226 Journal of CoMrARATixE Nei'holo(;y. 



quite narrow until the vagal tubers are passed, after which it 

 rapidly expands until at the exit of the ventral root of the 

 vagus it occupies the whole dorsal surface, not excepting the 

 lateral, lobes or cerebellar peduncles. Farther forward the 

 ventricle contracts into the aqueduct of Sylvius and passes 

 under the cerebellum, into which it sends a very narrow arm. 

 This cerebellar ventricle passes directly dorsad and does not 

 turn cephalad with the cerebellum. Immediately cephalad 

 of the volvula the aqueduct expands into the third ventricle, 

 which, at the same time, communicates laterally, by wide 

 openings, with the ventricles of the optic lobes near their 

 cephalic ends. The latter bodies are hollow throughout their 

 entire length, though the ventricle is much larger cephalad. 

 In this region the triangular ventral part of the third ventricle 

 is bridged over by the superior commissure, and from this 

 point forward severs its connection with the dorsal part. 

 The torus longitudinalis (fornix) very soon comes into contact 

 with the superior commissure, and thus separates the ven- 

 tricles of the two optic lobes again near their cephalic ends. 

 Beneath the superior commissure the third ventricle dips 

 suddenly ventrad in the form of a narrow cleft, reaching to 

 the surface. In its descent it sends two branches laterad into 

 the hypoaria, expands in the cinereum, and then sends an 

 arm caudad into the saccus vasculosus and another cephalad 

 into the infundibulum. The ventricle of the cinereum lies 

 in about the centre of that body. It is about one-third as 

 wide as the cinereum, but very thin dorso-ventrally. Ventrad 

 of this expansion the third ventricle gives oft' a very small 

 branch on each side (.5 mm. in diameter) which is lined with 

 very strong epithelium and extends caudad near the median 

 line between the hypoaria for some distance into the base of 

 the saccus vasculosus. The ventricles of the hypoaria are 

 crescent-shaped, diminishing caudad to sub-triangular and 

 oval. Just cephalad of the superior commissure the epiphysis 

 arises from the dorsal surface, as above described. Cephalad 

 of the dorsal sac the third ventricle passes into the cerebral 



