556 MAMMALS. 



there is a longitudinal band of fibres— the fornix. The corpus callosum 

 is readily disclosed by gently separating the hemispheres. The outer 

 wall arid floor of the anterior part of the cavity or ventricle of each hemi- 

 sphere is formed by a thick mass called the corpus striatum, and the 

 internal cavity is lessened by a prominent convex ridge called the hippo- 

 campus major. The ventricles of the cerebrum communicate with the 

 third ventricle between the optic thalami, by a small aperture called the 

 foramen of Munro. In front of the hemispheres two club-sh,aped 

 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 infiindibulum 

 at the end of which the pituitary body lies, lodged in a fossa of the 

 basisphenoid. Immediately in front of the infiindibulum, 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 mamillary 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 commissure runs, in the roof 

 of the hind part of the ventricle lies a small posterior commissure. 



The optic lobes are four-fold — corpora quadrigemina. They are 

 almost quite 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 is divided into a median and two lateral lobes, and is 

 marked by numerous folds mostly transverse. The two sides are con- 

 nected 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 without 

 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 following 

 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. 



