At the lower border of the interventricular foramen a small bundle of fibers leaves the columna of the fornix; this is rein forced by fibers from the anterior perforated substance and septum pellucidum and, bending backward, runs as medullary stria along the thalamus to the nucleus of the habenula; some of the fibers decussate through the stalk of the pineal body to the opposite nucleus habenulm and constitute the commissura habenularum. The columna of the fornix is joined by a small fasciculus from the intermediate stria of the olfactory tract, which runs backward to the hippocampus and uncus.
The upper surface of the body of the fornix is convex from before backward (Figs. 35 and 47). It forms the postero-medial part of the floor of the lateral ventricle. Along the median line it is joined to the corpus callosum by the septum pellucidum.
The septum pellucidum (Figs. 35, 46, 5o and 96), a double walled median partition, divides the superior chamber of the fore-brain cavity into lateral halves, the lateral ventricles. The septum pellucidum is crescentic in outline. Its convex border
fits into the concave surface of the body, genu and rostrum of the corpus callosum. Its concave border rests upon the fornix. Between the rostrum of the corpus callosum and the anterior commissure, the septum pellucidum is continuous with the gyms subcallosus with which it is associated in development and function.
The septum pellucidum, like the anterior commissure, corpus callosum and fornix, is developed from the thickened upper border of the lamina terminalis and the adjacent medial wall of the cerebral hemisphere in front of the interventicular foramen. These several structures extend upward and backward with the development and rotation of the hemispheres and together roof over the inter-brain. A lymph space, the cavum septi pellucidi, appears in the septum and is commonly called the fifth ventricle. The fore-brain cavity thus embraces four ventricles, viz.: Two lateral ventricles (the ventricles of the hemispheres), Cavum septi pellucidi (the ventricle of the septum), and Third ventricle (ventricle of the inter-brain).