

THE FORE-BRAIN OR PROSENCEPHALON 



anterior perforated substance and then bends abruptly medialward toward the 

 uncus of the hippocampal gyrus. The medial stria turns medialward behind 

 the parolfactory area and ends in the subcallosal gyrus; in some cases a small 

 intermediate stria is seen running backward to the anterior perforated substance. 



(c) The olfactory trigone (trigonum olfactorium) is a small triangular area in front 

 of the anterior perforated substance. Its apex, directed forward, occupies the 

 posterior part of the olfactory sulcus, and is brought into view by throwing back 

 the olfactory tract. 



(d) The parolfactory area of Broca (area parolfactoria) is a small triangular field 

 on the medial surface of the hemisphere in front of the subcallosal gyrus, from w T hich 

 it is separated by the posterior parolfactory sulcus; it is continuous below with 

 the olfactory trigone, and above and in front with the cingulate gyrus; it is limited 

 anteriorly by the anterior parolfactory sulcus. 



(e) The anterior perforated substance (substantia perforata anterior} is an irregularly 

 quadrilateral area in front of the optic tract and behind the olfactory trigone, 

 from which it is separated by the fissure prima ; medially and in front it is continuous 

 with the subcallosal gyrus ; laterally it is bounded by the lateral stria of the olfactory 

 tract and is continued into the uncus. Its gray substance is confluent above 

 with that of the corpus striatum, and is perforated anteriorly by numerous small 

 bloodvessels. 



2. The Uncus has already been described (page 826) as the recurved, hook-like 

 portion of the hippocampal gyrus. 



3. The Subcallosal, Supracallosal, and Dentate Gyri form a rudimentary arch- 

 shaped lamina of gray substance extending over the corpus callosum and above 

 the hippocampal gyrus from the anterior perforated substance to the uncus. 



(a) The subcallosal gyrus (gyrus subcallosus; peduncle of the corpus callosum) is 

 a, narrow lamina on the medial surface of the hemisphere in front of the lamina 

 terminalis, behind the parolfactory area, and below the rostrum of the -corpus 

 2allosum. It is continuous around the genu of the corpus callosum with the supra- 

 sallosal gyrus. 



(6) The supracallosal gyrus (indusium griseum; gyrus epicallosus) consists of a 

 thin layer of gray substance in contact with the upper surface of the corpus 

 callosum and continuous laterally with the gray substance of the cingulate gyrus. 

 It contains two longitudinally directed strands of fibers termed respectively the 

 medial and lateral longitudinal striae. The supracallosal gyrus is prolonged around 

 the splenium of the corpus callosum as a delicate lamina, the fasciola cinerea, 

 which is continuous below with the fascia dentata hippocampi. 



(c) The fascia dentata hippocampi (gyrus dentatus) is a narrow band extending 

 downward and forward above the hippocampal gyrus but separated from it by 

 the hippocampal fissure; its free margin is notched and overlapped by the fimbria 

 the fimbriodentate fissure intervening. Anteriorly it is continued into the notch 

 of the uncus, where it forms a sharp bend and is then prolonged as a delicate band, 

 the band of Giacomini, over the uncus, on the lateral surface of which it is lost. 



The remaining parts of the rhinencephalon, viz., the septum pellucidum, fornix, 

 and hippocampus, will be described in connection with the lateral ventricle. 



Interior of the Cerebral Hemispheres. If the upper part of either hemisphere be 

 removed, at a level about 1.25 cm. above the corpus callosum, the central white sub- 

 stance will be exposed as an oval-shaped area, the centrum ovale minus, surrounded 

 by a narrow convoluted margin of gray substance, and studded with numerous 

 minute red dots (puncta vasculosa) , produced by the escape of blood from divided 

 bloodvessels. If the remaining portions of the hemispheres be slightly drawn apart 

 a broad band of white substance, the corpus callosum, \vill be observed, connecting 

 them at the bottom of the longitudinal fissure; the margins of the hemispheres 

 which overlap the corpus callosum are called the labia cerebri. Each labrium is 



