THE MESENCEPHALON 507 



upper part of the longitudinal groove is occupied by the 

 pineal Body. The transverse groove curves round posterior 

 to each of the superior pair of quadrigeminal bodies and 

 separates them from the inferior pair. 



Brachia of the Corpora Quadrigemina. The corpora 

 quadrigemina are not marked off from the side of the mesen- 

 cephalon, for each body has in connection with it, on that 

 aspect, a prominent white strand, which is prolonged upwards 

 and anteriorly under the projecting pulvinar. The strands 

 are called the brachia of the corpora quadrigemina, and they 

 are separated from each other by a continuation, on the side 

 of the mesencephalon, of the transverse groove which inter- 

 venes between the two pairs of bodies. 



Corpus G-eniculatum ' Mediale. Closely connected with 

 the brachium of the inferior quadrigeminate body will be seen 

 the corpus geniculatum mediale. 



It is a little oval eminence, very PULVINAR CORP;GEN:LAT 



sharply denned, which lies on f X^ r-i-li^* 1 " R T ' 

 the side of the upper part of the 

 mesencephalon under shelter of 

 the pulvinar of the thalamus. ^^ * MEDIAL ROOT. 



Connections Of the Brachia 



and the Origin of the Optic 

 Tract/ It will now be seen that 

 the brachia are intimately con- 

 nected with the Optic tract. FIG. 209. Diagram of the Roots 

 ~, . ,, . , i . r of the Optic Tract. 



The inferior brachium proceed- 

 ing upwards from the lower quadrigeminal body advances 

 towards the corpus geniculatum mediale, and disappears 

 from view under cover of that prominence. Upon the 

 opposite side of the same geniculate body, the medial root 

 of origin of the optic tract arises, and the appearance is 

 such that the dissector might very naturally conclude that 

 the inferior brachium and the root of the optic tract are 

 continuous. That, however, is not the case. The superior 

 brachium is carried upwards and anteriorly between the 

 overhanging pulvinar and the corpus geniculatum mediale. 

 It partly enters the corpus geniculatum laterale, but a portion 

 of it is directly continuous with the lateral root of the optic 

 tract. 



The optic tract divides at its posterior end into a medial and a lateral 

 root. The medial root enters the corpus geniculatum mediale. The 



