THE BRAIX AND ITS MKMltHAXES. 749 



lamina necessarily has a free edge looking forward. On each side this lamina is 

 prolonged into a somewhat triangular-shaped, depressed area on the upper surface 

 of the optic thalaums. known as the trigonum habenulce, because the word "ha- 

 benula" (bridle) is often applied to this dorsal lamina of the stalk. The free edge 

 of the lamina is also prolonged as a delicate whitish band on to the optic thalamus, 

 on which it runs forward along the sharp margin which separates the superior 

 from the internal surface of the thalamus. These bands are the pineal strice or 

 peduncles of the pineal gland. Anteriorly each blends with the corresponding 

 anterior pillar of the fornix. 



Structure. The pineal gland consists of a number of follicles lined by epithe- 

 lium and connected together by ingrowths of connective tissue. The follicles 

 contain a transparent viscid fluid, and a quantity of sabulous matter, named 



'ulus cerebri, composed of phosphate and carbonate of lime, phosphate of 

 magnesia and ammonia, with a little animal matter. These concretions are 

 almost constant in their existence, and are found at all periods of life. At times 

 the sabulous matter is found upon its surface, and occasionally upon its peduncles. 

 In the foetal brain the pineal gland is a hollow protrusion from the posterior part 

 of the roof of the inter-brain. 



The Epithelial Roof. This stretches across the third ventricle from one pineal 

 to the other, and then is reflected downward to become continuous with the 

 epithelium covering the side of the ventricle (internal surfaces of thalami). The 

 epithelium is not continued on to that part of the superior surface of the thala- 

 mus which is adjacent to the pineal stria, and which is internal to the oblique 

 shallow groove, already referred to. which runs along this surface. External to 

 the groove, however, this upper surface is covered by epithelium, but this epithe- 

 lium belongs to the lateral ventricle. 



The roof epithelium of the third ventricle is continued anteriorly between the 

 pineal stria? until just before they join the down-curving anterior pillars of the 

 fornix. where it is interrupted by the foramen of Monro, around the margin of 

 which it passes into continuity with the epithelium, lining its own ventricle and 

 the lateral ventricle. 



The velum interposition (Fig. 458) is a vascular membrane, a prolongation 

 from the pia mater. It is of a triangular form, and separates the under surfaces 

 of the body of the fornix, posterior pillars of the fornix, and posterior part of 

 corpus callosum above (the last-named structures representing the Bunder surface 

 of the combined hemispheres"), from the cavity of the third ventricle and the 

 inner portions of the superior surfaces of the optic thalami below. Its anterior 

 extremity, or />./. is bifid, each bifurcation lying close to and behind the corre- 

 sponding anterior pillar of the fornix, and hence " in," but covered by epithelium, 

 the foramen of Monro. Its base lies beneath the posterior rounded border of the 

 corpus callosum above, and the optic thalami, the corpora quadrigemina, and 

 pineal gland below 



The velum interpositum is composed of two layers of pia mater, which sepa- 

 rate from each other at its base, the upper layer passing backward on the under 

 surface of the occipital lobes of the hemispheres, the lower layer passing down- 

 ward over the mid-brain, pons. and cerebellum. At the apex the two layers are 

 continuous with each other, as well as at the margins, which are free and lie along 

 and project a little beyond the groove, already referred to, which runs forward and 

 inward on the superior surface of the thalamus. Along this margin is the 

 ehoroid plexiu f t/><- lutiTul >, ///// .-1,\ which is covered by the mesial prolongation 

 of the layer of epithelium covering the outer portion of the upper surface of the 

 thalamus. After investing the margin of the velum interpositum the epithelium, 

 still prolonged mesially. is attached to the edge of the fornix, under which the 

 velum lies and beyond which it projects. As will be seen later, the fornix forms 

 part of the floor of this district of the lateral ventricle; hence the roughened 

 thickened margin (choroid plexus) of the velum interpositum really invaginates 

 into the lateral ventricle, that part of its floor which has become thinned out to a 



