THE CEREBRUM 505 



ventricles, and also with the fourth ventricle. The aquce- 

 ductus cerebri (Sylvius], a narrow channel which tunnels the 

 mesencephalon, brings it into communication with the fourth 

 ventricle. The opening of this canal will be seen at the 

 posterior part of the floor of the ventricle, immediately below 

 the posterior commissure. The interventricular foramen, which 

 puts it into communication with the two lateral ventricles, is a 

 Y-shaped aperture which lies at the anterior part of the ventricle, 

 and its two diverging limbs pass laterally and slightly upwards, 

 between the most prominent parts of the columns of the fornix 

 and the anterior tubercles of the thalami. They are usually 

 large enough to admit a crow-quill, and through them the 

 epithelial lining of the three ventricles becomes continuous. 



FORAMEN INTERVENTRICULARE 



ASSA INTERMEDIA 



AQU/tDUCTUS 



CEREBRI 

 (HYPOPHYSIS 



FIG. 208. Diagrammatic outline of the Third Ventricle 

 as viewed from the side. 



From each lateral part of the interventricular foramen a 

 distinct groove passes posteriorly, on the lateral wall of the 

 ventricle, to the mouth of the aquseductus cerebri. It is 

 termed the sulcus hypothalamicus. 



The outline of the third ventricle is seen to be very irregular when 

 it is viewed from the side in a median section through the brain (Fig. 207), 

 or as it is exhibited in a plaster cast of the ventricular system of the brain. 

 It presents several diverticula or recesses. Thus, in the anterior part of the 

 floor there is a deep funnel-shaped recess, recessus infundib-uli, leading down, 

 through the tuber cinereum, into the infundibulum of the hypophysis. 

 Another recess, recessus opticus, lies above the optic chiasma. Posteriorly, 

 two additional recesses are present. One, the recessus pinealis, passes 

 posteriorly, above the posterior commissure and the entrance of the aqure- 

 ductus cerebri, for a short distance into the stalk of the pineal body. 

 The second is placed above this, and is carried posteriorly for a greater 

 distance. Its walls are epithelial, and therefore it cannot be seen in an 

 ordinary dissection. It is termed the recessus suprapinealis. 



Dissection. The further study of the cerebral hemispheres should be 

 postponed until after the examination of the mid-brain or mesencephalon. 

 The membranes should be removed from the upper surface of the cere- 

 bellum, and the prominent anterior part of that organ may then be 



