NERVOUS SYSTEM. 709 
hemisphere is formed by a thick mass, called the corpus striatum, and 
the internal cavity is lessened by a prominent convex ridge, called the 
hippocampus major. The ventricles of the cerebrum communicate 
with the third ventricle, between the optic thalami, by a small 
aperture, called the foramen of Monro. In front of the hemispheres 
two club-shaped olfactory lobes project. The thin cortical layer of 
the cerebrum consists of grey (ganglionic) matter, and so does the 
thick corpus striatum, while the central part consists of white matter 
(nerve fibres), 
The thalamencephalon is entirely hidden, but gives origin as usual 
to the dorsal epiphysis, ending in a pineal body, which lies on the 
surface between the cerebrum and cerebellum, and to the ventral 
infundibulum, at the end of which the pituitary body lies, lodged in a 
fossa of the basisphenoid. Immediately in front of the infundibulum 
the optic nerves cross in a chiasma, from which optic tracts can be 
traced to the optic lobes. Immediately behind the infundibulum lies 
a rounded elevation, called the mammillary body. ‘ Anteriorly, on the 
ventral surface of each side of the thaiamencephalon, there is a rounded 
swelling, called the corpus geniculatum. The roof of the third 
ventricle is formed by a thin membrane or velum, with a plexus of 
blood vessels. In the anterior wall of the third ventricle lies the small 
anterior commissure; across the third ventricle the large middle com- 
missure runs; in the roof of the hind part of the ventricle lies a small 
posterior commissure. 
The optic lobes are fourfold—corpora quadrigemina. They are in 
large part covered by the cerebrum. Between them runs the iter con- 
necting the third ventricle and the fourth. The floor of this passage is 
formed by the thick crura cerebri which connect the medulla with the 
cerebrum. 
The cerebellum has a median and two lateral lobes (with accessory 
flocculi), and is marked by numerous folds, mostly transverse. The 
two sides are connected ventrally by the pons Varolii, lying across the 
anterior ventral surface of the medulla. 
The medulla oblongata lies beneath and behind the cerebellum, and 
is continued into the spinal cord. The cavity of the fourth ventricle is 
roofed by a thin membrane or velum, above which lies the cerebellum. 
‘ On the ventral surface the medulla is marked by a deep fissure, bordered 
by two narrow bands or ventral pyramids. : 
The spinal cord presents its usual appearance, with its dorsal sensory 
nerve-roots with ganglia, its ventral motor nerve-roots apparently with- 
out ganglia, and the spinal nerves formed from the union of these. The 
ganglia of the adjacent sympathetic system perhaps belong to the ventral 
roots of the spinal nerves. 
A large number of nerves pass down the neck. Of these the follow- 
ing are most important :— 
I. The eleventh cranial nerve or spinal accessory, leaving the skull 
with the ninth and tenth, and distributed to the muscles of the 
neck. 
2. The twelfth cranial nerve or hypoglossal, lying at first close to 
the ninth, tenth, and eleventh, turning, however, to the muscles 
of the tongue. 
