508 THE NERVOUS SYSTEM. 7 
from the buccal cavity. Its connexion with the latter is soon cut off, and it becomes 
encased within the cranial cavity in intimate association with the cerebral portion of the 
organ. Structurally, it consists of 
tubules or alveoli, lined by epithelial 
cells and surrounded by capillary 
vessels. Its structure is not unlike 
that of the thyroid body, and 
———Foramen of Monro —_— possibly it exercises a similar function. 
In giants, and in cases of acromegaly, 
the pituitary body is usually greatly 
Ventricle IIT. enlarged. 
COR PUS CALLOSUM 
Anterior commissure 
—— Corpus mammillare 
Lamina Cinerea.—This is a 
thin lamina which passes upwards 
from the optic chiasma in the 
great longitudinal fissure to be- 
Subarachnoid tissue 
in cisterna basalis 
~Infundibulum 
Pituitary body come connected with the rostrum 
of the corpus callosum. 
Cisterna pontis Anterior Commissure of the 
Cerebrum.— In the anterior part 
of the cleft, between the two optic 
thalami and immediately in front 
of the anterior pillars of the fornix, 
a round bundle of fibres crosses 
Schenoidaleinus. the mesial plane. This is the 
AR: r a, anterior commissure. It is much 
Fic. 377,—MESIAL SECTION THROUGH THE PITUITARY REGION 
~——~ Basi-oce ipital 
BN aaa aon larger than the posterior com- 
missure. 
Third Ventricle (ventriculus tertius)—This is the narrow cleft which 
separates the two optic thalami. Its depth rapidly increases from behind for- 
wards, and if may be said to extend from the pineal body behind to the lamina 
cinerea in front. Its jloor is formed by the structures already studied within 
the area of the interpeduncular space on the base of the brain, viz. the tuber 
cinereum, the corpora mammillaria, the gray matter of the locus perforatus posticus, 
and also to some extent behind this by the tegmenta of the crura cerebri. It is 
interesting to note that. the central gray matter which surrounds the Sylvian 
aqueduct is directly continuous with the gray matter of the locus perforatus posti- 
cus and tuber cinereum, and in this way it comes to the surface in the base of the 
brain. The optic chiasma crosses the floor in front and marks the place where the 
latter becomes continuous with the anterior wall of the cavity. The front wall of 
the third ventricle is formed by the lamina cinerea, which extends upwards from 
the optic chiasma. The anterior commissure, as it crosses from one side to the other, 
projects into the ventricle, but of course it is excluded from the cavity by the 
ventricular epithelial lining. It may be taken as indicating the place where the 
roof joins the anterior wall. The roof of the third ventricle is formed by a thin 
epithelial layer, continuous with the thin epithelial lining of the cavity, which 
stretches across the mesial plane from one tenia thalami “to the other. Applied 
to the upper surface of the epithelial roof is the fold of pia mater, termed the velum 
interpositum, and the roof is invaginated into the cavity along its whole length by 
two delicate choroid plexuses, which hang down from the under surface of this 
fold. When the velum interpositum is removed the thin epithelial roof is torn 
away with it, leaving only the lines of attachment in the shape of the tenia 
thalami. 
The lateral wall of the third ventricle is formed for the greater part of its 
extent by the inner surface of the optic thalamus, covered by a thick layer of 
central gray matter continuous with the Sylvian gray matter of the mesencephalon. 
A little in front of the middle of the ventricle the cavity is crossed by the middle 
or soft commissure, which connects the thalami with each other, and in front of this 
the anterior pillar of the fornix is seen curving downwards and backwards in the 
lateral wall. At first the bulging which it forms is distinctly prominent, but it 
