THE NERVOUS SYSTEM I4»7 



afis, which extends upwards from the optic commissure to the ros- 

 trum of the corpus callosum, and superiorly by the anterior pillars 

 of the fornix and the central portion of the anterior commissure. 



The posterior boundary is formed by the pineal body and the 

 posterior commissure, and under cover of the latter is the upper 

 opening of the aqueduct of Sylvius. The posterior boundary pre- 

 sents two recesses, pineal and suprapineal. The pineal recess 

 passes backwards above the posterior conmiissure into the 

 stalk of the pineal body, separating the stalk into two portions, 

 dorsal and ventral. The suprapineal recess is connected with 

 the back part of the epithelial roof of the ventricle, and passes 

 backwards over the pineal body. The third ventricle has thus 

 four diverticula, namely, the optic recess, the infundibular recess, 

 (both of which recesses are associated with the floor), the pineal 

 recess, and the suprapineal recess. The cavity communicates 

 with the fourth ventricle by means of the aqueduct of Sylvius, 

 and with the two lateral ventricles by means of the foramina of 

 Monro. 



Foramina of Monro. — ^These are two in number, right and left, 

 and they lead from the lateral ventricles into the third ventricle. 

 Each foramen is situated between the anterior pillar of the fornix 

 in front and the anterior tubercle of the optic thalamus behind. 

 From this point the foramen of each side leads inwards and slightly 

 downwards, and opens into the third ventricle at the anterior 

 and upper part of the corresponding lateral wall. By means of the 

 foramina of Monro the lateral ventricles communicate with the 

 third ventricle, and, through that ventricle, with each other. The 

 choroid plexuses of the lateral ventricles also become continuous 

 with each other and with those of the third ventricle through these 

 foramina. From each foramen a furrow, called the sulcus of 

 Monro, leads, as stated, in a backward direction, over the inner 

 surface of the optic thalamus towards the upper opening of the 

 aqueduct of Syhaus. 



Development. — The posterior and greater part of the third ventricle repre- 

 sents the cavity of the thalamencephalon or diencephalon ; and the antericH" 

 part, in the region of the foramina of Munro, represents the mesial portion 

 of the cavity of the telencephalon. 



The foramen of Monro, of each side, represents the original wide communi- 

 cation between the cavity of the cerebral vesicle and the cavity of the 

 mesial portion of the telencephalon. 



Optic Thalami. — ^The optic thalami are two large ovoid masses 

 of grey matter which He obliquely, with their long axes directed 

 backwards and outwards, for the most part on the sides of the third 

 ventricle. Their anterior extremities are near each other, but their 

 posterior extremities stand apart, the corpora quadrigemina being 

 situated between them. Over their anterior two-thirds they are 

 separated from each other by the third ventricle. 



The optic thalamus presents four surfaces — superior, inferior, ex- 

 ternal, and internal ; and two extremities — anterior and posterior. 



The superior surface is limited externally by an oblique groove. 



