THE NERVOUS SYSTEM 1423 



the olfactory lobe, and, it may be, from the anterior pillar of the 

 fornix. The axons of the ganglionic cells issue from the ventral 

 surface of the ganglion and form a bundle, called the fasciculus 

 retroflexus. This bundle passes downwards and forwards in the 

 tegmentum of the cms cerebri, l>'ing on the mesial side of the red 

 nucleus. Its fibres terminate in connection with the cells of the 

 ganglion inter pedunculare, which is situated in the lower part of the 

 locus perforatus posticus, directly above the pons Varolii. 



The ganglia habenulanun are connected with each other by 

 fibres which constitute the sitperior commissure or commissure of 

 the habenular ganglia. These nbres cross in the dorsal part of the 

 base of the pineal body, and are on a higher plane than the posterior 

 commissure. 



The striae medullares, or habenulae, and the ganglia habenulanun 

 are associated with the rhinencephalon or olfactory brain. 



Posterior Commissure. — ^This is a band of white fibres which is 

 situated at the back part of the third ventricle. It lies in the 

 posterior wall of the ventricle, directly above the upper opening 

 of the aqueduct of Sylvius, and underneath the base of the pineal 

 body. Its fibres are regarded as arising from a nucleus in the grey 

 matter of the lateral wall of the third ventricle, near the upper 

 opening of the aqueduct of Sylvius. Some of the fibres of either 

 side, after crossing, may descend in the tegmentum of the cms 

 cerebri, as part of the dorsal longitudinal bundle of that side, and 

 so reach the medulla oblongata. 



Hypothalamus. — ^The hypothalamus consists of two parts — 

 mammillary and optic. The pars mammillaris hypothalami repre- 

 sents the two corpora mammillaria. The pars optica hypothalami 

 includes the following structures: 



1. Tuber cinereum. 



2. Infundibulum. 



3. Posterior or cerebral lobe of the pituitary body. 



4. Optic commissure. 



5. Lamina terminalis. 



The corpora mammillaria are two small, white, pea-like bodies, 

 which lie side by side directly in front of the locus perforatus 

 posticus. Each is composed of white matter externally, and 

 of a grey nucleus internally. The white matter is derived from 

 the corresponding anterior pillar of the fornix, the fibres of which 

 terminate in connection with the cells of the grey nucleus. This 

 grey nucleus contains many cells, the axons of which give rise to 

 two fascicuh, namely, the bundle of Vicq d'Azyr, and the peduncle of 

 the corpus mammillare. The bundle of Vicq d'Azyr, which is appar- 

 ently a continuation of the anterior pillar of the fornix, enters the 

 optic thalamus, and its fibres terminate in connection with the cells 

 of the anterior nucleus. The peduncle of tlie corpus mammillare 

 passes downwards and backwards in the grey matter of the floor 

 of the third ventricle to the tegmental region of the mesencephalon, 

 but the mode of termination of its fibres is not known. 



