DISSECTION OF THE BRAIN, OR ENCEPHALON. 259 



The 3rd ventricle has a ciliated lining continuous with that of the 

 4th through the aqueduct of Sylvius, and with that of the lateral 

 ventricles through the foramen of Monro. In the foetus the cavity 

 communicates through the tuber cinereum and infundibulum with the 

 pituitary body. 



The Foramen of Monro, or Foramen Commune Anterius, is the 

 common point of communication between the 3rd and lateral ventricles. 

 It might be described as a short vertical shaft ascending from the fore- 

 part of the 3rd ventricle, and opening under the fornix, which is thrown 

 over it like an arch. Beneath this arch the lateral ventricles communi- 

 cate with one another and with the 3rd ventricle. 



The Corpora Quadrigemina are two pairs of bodies superposed to the 

 crura cerebri behind the optic thalami. The anterior pair of bodies, or 

 nates, are larger than the posterior pair, or testes, from which they are 

 separated by a groove. Between the right and left nates there is a welJ- 

 defined groove, but the groove between the testes is faint or not observ- 

 able. The nates are grey on their surface, but the testes are white. 

 The bodies were named nates and testes from a fancied resemblance 

 to the hips and testicles of a man, but these terms are far from express- 

 ing the relative size of the two bodies. 



The Aqueduct of Sylvius, or Iter, is a tunnel which, commencing 

 posteriorly in the 4th ventricle, beneath the valve of Vieussens, 

 extends forwards beneath the corpora quadrigemina, and opens into the 

 hinder part of the 3rd ventricle. It possesses a ciliated lining con- 

 tinuous with that of the ventricles which it connects. 



Optic Tracts and Corpora Geniculata. — The optic tracts have 

 already been seen at the base of the brain, where they form the 

 anterior boundaries of the interpeduncular space. When followed 

 backwards, each tract will be found to turn round the crus cerebri and 

 join the optic thalamus. At the point of junction two eminences are 

 placed, an outer, or anterior, and an inner, or posterior. These are 

 named respectively the corpus geniculatum externum and internum. 

 They are composed of grey matter from which some fibres of the optic 

 tract pass. Other fibres of the tract come directly from the optic 

 thalamus, and still others from the corpora quadrigemina. 



THE CRANIAL OR ENCEPHALIC NERVES (PLATE 33). 



In the examination of the base of the brain the roots of the cranial 

 nerves have already been noticed, but it will be advantageous to 

 describe them here as a series. The cranial nerves are distinguished 

 by special names, and also by numerical designations. It must be 

 observed, however, at the outset that there are two different systems 

 of enumeration in use amongst anatomists, the first of which recognises 



