748 THE NERVOUS SYSTEM. 



across the middle line with the inner nucleus of the opposite side by the middle 

 commissure of the third ventricle. The outer nucleus is continued into the 

 pulvinar. 



There are two other smaller nuclei in the optic thalamus one the nucleus of 

 the anterior tubercle, and the other, just beneath the trigonum habenulce (see 

 below), the ganglion of the habenula. There are also two bundles of fibres in 

 addition to those just described. One of these runs through the anterior part of 

 the optic thalamus. It is the anterior pillar of the fornix, and will be again 

 referred to, as will also the other, much smaller, the bundle of Vicq d' Azyr, whose 

 fibres run downward from their origin in the cells of the nucleus of the anterior 

 tubercle, just mentioned. 



The third ventricle (Figs. 442, 447, 451) is the fissure placed between the optic 

 thalami and extending to the base of the brain. It is bounded, above, by the 

 posterior commissure and the under surface of the velum interpositum, lined with 

 epithelium, from which are suspended the choroid plexuses of the third ventricle. 

 Its floor, somewhat oblique in its direction, -is formed, from before backward, by 

 the lamina cinerea, the tuber cinereum and infundibulum, the locus perforatus 

 posticus (posterior perforated lamina) and behind these by the tegmentum of the 

 mid-brain ; its sides are formed by the internal surfaces of the optic thalami. It 

 is bounded, in front, by the lamina cinerea, while the extreme upper part of its 

 " anterior boundary " is a layer of epithelium covering, posteriorly, and through 

 which are seen, from within the ventricle, the anterior pillars of the fornix and 

 middle part of the anterior commissure. 



These last-named structures belong to the hemispheres, and the epithelium 

 covering them posteriorly is the same layer as that which lies in contact with the 

 lamina cinerea, which itself, on reaching the anterior commissure, passes in front 

 of it, and is continuous with the corpus callosum of the hemispheres. This extreme 

 upper part of the lamina cinerea is often called the lamina terminalis. It is the 

 representative in the adult brain of the anterior end of the primary fore-brain, 

 around and in front of which have grown the anterior parts of the hemisphere 

 vesicles to form the frontal lobes of the hemispheres. 



The lateral extension of this epithelial layer is through the corresponding fora- 

 men of Monro, which lies just behind each anterior pillar of the fornix. 



The various structures which enter into the formation of the third ventricle 

 will now be described more in detail, beginning with those of the ROOF (Fig. 442). 



The posterior commissure is a distinct rounded bundle of white fibres running 

 transversely just above the opening of the Sylvian aqueduct. The pineal gland 

 is placed above it and connected to its upper surface. It is made up of: (1) the 

 combined upper ends of the two posterior longitudinal bundles (see tegmentum of 

 mid-brain) as each bends to the opposite side in order to pass through the opposite 

 optic thalamus and reach the white substance of the hemisphere ; (2) commissural 

 fibres between the optic thalami ; (3) fibres from one anterior corpus quadrigem- 

 inum to the opposite upper fillet. 



The pineal gland (epiphysis cerebri], so named from its peculiar shape (pinus, 

 a fir-cone), is a small reddish-gray body, conical in form (hence its synonym, 

 conarium), placed immediately behind the posterior commissure and between the 

 anterior corpora quadrigemina. It is retained in its position by a duplicature of 

 pia mater derived from the under layer of the velum interpositum, which almost 

 completely invests it. The pineal gland is about four lines in length and from 

 two to three in width at its base, and is said to be larger in the child than in the 

 adult, and in the female than in the male. Its base is connected to the optic 

 thalami by a stalk, which consists of two laminre, an upper and a lower, the inter- 

 vening space, apex toward the gland, being known as the pineal recess. The 

 lower or ventral lamina is derived from the lower aspect of the posterior commis- 

 sure, and is reflected upward and backward to meet the upper or dorsal layer at 

 the base of the gland. The dorsal lamina is the direct continuation backward of 

 the epithelial roof of the third ventricle. When this is torn away the dorsal 



