THE BASE OF THE BRAIN 329 



The taenia semicircularis lies in a groove between the caudate 

 nucleus and the optic thalamus. In front it joins the anterior 

 pillar of the fornix; behind it enters the nucleus amygdala?. 

 Beneath it is the vena corporis striati. 



The choroid plexus is a very vascular fringe covered with 

 epithelium continuous with that of the ependyma, and forms 

 the border of the velum interpositum. It extends from the 

 foramen of Monro, where it is continuous with the other, 

 across the floor of the body of the ventricle and into the middle 

 cornu. 



The corpus fimbriatum is the narrow band of white matter 

 on the hippocampus major into which is prolonged the posterior 

 pillar of the fornix. 



The hippocampus major is a curved white prominence in 

 the floor of the middle horn. Its lower part presents the appear- 

 ance of a paw from its grooves and eminences, hence called 

 the pes hippocampi. The hippocampus major is caused by 

 the dentate fissure, and the gray matter contained in this 

 fissure (dentate convolution) projects as a free margin the 

 fascia dentata. 



The eminentia collateralis (pes accessorius) is formed by the 

 collateral fissure. 



The great transverse fissure of Bichat, separating the cere- 

 brum and cerebellum, lies between the fornix and the splenium 

 of the corpus callosum above and the corpora quadrigemina 

 below; laterally, it lies between the back part of the optic 

 thalamus below and the corpus fimbriatum and fascia dentata 

 above: It is really a cleft into the ventricle, produced by 

 pulling out the pia forming the velum interpositum. 



The septum lucidum separates the lateral ventricles. It 

 is composed of two layers, a small space containing fluid being 

 left between them called the fifth ventricle. It is attached 

 above to the corpus callosum, below to the anterior pillars 

 of the fornix and the reflected portion of the corpus callosum. 



The Base of the Brain 



On the under surface of the encephalon are found the base 

 of the frontal, resting on the orbital plate of the frontal bone 

 and the temporal lobe, of which there are a middle and a pos- 



