128 THE CEREBRUM. 



It is a narrow, vertical cleft about an inch in length from before 

 backward and a quarter of an inch broad at its widest part It 

 separates the thalami, and extends almost to the inferior surface 

 of the cerebrum. The roof (Figs. 29, 36, 37, 40 and 42) follows the 

 curve of the fornix and arches from the posterior commissure 

 forward to the anterior commissure. There is a little recess 

 above the anterior commissure and between the columnae of the 

 fornix, bounded in front by the inferior angle of the septum pel- 

 lucidum, called the recessus triangularis, in which the roof and 

 anterior wall meet. The anterior wall extends from the trian- 

 gular recess down to the optic recess, at the angle between the 

 lamina cinerea and the optic chiasma. This angle is so named 

 because, on either side of it, there is a lateral extension of the 

 third ventricle between the lamina terminalis and the columna 

 of the fornix, which is located in the root of the embryonic optic 

 vesicle. The floor (Fig. 28) describes two arches, convex toward 

 the ventricle. The first arch, very convex and short, stretches 

 between the optic recess and the infundibulum, in which the 

 floor reaches its lowest point. The distance from the infundib- 

 ulum to the anterior orifice of the cerebral aqueduct is spanned 

 by the second arch. It is long and flat. Its posterior extremity 

 is but a sixteenth of an inch below the posterior commissure; the 

 anterior orifice of the cerebral aqueduct separates them. The 

 ventricle is thus contracted behind to the size of the cerebral 

 aqueduct, with which it is continuous. The lateral walls (Figs. 

 28 and 29) are close together throughout. At one point near the 

 middle they come together and are joined by the massa inter- 

 media (middle commissure). Antero-superiorly, the lateral wall 

 is perforated by the interventricular foramen (of Monro). That 

 foramen constitutes the slight separation between the front of the 

 thalamus and the columna of the fornix. It opens into the lateral 

 ventricle at the junction of the anterior horn with the central part. 

 The ependyma which lines the third ventricle is continuous through 

 the interventricular foramen with the lining of the lateral ventricle. 

 But one layer of the ependyma is present in the roof of the ven- 

 tricle; that is the epithelial layer. The third ventricle, like all 

 true ventricles, is occupied by cerebro-spinal fluid. 



