364 



Terse and longitudinal fibres are much more numerous. Those which 

 extend transversely from side to side decussate in the middle line, 

 and, through the under half of the lobes, form a dense layer of sepa- 

 rate bundles ; between these bundles, and at right angles to them, 

 the other set of transverse fibres radiate from the arched transparent 

 substance beneath, and after penetrating to various depths, turn 

 round to become continuous partly with them and partly with the 

 longitudinal fibres which run chiefly along the superficial half of the 

 lobes. The same system of latero- transverse bundles is continued 

 in front of the nates to form the posterior commissure. A large 

 proportion of the optic nerve is connected with the nates. The 

 optic tract, on reaching the back of the thalamus opticus, divides into 

 two parts : one of these, after bending suddenly downwards and 

 somewhat inwards, enters the corresponding lobe of the nates, spread- 

 ing amongst its cells, and forming its longitudinal as well as some 

 of its transverse fibres ; the other division of the optic tract winds 

 inwards and somewhat backwards over the thalamus, which it enters, 

 diverging, between streaks of longitudinal fasciculi, in a series of 

 separate bundles, which, in their turn, diverge and distribute their 

 fibres in every direction amongst a dense collection of cells. The 

 cells are various in size and shape ; a considerable proportion are 

 round, oval, or fusiform in the direction of the bundles which first 

 enter. Some of the fibres of the optic tract are connected with the 

 corpus geniculatum internum, which is crowded with cells of a me- 

 dium or rather small size, and for the most part round or oval. 



Structure of the Pineal Gland. The structure and relations of 

 this body are of great interest, and deserve close attention. It is 

 attached by its base to the posterior commissure. The arched trans- 

 parent lamina which forms the roof of the canal, or iter, beneath the 

 corpora quadrigemina, decreases in thickness or depth as it approaches 

 the anterior extremity of the canal, beyond which it is reduced to 

 the epithelial layer which lined it. This epithelium is now re- 

 flected round the front of the posterior commissure, and from thence 

 under the base of the pineal gland. When carefully examined, it is 

 found, along its border, to be composed of narrow fusiform cells, in- 

 termixed with round and oval granular nuclei, of which the average 

 diameter is about the 2800th of an inch. From both kinds of 

 these cells or nuclei fibres proceed and cross each other in every 



