THE NERVOUS SYSTEM 1433 



White Matter of the Tegmentum. — ^The principal tracts of the 

 white matter on either side are as follows : 



1. Superior cerebellar peduncle. 



2. Dorsal longitudinal bundle. 



3. Ventral longitudinal bundle, or tecto-spinal tract. 



4. Rubro-spinal tract. 



5. Mesial fillet (chief sensory tract). 



6. Lateral (acoustic) fillet. 



Superior Cerebellar Peduncle. — ^The fibres of this peduncle emerge, 

 for the most part, through the hilum of the nucleus dentatus in the 

 cerebellar hemisphere. The two peduncles, right and left, having 

 emerged from the hemispheres, pass upwards on the lateral parts 

 of the dorsal surface of the pons Varolii in a converging manner 

 towards the lower pair of the quadrigeminal bodies, being connected 

 by the valve of Vieussens . On entering the mesencephalon, the two 

 peduncles decussate across the raphe beneath the lower quadri- 

 geminal bodies. This decussation extends as high as the upper 

 quadrigeminal bodies, and it involves almost all the fibres of the 

 two peduncles. Each peduncle, having gained the opposite side, 

 ascends in the upper part of the tegmentum as a longitrudinal tract, 

 and soon comes into contact with the red nucleus. Many of its 

 fibres enter this nucleus and terminate in aborizations around its 

 cells. Other fibres of the peduncle encapsule the nucleus, and 

 then ascend through the subthalamic tegmental region to the 

 anterior part of the optic thalamus, within which they terminate 

 in arborizations around the cells of the anterior nuclear area (chief 

 sensory nucleus). From the cells of the red nucleus relays of fibres 

 proceed upwards, which carry on those fibres of the peduncle which 

 terminate within the nucleus, and these relays ascend with those 

 fibres of the peduncle which encapsule the red nucleus to the optic 

 thalamus. From the cells of the optic thalamus other relays of 

 theilamo-corticipetal fibres arise, which pass through the internal 

 capsule to the cerebral cortex, more particularly the cortex of the 

 ascending parietal or post-central gyrus, which lies directly behind 

 the fissure of Rolando. 



The superior cerebellar peduncle of one side connects the cere- 

 bellar hemisphere of that side with the post-Rolandic region of the 

 cerebral cortex of the opposite side, the red nucleus and the optic 

 thalamus being cell-stations in the path of the fibres. 



A few of the fibres of each superior cerebellar peduncle do not 

 take part in the decussation beneath the lower pair of quadrigeminal 

 bodies, but ascend to the red nucleus of their own side. 



Before the fibres decussate, or after the decussation has taken 

 place, each of them furnishes a descending branch. These de- 

 scending branches form the descending cerebeUar bundle of Cajal, 

 which traverses the dorsal part of the pons Varolii and the medulla 

 oblongata, giving off collaterals to the motor nuclei of these parts. 

 According to Cajal, the fibres of this bundle enter the anterior 



