372 NERVOUS SYSTEM. 



canal formed in the latter. In order to see this arrangement, a 

 part of the corpus striatum must be removed. The anterior 

 commissure resembles a nerve in its structure, as it is surround- 

 ed by a very delicate sheath, and is divided into fasciculi of 

 fibres. It will now be understood that three commissures are 

 found in the third ventricle, the Anterior Commissure the Pos- 

 terior, which is just in front of the Pineal Gland,* and the Soft 

 Commissure, being a cineritfc>us adhesion of the Thalami.f 



Just behind and below the anterior commissure, the base of 

 the infundibulum opens into the third ventricle; this place is the 

 Iter ad Infundibulum. At the posterior extremity of the third 

 ventricle, just below the posterior commissure, which has been 

 described as a process of the pineal gland, the communication 

 exists with the fourth ventricle. This passage is the Aqueduct 

 of Sylvius, and leads obliquely downwards and backwards un- 

 der the valve of the brain. 



The third ventricle communicates freely with the lateral ven- 

 tricle through the aperture called the Foramen of Monro, which 

 is situated precisely at the place where the plexus choroides ter- 

 minates; that is to say, under the anterior crus of the fornix. 

 Doubts have, from time to time, been suggested in regard to the 

 natural existence of this communication; it only requires a mo- 

 derate degree of accurate observation to dispel them: they have 

 arisen, probably, from the aperture being shut up by the occa- 

 sional adhesion of the plexus choroides to the contiguous sur- 

 face of the brain. 



The Fourth Ventricle (Ventriculus Quartus, Cerebelli) has 

 been, in a great degree, described in the account of the neigh- 

 bouring parts; it will, therefore, be very readily understood on 

 the present occasion. It is an irregular triangular cavity, the 

 base of which is downwards. It is bounded in front by the tuber 

 annulare, and the medulla oblongata, behind by the fundamental 

 portion of the cerebellum, and above by the valve of the brain 

 and the tubercula quadrigemina ; it is under the latter that the 

 communication between it and the third ventricle is found. Its 

 lateral parietes are formed by the medullary prolongations from 

 the cerebellum to the tubercula quadrigemina. This caviiy, as 



* See Pineal Gland. t See Thalami. 



