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 thalamencephalon, 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 : 



1. 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. 



