THE INTERNAL STRUCTURE OF THE MESENCEPHALON l6l 



conjunctivum, and stains deeply in Weigert preparations, but farther rostrally 

 these fibers are less numerous and the nucleus takes on more and more the ap- 

 pearance of gray substance. 



Afferent fibers reach the red nucleus chiefly through the brachium con- 

 junctivum, but it also receives fibers from the cerebral cortex of the frontal 

 lobe and others from the corpus striatum (Fig. 115). These descending fibers 

 help to form the capsule of the nucleus and are most abundant along its medial 

 surface. 



Efferent Fibers. From the cells of the red nucleus arise the fibers of the 

 rubrospinal tract, which after crossing the median plane descend into the spinal 

 cord. Other cells give origin to fibers, which decussate along with those of the 

 rubrospinal tract and terminate in the nuclei of the reticular formation and in 

 the nucleus of the lateral lemniscus. These form the tractus rubroreticularis 

 (Fig. 115). Other fibers from the red nucleus reach the thalamus. 



The nerve-cells which are found in the red nucleus vary greatly in size. The smaller 

 ones have the character of the cells of the reticular formation and send their axons into the 

 tegmentum of the same and the opposite side. Another group of very large cells furnishes 

 the axons that constitute the rubrospinal tract. This collection of large cells is phylogenetic- 

 ally the older and forms the chief part of the red nucleus in the lower mammals. But in 

 man, where the two parts are rather sharply differentiated, the chief mass is composed of 

 the smaller cells. 



The red nucleus may be regarded as an especially highly developed portion of the motor 

 nuclei of the tegmentum. In the lower mammals it serves as a center through which the 

 cerebellum can influence the motor functions of the spinal cord and medulla oblongata. 

 In man it has the same function, but is also more closely linked with the reticular formation 

 of the pons by way of the rubroreticular tract. It is a significant fact that in man where 

 the rubrospinal tract is relatively small the rubroreticular tract is especially well developed. 

 This suggests the possibility that impulses from the red nucleus may be relayed through the 

 reticular nuclei of the pons to the spinal cord (Fig. 115). 



The Tegmental Decussations. At the level of the superior colliculus and 

 between the two red nuclei the median raphe presents an unusual number of 

 crossing fibers (Fig. 116). Among these are included the dorsal tegmental de- 

 cussation (fountain decussation of Meynert) and the ventral tegmental decussa- 

 tion (fountain decussation of Forel). The latter is composed of fibers from the 

 red nucleus, which, after crossing the median plane, descend through the brain 

 stem into the lateral funiculus of the spinal cord as the rubrospinal tract (Fig. 

 115). The dorsal tegmental decussation is composed of fibers which arise in the 

 superior colliculi of the corpora quadrigemina, sweep in broad curves around the 

 central gray stratum, and after crossing the median plane in the dorsal part of 

 the raphe, go to form the tectobulbar and tectospinal tracts. 



