588 



THE NEKVOUS SYSTEM. 



Decussating 



fibres ~~ 



Inferior 

 colliculus 



Mesencephalic 



root of tri- 



geminal nerve 



Trochlear 



nerve 



Medial 



longitudinal 



bundle 



Lateral 



lemniscus 



Decussating 



brachia 



conjunctiva 



Medial 

 lemniscus 



capsule, which is thicker on the medial than on the lateral side (Fig. 521). These 

 are prolonged into the thalamus, and end ultimately in connexion with the ventral 

 thalamic cells. The brachium conjunctivum is, therefore, a great efferent tract 

 which issues from the nucleus dentatus of the cerebellum, crosses the median plane 

 in the inferior part of the mesencephalon, and ends in the red nucleus and the 

 ventral part of the thalamus. 



Nucleus Ruber. The red nucleus is a rounded nuclear mass, of a reddish tint 

 in the fresh brain, which lies in the superior part of the tegmentum, and in the 

 path of the brachium conjunctivum. In transverse section it presents a circular 

 outline. It begins at the level of the inferior border of the superior colliculus 

 and it extends upwards into the hypothalamus. At first it is small and is placed 

 at a little distance from the median plane ; but, as it proceeds upwards, it 

 increases in bulk and approaches more nearly to the median raphe, and to its 



fellow of the opposite side. The 

 curved emerging bundles of the 

 oculomotor nerve pass through it 

 on their way to the surface. The 

 relation which the fibres of the 

 opposite brachium conjunctivum 

 present to it has been described. 

 These fibres traverse its inferior 

 part in such numbers that in 

 Weigert-Pal specimens it presents 

 a very dark colour ; but higher up, 

 as the fibres gradually end in 

 nuclear mass, they become less 

 numerous in its midst, and the 

 nucleus assumes a paler tint. 



Numerous fibres which descend 

 from the cerebral cortex, and others 

 from the corpus striatum, enter the 



red nucleus. It also sends out 



FIG. 522. SECTION THROUGH THE INFERIOR COLLICULUS , , . , -,. 



AND THE TEGMENTUM OF THE MESENCEPHALON BELOW "Dres which proceed in two direc- 



THE LEVEL OF THE NUCLEUS OF THE TROCHLEAR tions : (1) upwards into the thala- 



NERVE IN THE ORANG. (The decussation of the mus . ( 2 ) downwards to the spinal 



bracnia conjunctiva and the course of the trochlear j 11 mi_ A.-L. ^ .ci_ 



nerve in the central gray matter are seen.) medulla. I he thalamic tlbres may 



be regarded as carrying on the 



continuity of the path of the brachium conjunctivum after its nodal interruption in 

 the red nucleus. The fibres to the spinal medulla, called the rubro-spinal tract 

 and first described by Monakow, cross to the opposite side and then descend in the 

 tegmentum to reach the lateral funiculus of the spinal medulla (Fig. 473, p. 534). 

 Fasciculus Longitudinalis Medialis. The medial longitudinal fasciculus is a 

 very conspicuous tract of longitudinal fibres which extends throughout the whole 

 length of the medulla oblongata, pons, and mesencephalon, in the formatio reti- 

 cularis or tegmental part of each. Below, at the level of the decussation of the 

 pyramids, it becomes continuous with the fasciculus anterior proprius of the spinal 

 medulla (p. 562), whilst, by its opposite or superior end, it establishes intricate 

 connexions in the region immediately above the mesencephalon. Throughout its 

 whole length it lies close to the median plane and its. fellow of the opposite side. 

 In the mesencephalon it is applied to the ventral aspect of the central gray 

 matter, whilst in the pons and medulla oblongata it is situated immediately 

 subjacent to the gray matter of the floor of the fourth ventricle. One of its 

 most salient features is the intimate association which it presents with the three 

 motor nuclei from which the nerves for the supply of the muscles of the eyeball 

 take origin, viz., the oculomotor nucleus, the trochlear nucleus, and the abducent 

 nucleus. The first two of these are closely applied to its medial and dorsal aspect, 

 whilst the abducent nucleus is placed on its lateral side. Into each of these nuclei 

 it sends many collaterals, and probably also some of its constituent fibres, and these 

 end around the nuclear cells. It would appear, therefore, that one of the most 



