1432 A MANUAL OF ANATOMY 



from each other by a median raphe, which is continuous with that 

 of the pons VaroHi. In the lower part of the mesencephalon this 

 raphe is indistinct on account of the decussation which takes place 

 across the median plane between the superior cerebellar peduncles, 

 underneath the lower pair of quadrigeminal bodies. 



The dorsal surface of each tegmentum extends on either side of 

 the grey matter of the aqueduct of Sylvius, and becomes continuous 

 with the basal parts of the upper and lower quadrigeminal bodies 

 of the corresponding side. The ventral surface is separated from 

 the crusta by the substantia nigra. 



Structure of the Tegmentum. — Each tegmentum, as stated, is 

 continuous inferiorly with the formatio reticularis of the dorsal 

 portion of the pons Varolii. It consists of bundles of longitudinal 

 fibres, separated by bundles of curved fibres which pass trans- 

 versely. The intervals between these bundles are occupied by 

 grey matter. 



Grey Matter. — ^The grey matter of the tegmentum contains the 

 red nucleus. 



The red nucleus {nucleus ruber) is a round reddish mass, which is 

 situated in the centre of the upper part of the tegmentum, and it 

 lies in the path of the superior cerebellar peduncle of the opposite 

 side. It corresponds to the position of the upper quadrigeminal 

 body, and is prolonged upwards into the subthalamic tegmental 

 region. Some of the fibres of the superior cerebellar peduncle of 

 the opposite side surround the red nucleus in the form of a capsule 

 on their way to the optic thalamus. Other fibres of that peduncle 

 enter the red nucleus and terminate in arborizations around its 

 cells. 



The axons of the cells of the red nucleus form two sets of nerve- 

 fibres — ascending and descending. The ascending fibres pass to the 

 optic thalamus, in company with those fibres of the superior cere- 

 bellar peduncle which encapsule the red nucleus. These ascending 

 fibres form relays which carry on those fibres of the superior cere- 

 bellar peduncle which terminate within the red nucleus, that nucleus 

 being a cell-station in their path. The descending fibres constitute 

 the rubrospinal tract, or bundle of Monakow. The fibres of this 

 tract cross the median plane in the raph6, and by their decussation 

 with those of the opposite side they constitute the fountain-de- 

 cussation of Forel, in contradistinction to the fountain-decussation 

 of Meynert. The latter decussation is on a higher level, and in- 

 volves the fibres of the ventral longitudinal bundles, or tectospinal 

 tracts, which derive their fibres from the cells of the stratum opticum 

 and stratum lemnisci of the upper quadrigeminal bodies. The 

 rubro-spinal tract of either side descends through the pons Varolii 

 and medulla oblongata into the lateral column of the spinal cord, 

 where each constitutes the prepyramidal tract, which lies on the 

 ventro-lateral aspect of the crossed pyramidal tract. (The tecto- 

 spinal tract, or ventral longitudinal bundle, on either side, descends 

 into the anterior column of the spinal cord.) 



