752 THE NERVOUS SYSTEM. 



They are covered with epithelium, and are derived from the lower layer of the 

 velum interpositum. Of the arteries of the velum interposituru, some branches 

 from the superior cerebellar and posterior cerebral enter from behind beneath the 

 corpus callosum. The veins of the velum interpositum, the vence Galeni, two in 

 number, run between its layers ; they are formed by the venae corporis striati and 

 the veins of the choroid plexuses ; the venae Galeni unite posteriorly into a single 

 trunk, which terminates in the straight sinus (Fig. 383). 



Openings. The third ventricle has four openings connected with it. In front 

 are the two foramina of Monro, one on each side, through which the third com- 

 municates with the lateral ventricles. Behind is a third opening, that of the 

 aqueduct of Sylvius, or iter a tertio ad quartum ventriculum. The fourth, sit- 

 uated in the anterior part of the floor of the ventricle, is a deep pit, which leads 

 downward to the funnel-shaped cavity of the infundibulum (iter ad infundibulum). 

 A fifth opening exists in the foetus which communicates behind with the cavity 

 in the pineal gland. 



The lining membrane of the lateral ventricles is continued through the 

 foramen of Monro into the third ventricle, and extends along the iter a tertio 

 (aqueduct of Sylvius) into the fourth ventricle ; at the bottom of the iter ad 

 infundibulum it ends in a cul-de-sac. 



The Optic Tracts (Figs. 415, 443). These are two well-marked flattened bun- 

 dles of fibres which lie along the upper parts of the crustae. They are attached 

 only by their upper edges, which also serve to mark the transition between upper 

 limit of crusta and internal capsule of hemisphere. These edges also mark the 

 limit of separation, without tearing, between the temporo-sphenoidal lobes of the 

 hemispheres, which, at the base of the brain, overlap the optic tracts and the 

 crustae. Each tract was originally a hollow outgrowth (optic vesicle) from the 

 back part of the fore-brain. Anteriorly each unites with the other to form the 

 optic commissure. The fibres of each are described in connection with the optic 

 nerve (which see). 



THE HEMISPHERES. 

 General Considerations and Development. 



The two hemispheres are by far the largest portion of the encephalon, each 

 one in bulk exceeding somewhat all the other divisions of the brain. Together 

 with the fore part of the third ventricle they form what is called by some writers 

 the prosencephalon or fore-brain. 



Each hemisphere is an enormously developed " hemisphere vesicle " whose 

 cavity is the lateral ventricle, and whose walls, originally smooth, thin, and 

 spherical, are convoluted, elongated in various directions, arid, for the most part, 

 exceedingly thick. Although the two hemispheres in the adult brain are con- 

 nected with each other by means of the corpus callosum and anterior commissure, 

 this connection is merely between the adjacent walls, and in no wise involves the 

 cavities, each cavity being as distinct from the opposite one as it is in early foetal 

 life before the intermural connection is established. Each lateral ventricle is, 

 therefore, a complete cavity, communicating only with the third ventricle through 

 the corresponding foramen of Monro. 



The development of each hemisphere vesicle may be described approximately 

 as follows (Figs. 412, 413, 414) : After becoming a rounded hollow projection 

 from the side of the prosencephalon, each hemisphere vesicle expands in an 

 anterior direction and approaches close to its fellow. At the same time it grows 

 upward over the inter-brain and backward along its side, while from this latter 

 portion two projections may be said to take place one still farther backward, 

 covering over the dorsal surface of the mid-brain and cerebellum ; and the other, 

 downward and forward, overlapping somewhat the external surface of the portion 

 from which it is derived, until its lower end projects below, and also overlaps the 

 ventral surface of the mid-brain (crustae). We can thus distinguish four main 

 divisions of the developing hemisphere vesicle : an anterior, a superior, the latter 



