rendered famous by the vague theory of. 
Descartes, which viewed it as the chief. source of nervous power, is placed 
just behind the third ventricle, resting in a superficial groove which passes 
along the median line between the corpora quadrigemina. It is of a grey- 
ish color, heart-shaped, with the apex directed backwards and downwards. 
A process from the deep layer of the velum interpositum envelopes it and. 
retains it in place. From each angle of the base there passes off a band of 
white matter to the inner surface of each optic thalamus, called the pedun- 
cles or haben ot the pineal gland. The only connexion with the brain is 
by means of these habenze. In the adult, grains of sandy matter are usually 
found in the pineal gland, which are mostly collected in a cavity towards 
its base ; sometimes, however, situated on the surface. 
- The soft commissure is a lamina of light grey matter, situated in she 
tains ventricle, and extending between the optic thalami of opposite sides.. 
It forms a transverse horizontal plane dividing the ventricle into two 
portions. 
_ We come next to consider that part of the. brain which. les intermediate 
to the cerebrum, cerebellum, and medulla oblongata, and conveniently 
termed the mesocephalon. 'The pineal body rests upon its upper surface; 
but from its internal relation to the third ventricle and optic thay we 
found its description with them most convenient. | 
. 8. THE MEsocEPHALON. The limits of the mesocephalon cannot st aati 
be defined, as it is continuous with the crura cerebri above, with the crura 
cerebelli behind, with the medulla oblongata below, and with the cerebellum 
itself above and behind. Many treat of it as one mass with the medulla 
oblongata, We shall consider it as composed of the tubercula eee” 
gemina and the pons Varolu. 
The corpora, or tubercula quadrigemina, are four eminences seen imme- 
diately behind the third ventricle. A transverse furrow separates them 
into an anterior, the nates, and a posterior pair, the testes; a longitudinal 
furrow along the median line divides the right and left pairs from each 
other. The aed body rests in the anterior extremity of the lonigitiedenng 
depression. 
The posterior crura of the corpora quadrigemina are connected with the 
cerebellum by two columns of white matter, one of which passes into the 
- central white substance of each cerebellar hemisphere; these are the pro- 
cessus cerebelli ad testes. 'They enter into the formation of the crura cerebelli. 
The interval between the two processus cerebelli is occupied by a horizon- 
tal stratum of nervous matter called the valve of Vieussens, or of the fourth 
ventricle. 
The pons Varolii, or great commissure of the cerebellum, is somewhat 
square, and placed obliquely on the cuneiform process between the cere- 
brum and the cerebellum. The fourth ventricle, the aqueduct of Sylvius, 
and the corpora quadrigemina are on its superior and posterior surface. 
Its superior extremity receives the crura cerebri, which it surrounds like a 
ring, and is hence sometimes called the annular protuberance; the crura 
cerebelli are attached to its sides, and the medulla oblongata to its: lower 
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