THE BRAIN 127 



The tegmentum consists of white longitudinal fibres and 

 interspersed grey matter, continuous with the formatio reticu- 

 laris of the medulla and pons, and passing above into the 

 optic thalamus. Two more or less definite white longitudinal 

 tracts are here passing upwards, the fillet and the superior 

 cerebellar peduncle, which latter decussates with its fellow 

 beneath the aqueduct. 



The corpora quadrigemina are four small bodies placed in 

 pairs, superior and inferior corpora quadrigemina. 



Relations : 



In front. Above. Below, 



3rd ventricle. Posterior border of Aqueduct of 



Posterior commissure. corpus callosum. Sylvius. 



The corpora quadrigemina of opposite sides are separated 

 by a longitudinal groove, which is connected below with the 

 superior medullary velum ; the superior corpora quadri- 

 gemina are partly concealed by the overlying pineal gland. 

 From the outer side of each corpus there proceeds a white 

 tract, the brachium. The brachia from the inferior corpora 

 go to the inner geniculate body, and the brachia from 

 the superior bodies pass into the optic tract between 

 the inner geniculate body on the one hand, and the 

 optic thalamus and the external geniculate body on the 

 other hand. 



Structure. The inferior bodies consist of a grey nucleus 

 enclosed in a thin capsule of white fibres. The white 

 fibres, separating the nuclei and the grey matter of the 

 aqueduct, are derived from the fillet of the olivary body. 

 The nuclei of each side are connected by commissural grey 

 ;Jiatter. 



The superior bodies consist of a grey nucleus intersected by 

 white longitudinal fibres. 



THE THALAMENCEPHALON. 



The third ventricle is the narrow space between the optic 

 thalami, extending from the opening of the Sylvian aqueduct 

 under the posterior commissur0, to the foramen of Monro. 

 It is much deeper in front than behind. 



The third ventricle communicates above by the Y-shaped 

 foramen of Monro with the lateral ventricles and behind with 

 the 4th ventricle by the aqueduct of Sylvius. 



