224 THE NERVOUS SYSTEM 



lamina terminalis is to be regarded as forming the rostral end of the brain; 

 and the part of the third ventricle, which lies behind it and dorsal to the optic 

 chiasma, belongs to the telencephalon. The anterior commissure is a bundle of 

 fibers which crosses the median plane in the lamina terminalis and serves to 

 connect certain parts of the two cerebral hemispheres, which are associated with 

 the olfactory nerves. The anterior commissure and the lamina terminalis form 

 the rostral boundary of the third ventricle, and between the latter and the optic 

 chiasma is a diverticulum, known as the optic recess. 



The third ventricle is a narrow vertical cleft, the lateral walls of which are 

 formed for the greater part by the medial surfaces of the two thalami. Ventral 

 to the massa intermedia is seen a groove known as the hypothalamic sulcus, which 

 if followed rostrally leads to the interventricular foramen, while in the other 

 direction it can be traced to the cerebral aqueduct. Below this groove the 

 lateral wall and floor of the ventricle are formed by the hypothalamus. 



In the floor of the ventricle there may be enumerated the following structures, 

 beginning at the rostral end: the optic chiasma, infundibulum, tuber cinereum, 

 mammillary bodies, and the subthalamus. 



The roof of the third ventricle is formed by the thin layer of ependyma, which 

 is stretched between the striae medullares thalami of the two sides, and whose 

 torn edge, in the dissected specimen, is represented by the tania thalami (Figs. 

 85, 155, 159). Upon the outer surface of this ependymal roof is a fold of pia 

 mater in the transverse fissure. This is known as the tela chorioidea; and from 

 it delicate vascular folds are invaginated into the ventricle, carrying a layer of 

 ependyma before them by which they are, in reality, excluded from the cavity. 

 These folds are the chorioid plexuses. There are two of them extending side by 

 side from the interventricular foramina to the caudal extremity of the roof. 

 Here they extend into an evagination of the roof above the pineal body, known 

 as the suprapineal recess. 



There are three openings into the third ventricle. The aqueduct of the cere- 

 brum opens into it at the caudal end; while at the opposite extremity it com- 

 municates with the lateral ventricles through the two interventricular foramina. 



THE VISUAL APPARATUS 



Development of the Retina and Optic Nerve. There is but one pair of 

 nerves associated with the diencephalon, and these, the optic nerves, are not 

 true nerves, but fiber tracts joining the retinae with the brain. It will be re- 

 membered that the retina develops as an pagination of the lateral wall of the 

 prosencephalon in the form of a vesicle whose cavity is continuous with that of 



