THE BRAINi 387 



The whole surface of the cerebellum is 

 marked by close-set folds, mainly transverse in 

 direction. 



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 medullse anterius, or valve of 

 Vieussens, is a thin transparent membrane 

 connecting the hinder border of the optic 

 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 medullse 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 

 hemispheres and the cerebellum, but is very 

 liable to be torn away with the dura mater. It 

 has been shown to be a degenerate eye-like organ. 



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 



