14 BEAN. 



XI. Anteriorly, the velum interpositum is attached to the lamina terminalis. This 



is relatively thick above, but becomes thinner below and ends by becoming 

 attached below to the front edge of the optic commissure, llie ventricle 

 forms a slight prolongation forward above the optic commissure which is 

 the optic recess. 



1. Note, entering the upper part of the lamina terminalis, the pillars of the 



forn ix. 



2. Just behind the upper part of the lamina terminalis and bounded above by 



the velum interpositum is the interventricular foramen of Miinro. 



3. In the lamina terminalis, a little below the level of the foramen of ^lunro, 



is the anterior commissure. 



XII. The floor of the III ventricle is formed of, in front : 



1. The optic commissure. 



2. Behind, the tiiber cinereum from which the infundibulum projects down- 



ward. 



3. From behind the tuber cinereum and just in front of the mammillary body, 



the floor ascends obliquely to join the floor or the aqueduct of the 

 cerebrum. 



XIII. In the lateral walls of the III ventricle are the large optic ihalami. 



1. Connecting these and passing across the middle of the ventricle is the 

 intermediate mass. 



On the mesial surface of the forebrain are seen : 



I. The corpus callosum. Note: 



1. The posterior end slightly rolled under, the splenium. 



2. Anterior to this a narrow, constricted portion, the Isthm us. ( 1 ) . 



3. The trunk or body forming the main part of the callosum. 



4. The prominent anterior end, the knee. 



5. A portion bending back from the knee to join the lamina terminalis, the 



beak. 



II. The fornix. 



1. The crura come from the floor of the lateral ventricles and bend down in 



the lamina terminalis. 



2. The greater portion of that seen is the body of the fornix composed of fibers 



extending between the two crura. 



3. Note that the body of the fornix behind fuses with the splenium of the 



corpus callosum. 



III. Across the triangular area inclosed by the callosum and the fornix stretches 



a thin membrane — the transparent septum — which separates the two lateral 

 ventricles. 

 1. The septum is really double, i. e., consists of 2 parallel meiAbranes. The 

 space between the two is termed the V. ventricle. 



r 



In one half-brain, cut away the roof down to and i^arallel with the 

 upper surface of the corpus callosum. In doing this you will jDrobably 

 have removed a portion of the lateral ventricle. Eemove the remainder 

 of the roof of the ventricle emitting at first forward and taking care not 

 to cut away the corpus callosum. You will thus expose the body of the 

 ventricle and an anterior prolongation of it which extends forward and 

 downward, the anterior liorn. 



Now cutting backward you will find that tlie ventricle soon turns and 

 extends outward and then downward and finally forward to the tip 



