THE CEREBRUM 319 



ing as a ventral diverticulum from the primitive neural tube. 

 The prehypophysis is the larger. 



The lamina cinerea, or terminalis, is a thin, easily torn lamina 

 between the optic chiasm and anterior commissure, bounded 

 laterally by the cerebral hemispheres. 



Optic tract (see Optic Nerve). 



Optic chasm (see Optic Nerve). 



The Third Ventricle 



The third ventricle is derived from the primitive fore-brain 

 vesicle, except that portion which also enters into the forma- 

 tion of the lateral ventricles. It is a narrow space between 

 the two thalami and hypothalamic gray, limited in front by 

 the terma, behind continuous with the aqueduct (Sylvian), 

 and laterally is continuous with the lateral ventricles through 

 the foramen of Monro. The roof is covered by an ependymal 

 layer, fused with the tsenia thalami, and the ependymal layer 

 is reinforced by a vascular fold of pia mater, the two together 

 constituting the velum interpositum; also the fornix. The 

 floor is formed by the tuber cinereum, corpora albicantia, 

 and posterior perforated substance, as well as the optic chiasm 

 and a portion of the tegmentum of the crura cerebri. 



In its cavity are three commissures: the anterior, in front of 

 the anterior pillars of the fornix, piercing on each side the corpus 

 striatum; the middle, or soft, of gray matter, connecting the 

 thalami; and the posterior, also connecting the optic thalami 

 behind, and lying in front of and beneath the pineal gland. 



In front are the two openings, one on each side, of the fora- 

 men of Monro. 



The Cerebrum 



The cerebrum is the largest part of the brain, and consists 

 of two lateral halves, or hemispheres, separated by the great 

 longitudinal fissure and connected to each other by a great 

 commissure, the corpus callosum. The latter constitutes a 

 great system of association fibers for the bilateral coordination 

 of corresponding cortical parts. The hemispheres are sub- 

 divided into lobes, and the latter present over their entire 

 surfaces convoluted eminences, the gyres, or convolutions, sepa- 

 rated by depressions, the sulci or fissures. 



