618 



THE NERVOUS SYSTEM. 



The third ventricle communicates with both of the lateral ventricles, and also 

 with the fourth ventricle. The aquseductus cerebri, the narrow channel which 

 tunnels the mesencepbalon, brings it into communication with the fourth ventricle. 

 The opening of this aqueduct is placed at the posterior part of the floor of the third 

 ventricle, immediately below the posterior commissure. The foramina inter- 

 ventricularia bring it into communication with the lateral ventricles. These 

 apertures are placed at the upper and anterior parts of the side walls, and lead 

 laterally and slightly upwards between the most prominent parts of the columns 

 of the fornix and the anterior tubercles of the thalami. They are just large 

 enough to admit a crow-quill, and through these passages the epithelial lining of 

 the three ventricles becomes continuous. From the foramen a distinct groove on 





--OPTIC RECESS 



^INFUNDIBULAR 

 RECESS 



'FOURTH 

 VENTRICLE 



FIG. 545. PROFILE VIEW OF A CAST OF THE VENTRICLES OF THE BRAIN (from Retzius). 

 This figure faces in the direction opposite to that of Fig. 544. 



R.SP. Recessus suprapinealis. 

 R.P. Recessus pinealis. 



A.S. Aqiueductus cerebri. 

 F.M. Foramen interventriculare. 



the side wall of the ventricle leads backwards towards the mouth of th 

 aqueduct. It is termed the sulcus hypothalamicus, and is of interest, inasmuch t 

 it is considered by His to represent in the adult brain the furrow which dividii 

 the side wall of the embryonic brain-tube into an alar and a basal lamina. 



The outline of the third ventricle, when viewed from the side in a median secti( 

 through the brain (Fig. 544), or as it is exhibited in a plaster cast of the ventricular syste 

 of the brain (Fig. 545), is seen to be very irregular. It presents several diverticula jj 

 recesses. Thus, in the anterior part of the floor there is a funnel-shaped pit or recess, leadrii 

 down through the tuber cinereum into the infundibulum of the hypophysis. Anotb * 

 recess, the recessus opticus, leads forwards immediately in front of this, above the op*': 

 chiasma. Posteriorly two diverticula are present. One, the recessus pinealis, passes ba(| 

 wards above the posterior commissure and the mouth of the aquseductus cerebri foi 

 short distance into the stalk of the pineal body. The second is placed above this and < 

 carried backwards for a greater distance. It is a diverticulum of the epithelial roof, ai 

 therefore, is difficult to demonstrate. It is termed the recessus suprapinealis. 



