132 THE CEREBRUM. 



-Posterior perforated substance (of mid-brain), 



Tegmenta (of mid-brain). 

 Posteriorly 



Ventricle is continuous with cerebral aqueduct. 

 Lateral walls 



Thalamus, and reflected hypothalamic substance, 



Columna of the fornix, and 



Foramen interventriculare between them. 

 Roof. A band of white fibers passes across the back part of 

 the third ventricle and supports the posterior end of the roof 

 epithelium. That band is the posterior commissure (commissure, 

 posterior, Figs. 28 and 40). It crosses immediately in front of 

 the corpora quadrigemina. Beneath it is the anterior orifice of 

 the cerebral aqueduct. The pineal body is above and behind it, 

 and the commissure fuses with the ventral pineal lamina. The 

 posterior commissure stretches from the central gray substance 

 of the mid-brain on one side, over the aqueduct, to the gray sub- 

 stance of the opposite side and also contains decussating fibers 

 of the medial longitudinal bundle (Heald). The commissure 

 is in need of further investigation. 



The roof epithelium (Figs. 42 and 37) of the third ventricle 

 stretches from the posterior commissure to the anterior commissure, 

 and laterally, is attached to the upper internal border of the thal- 

 amus. It is the superficial layer of the ependyma; but it is, here, 

 the only adult representative of the roof of the diencephalon. 

 The roof epithelium presents two longitudinal folds suspended 

 in the ventricle. The lower layer of the chorioid tela of the third 

 ventricle, invests the roof epithelium superiorly; and, dipping down 

 into the longitudinal folds, that inferior layer forms the chorioid 

 plexuses of the third ventricle, which are continuous with those 

 of the lateral ventricles through the interventricular foramina. 

 At the back part in the middle line, there is a pouch-like evagina- 

 tion of the roof of the diencephalon in the embryo, which develops 

 into the pineal body; and there remains a slight pit, called the 

 pineal recess, in the adult condition. A second evagination occurs 

 just above the pineal recess, which forms the epipineal recess. 

 Pineal B'ody. (Corpus pineale, Figs. 40, 43 and 76). It is a 



