604 ELEMENTARY PHYSIOLOGY less. 



as an opening into the hinder end of the cavity of the 

 fourth ventricle, while the front end of the cavity ia 

 prolonged into a narrow passage, the aqueduct of 

 Sylvius, which leads into a much larger cavity 

 known as the third ventricle, and marked by a 3. 

 Above this aijueduct are four largely developed masses of 

 tissue, but of these tiL-a only are seen in the section at QA , 

 QF, since the four are arranged in two pairs, one pair 

 being placed each side of the middle line of the brain ; 

 from their number (four) these structui'es have received 

 the name of corpora quadrigemina. In front of the 

 corpora quadrigemina is a small structure, seen in section, 

 the pineal gland, P. The posterior corpus quadri- 

 geminum is continuous with a thin layer of nervous 

 tissue, which leads back into the cerebellum ; this 

 forms an overhanging roof to the front end of the fourth 

 ventricle, and is known as the valve of Vieussens 

 (Fig. 161, V.V.). The floor of the third ventricle is 

 produced forwards and downwards into a funnel-shaped 

 space, to the tip of which is attached a body of a glandular 

 nature known as the pituitary body (Fig. 160, P, and 

 Fig. 161, //). The roof of the third ventricle is provided 

 by a layer of tissue seen in section and known as the 

 fornix (Fig. 161, F) ; this is connected posteriorly with 

 the hinder end of the corpus callosum, and in front it 

 curves downwai'ds and backwards into the lateral wall (A the 

 third ventricle towards the corpus albicans, M. The 

 vertical space between the fornix and the corpus callosum is 

 filled in by a thin donhle layer of nervous tissue ; this is 

 known as the septum lucidum. It lies in the plane of 

 the paper on which the figure is printed, but only a small 

 portion of it is shown at SL. The remaining part has been 

 cut away in order to reveal a feature of which, so far, no 

 mention has been made, viz., the darkly shaded cavity AC, 

 LV., lying in the middle of the cerebral hemisphere, and 

 known as the (riglit) lateral ventricle. The cavity of 

 this ventricle communicates with that of the third ventricle 

 by a small opening at x, the foramen of Monro, Since 



