DISSECTION OF THE BRAIN, OR ENCEPHALON. 253 



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

 ing the rehxtive 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 Vieusseus, 

 extends forwards beneath the corpora quadrigemina, and opens into tlie 

 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 cms 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 externwn and internum. They are composed of 

 grey matter from which some fibres of the optic ti'act pass. Other 

 fibres of the tract come directly from the optic thalamus, and 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, how- 

 ever, at the outset that there are two difierent systems of enumeration 

 in use among anatomists, the first of which recognises twelve, and the 

 other nine, pairs of nerves. This diversity of nomenclature is apt to lead 

 to confusion, but fortunately this confusion does not extend to veterinary 

 anatomy, in which, both at home and abroad, the first and more natural 

 of these methods is exclusively employed. This system is also that 

 employed by human anatomists on the continent, but by British human 

 anatomists the number of cranial nerves is stated as nine pairs. The 

 following table exhibits in the central column the special names of 

 the nerves, and in the side columns their numerical designations under 

 the two systems : — 



1st pail- . . Olfactory nerves 1st pair. 



2iid ,, . . Optic nerves ...... 2nd ,, 



3rd ,, . . Oculo-motor nerves .... 3rd ,, 



4tli ,, . . Pathetic or Trochlear nerves . . 4th „ 



5th ,, . . Trifacial or Trigeminal nerves . . 5th ,, 



6th ,, . . Abducent nerves 6th ,, 



7th , , . . Facial nerves (Portio dura) . . . | 

 8th „ . . Auditory nerves (Portio mollis) . . ) " 



9th ,, . . Glosso-pharyngeal nerves . . . -, 

 10th ,, . . Pneumogastric or Vagus nerves . . K%i\x 

 11th ,, . . Spinal Accessory nerves . . . j 

 12th ,, . . Hypoglossal nerves .... 9th ,, 



The Olfactory or 1st nerve. The fibres of this nerve leave the surface 



