704 



THE XFAIVOUS SYSTEM 



velum iutt'iposituui, and reflecting it backwards. This must be done cautiously, as the pineal 

 body is closely eiit'ulded in the posterior part of the under layer ol' that membrane, and an 

 attempt should be made to retain it in its proper place. The dissector should thoroughly 

 examine the cavity and floor of the third ventricle and the free surfaces of the optic thalami, 

 but he is recommended to dissect the deep surfaces of the latter bodies in connection with the 

 luesencejthalon. 



The Thalamencephalon 



The THIRD VENTRICLE is a naiTow cavity whose lateral walls are so closely 

 a})proxirnated that in I'orunal sections it appears as a vertical slit. It communi- 

 cates in front with the lateral ventricles by the foramina of Monro. Behind, it is 

 continuous with the fourth ventricle through a narrow passage, the aqueduct of 

 Sylvius. The opening into the aqueduct is bounded above by a transverse white 

 bundle of fibres, the posterior commissure ; and, above this, a prolongation of the 



Fig. 421. — Coronal Section through the Anterior Part of the Third Ventricle. 

 (From a mounted specimen in the Anatomical Department of Trinity College, Dublin.) 



COB PL'S CALLOSUM 



SEPTUM LUCID CM 



ASTERIOR 

 PILLAR-^ (IF'y 

 FOR MY 



T.e.VIA 

 <EMICIR- 

 rfLARIS 

 IPERCULrM 

 < I.XUSTRVit 

 PUT AMES 



i;i.OBus 



PALLIDVS 

 iS'TERIOR 

 COMMIS- 

 SURE 

 iMYGDA- 

 LOW 

 SUVLECS 



PITUITARY BODY 



ventricular cavity extends into the stalk of the pineal body, in which it ends 

 blindly. The third ventricle is covered by the corpus callosuni, the fornix, and the 

 velum interpositum; its true roof, however, is formed by the epithelium lining the 

 under surface of the last-named structure. The central joart of its cavity is crossed 

 transversely by the middle or grey commissure. It is bounded, laterally by the 

 optic thalami, and in front l)y the anterior pillars of the fornix and by the anterior 

 commissure. It is limited behind by the commencement of the aqueduct of 

 Sylvius, by the posterior commissure, and by the reflexion of epithelium from the 

 U]iper surface of the pineal body on to the under surface of the velum interpositum. 

 In the anterior part of the ventricle the floor is marked off from the lateral walls 

 by the anterior jjillars of the fornix, which, after forming part of the anterior 

 l)Oundary, sweep l)ackwards with an inclination downwards: m this situation they 

 are covered by a thin layer of the central grey sulistance of the ventricle, and 

 ]»roject as slight ridges. The third ventricle is relatively shallow for about the 

 posterior third of its extent, but in its anterior two-thirds it presents a deep recess 

 which is divided into an anterior and a posterior part by a prominent ridge. This 



