THE BRAIN 343 



f. The medulla oblongata is the part of the brain 

 beneath and behind the cerebellum. It is some- 

 what flattened dorso-ventrally, is widest in front, 

 and gradually narrows behind, passing into the 

 spinal cord. 



i. The velum medullae anterius, or valve of j 

 Vieussens, is a thin transparent membrane, * 

 connecting the hinder border of the opticf^ 

 lobes with the cerebellum, and roofing over } ^ ( 

 the anterior part of the fourth ventricle. It 

 lies beneath the anterior part of the cere- 

 bellum, which must, be gently pressed back- 

 wards to expose it. 



ii. The velum medullae posterius is a thin trans- 

 parent membrane, forming the roof of the 

 posterior part of the fourth ventricle : it is 

 covered in front by the cerebellum. 



g. The pineal body is a small rounded median body, 

 lying on the optic lobes, and connected by a 

 stalk with the roof of the thalamencephalon. 



It is visible on the dorsal surface, in the angle 

 between the hinder ends of the cerebral hemi- 

 spheres and the cerebellum, but is very liable to 

 be torn away with the dura mater. It has 

 recently been shown to be a degenerate eye. 



2. The ventral surface of the brain. 



a. The cerebral hemispheres, as seen from below, are 



closely apposed in front, but diverge posteriorly. 



About the middle of its length the inner 

 border of each hemisphere presents a notch, 

 which is continued outwards across the surface 

 of the hemisphere as a shallow groove, the 

 Sylvian fissure, separating the frontal from the 

 temporal lobe. 



b. The olfactory lobes lie, in their hinder portions, 



along the under surface of the frontal lobes of 

 the hemispheres, extending back as far as the 



