518 



NERVOUS SYSTEM. 



The corpus callosum terminates at the base of the brain by a concave margin, 

 which is connected with the tuber cinereum through the intervention of a thin 

 layer of gray substance, the lamina cinerea. This may be exposed by gently 

 raising and drawing back the optic commissure. A broad white band may be 

 observed on each side, passing from the under surface of the corpus callosum 

 backwards and outwards, to the commencement of the fissure of Sylvius; these 

 bands are called the peduncles of the corpus callosum. Laterally, the corpus 

 callosum extends into the anterior lobe. < 



The lamina cinerea is a thin layer of gray substance, extending backwards 

 from the termination of the corpus callosum above the optic commissure to the 

 tuber cinereum ; it is continuous on either side with the gray matter of the anterior 

 perforated space, and forms the anterior part of the inferior boundary of the third 

 ventricle. 



Fig. 265. Base of the Brain. 



The olfactory nerve, with its bulb, is seen on either side of the longitudinal 

 fissure, upon the under surface of each anterior lobe. 



The fissure of Sylvius separates the anterior and middle lobes, and lodges the 

 middle cerebral artery. At its entrance is seen a point of medullary substance, 

 corresponding to a subjacent band of white fibres, connecting the anterior and 

 middle lobes, and called the fasciculus unciformis ; on following this fissure out- 

 wards, it divides into two branches, which inclose a triangular-shaped prominent 

 cluster of isolated convolutions, the island of Reil. These convolutions, from 

 being covered in by the sides of the fissure, are called the gyri operti. 



