750 THE NERVOUS SYSTEM. 



layer of epithelium, and which stretches from the edge of the fornix outward over 

 the thalaraus (outer part of its superior surface) to the teenia semicircularis (a 

 structure forming part of the floor of the lateral ventricle), which lies along that 

 margin of the optic thalamus which separates its superior from its external surface. 



Tela Choroidea Superior. From the preceding it is clear that the under layer 

 of the velum interpositum has three districts a mesial and two lateral, the latter 

 resting on the upper surfaces of the thalami, the former on the " roof epithelium " 

 of the third ventricle, with which it forms practically one membrane. This mem- 

 brane is the tela choroidea superior, and is exactly similar to the tela choroidea 

 inferior of the fourth ventricle. 



The reason, on embryological grounds, for the existence of two layers of the 

 velum interpositum will be given in describing the choroid plexuses of the lateral 

 ventricles. 



Of the structures forming the FLOOR (Figs. 442, 446) of the third ventricle, the 

 tegmentum of the mid-brain has been described. The rest of the floor, including 

 the " anterior boundary," is a gray lamina prolonged from the substantia nigra, 

 and its ventral surface appears at the base of the brain, where, however, the 

 tegmentum cannot be seen (Fig. 473). Various portions of this lamina have 

 received diiferent names. Each of these will now be considered, beginning pos- 

 teriorly. 



The posterior perforated lamina succeeds the tegmentum. It is the anterior 

 part of that portion of the substantia nigra which appears in the interval between 

 the diverging crustae of the mid-brain on each side and the upper margin of the 

 tuber annulare of the pons Varolii posteriorly and below. Together with that 

 portion it is often called the posterior perforated space (pons Tarini). It reaches 

 forward as far as the mamillary tubercles, beyond which the gray lamina is known 

 as the tuber cinereum. 



The " space " is perforated by numerous small orifices for the passage of the 

 postero-median ganglionic branches of the posterior cerebral and posterior commu- 

 nicating arteries. 



The corpora albicantia, or mamillaria, or mamillary tubercles, are two small, 

 round, white masses, each about the size of a small pea, placed side by side imme- 

 diately behind the tuber cinereum. Each projects downward from the under 

 surface of the optic thalamus, the exceedingly narrow interval between them being 

 bridged over by a gray commissure continuous with the posterior perforated lamina 

 behind and the tuber cinereum in front. Each is composed externally of white 

 substance and internally of gray matter, the gray matter of the two being con- 

 nected by the transverse commissure of the same material just mentioned. 



The fibres of the white substance terminate in the cells of the gray matter, 

 and they are derived from two distinct bundles : one, deeply situated in the sub- 

 stance of the optic thalamus, is the bundle of Vicq d'Azyr, already mentioned ; 

 the other, much larger, is the anterior pillar of the fornix, which, after bending 

 sharply downward around the foramen of Monro, passes obliquely, downward and 

 backward, through the substance of the anterior portion of the thalamus, to ter- 

 minate in, and thus help to form, the corresponding corpus albicans or bulb of the 

 fornix. In its course through the thalamus it lies quite near the internal surface, 

 and may even cause a slight projection on the side of the third ventricle. 



The tuber cinereum is the next portion of the general lamina of the floor. 

 It is wider than the posterior perforated lamina, and blends laterally with the sub- 

 stance of the lower and anterior part of the thalamus, while antero-laterally, pass- 

 ing dorsal to the optic tract, it extends beyond the limits of the thalamus into the 

 gray matter of the anterior perforated space on the under surface of the hemi- 

 sphere. Anteriorly, it is attached to the posterior edge of the optic commissure. 

 From the middle of its under surface a conical tubular process of gray matter, 

 about two lines in length, is continued downward and forward, to be attached to 

 the posterior lobe of the pituitary body : this is the infundibulum. Its canal, 

 which is funnel-shaped, communicates with the third ventricle. 



