668 MAMMALIA. 



The cerebral hemispheres of the rabbit are very slightly convoluted, 

 and they leave the cerebellum quite uncovered. They are connected 

 transversely by a broad bridge the corpus callosum and beneath this 

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

 is readily disclosed by gently separating the hemispheres. The outer 

 wall and 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-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 basi- 

 sphenoid. 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 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 fourfold corpora quadrigemina. They are almost 

 quite covered by the cerebrum. Between them runs the iter connecting 

 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 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 : 



(l) The eleventh cranial nerve or spinal accessory, leaving the skull 

 with the ninth and tenth, and distributed to the muscles of the 

 neck. 



