THE CEREBRUM. 371 



The Plexus Choroides, which was stated to bound the Velum 

 Interpositum on each side, and to descend along the Hippocam- 

 pus Major to the fissure of Sylvius, or rather to ascend from 

 this point, and to terminate in the vein on the side of the'jhid- 

 dle line of the Velum, is narrow at its termination, but increases 

 continually in breadth as it is traced towards its commencement. 

 The middle part, however, where it makes Us turn, is an excep- 

 tion to this rule, for there it is larger in every way than else- 

 where: its vessels being more capacious and more tortuous. Pre- 

 cisely at this point a vesicle or more is very frequently found, 

 considered by some as a hydatid of the brain; in some cases it 

 is filled with calcarious matter instead of with water. The 

 Glandulae Pacchioni, as stated, also prevail at this margin. 



On the under surface of the Velum Interpositum, adhering to 

 it, there is on each side a small venous plexus which goes from 

 before backwards, and terminates in the vena galeni, near its junc- 

 tion with its fellow. It receives the blood of the third ventri- 

 cle. 



There is also the same sort of plexus in the fourth ventricle. 



Upon the removal of the Velum Interpositum, or its elevation, 

 the whole upper surface of the thalami optici is exposed. The 

 third ventricle is also brought into view, being placed imme- 

 diately between the thalami optici. It is a narrow oblong cavi- 

 ty, bounded below by the pons tarini, crura cerebri and the 

 eminentise mammillares, and above by the velum interposi- 

 tum, and the fornix. The anterior crura of the fornix are at its 

 fore part, and just before them is the anterior commissure (Com- 

 missura Anterior.) This body is a transverse fasciculus of 

 medullary matter, which passes from one hemisphere to the 

 other through the anterior margins of the thalami optici. Its 

 middle part is rounded and free, but its extremity penetrates 

 on each side into the substance of the anterior inferior portion 

 of the corpus striatum, and spreading out gradually, describes 

 a curve with its convexity forwards, which terminates in the 

 Tsenia Hippocampi of the inferior cornu of the lateral ventricle. 

 This fasciculus, in penetrating the corpus striatum, does pot mix 

 with its substance, but, in the early part of its course, goes in a 



suchfan arrangement would be contradictory to that of the tunica arachnoidea 

 on the surface of the brain, \vhich never dips into fissures. 



