Gross Axatomy 15 



and related parts (chapter XIII), Two eminences appear on 

 its lateral surface, the smaller, posterior one being the medial 

 geniculate body, the larger, anterior one the lateral geniculate 

 body, and in front of this the diencephalon is attached laterally 

 to the cerebral hemisphere (corpus striatum). A deep, oblique 

 groove, into which fits the fimbria of the hippocampus (vide 

 infra), marks roughly the boundary between thalamus and 

 corpus striatum on the dorsal surface. 



The lateral geniculate body is covered with white fibres 

 of the optic tract, w^hich may be follow^ed dow^n and forward 

 to the optic chiasma, the decussation on the ventral surface of 

 the brain of the two optic tracts. From the chiasma the 

 tracts continue forwards as the optic nerves. 



Behind the optic chiasma lies the pituitary body, or 

 hypophysis, which is attached to the base of the brain by 

 a thin, hollow stalk, the infundibulum. The hypophysis is 

 frequently left behind when the brain is removed from the 

 skull, the infundibulum being broken through, in w^hich case 

 the cavity of ihe infundibulum appears as a small median 

 slit opening into the third ventricle. This slit is surrounded 

 by an elevation of gray matter, the tuber cinereum, and this 

 is continuous posteriorly with the mamillary body, a con- 

 spicuous mass projecting postero-ventrally over the anterior 

 part of the interpeduncular fossa. There are paired mamil- 

 lary bodies in the human brain, but in the rat they are repre- 

 sented by a single median mass. 



If one hemisphere be removed, there may be observed on 

 the part of its medial surface which lies in contact with the 

 diencephalon a distinct curved groove, the hippocampal 

 fissure, and parallel and a little anterior to it a conspicuous 

 ridge of white fibres, the fimbria hippocampi (PI. l\.). Be- 

 tween these is a fainter groove, which marks the div^ision 

 between the fascia dentata or dentate gyrus, lying between 

 it and the hippocampal fissure, and the hippocampus proper, 

 covered with a thin laver of white fibres which join the 



