52G THE ORGANS 



pupillae, via ciliary sympathetic ganglion) are uncertain. From a priori grounds, 

 the innervation of the intrinsic muscles by the small-celled groups and the other 

 muscles by the large-celled groups would seem probable. Some of the fibres, 

 usually stated to be from the posterior dorsal-lateral group, decussate. Recent 

 observations (Cajal) would indicate that the decussating fibres come from the 

 ventro-mesial lateral groups. What is perhaps the prevailing view as to these 

 relations is shown in Fig. 354. The various fibres pass ventrally in a number of 

 bundles, some passing mesial to, some traversing, and some passing lateral to 

 the superior cerebellar peduncle and red nucleus. Ventral to these the root 

 fibres come together and emerge on the ventral aspect of the midbrain (Figs. 

 355 and 356.) 



The nucleus of nerve III receives terminals and collaterals from the medial 

 longitudinal fasciculus, i.e., from the ascending axones from Deiters' nucleus and 

 the descending axones from the nucleus of the medial longitudinal fasciculus, 

 possibly collaterals from the colliculo-spinal tract, collaterals or terminals from 

 the superior cerebellar peduncle and from the reticular formation. Its exact 

 relation to efferent' pallial fibres is not known. Fibres in the medial longitudinal 

 fasciculus connect the nuclei of III, IV and VI (synergic movements of eye 

 muscles). 



Immediately caudal to the nucleus of nerve III (not in the plane of the 

 section) is the nucleus of the nerve IV occupying a position similar to the lateral 

 groups of the nerve III nucleus. The course of the root fibres of the IV has been 

 mentioned (preceding section). It receives terminals similar to those received 

 by III. 



Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The mesen- 

 cephalic trigeminal root is sometimes distinguishable on the lateral border of the 

 central gray. 



The lateral lemniscus has partly or wholly terminated in the inferior coUiculus 

 at a lower level. Fibres from the latter form its arm or brachium, representing 

 another link of the cochlear path. In the present section the fibres of the 

 brachium of the inferior coUiculus are seen entering the medial geniculate body 

 (which is a part of the thalamus). Axones of the cells of this body constitute the 

 last relay of the cochlear path (III, p. 4S2) to the temporal cortex cerebri (not 

 present in this section). As already stated, there is doubt as to how far this 

 path is interrupted in various nuclei along its course such as the superior olives, 

 trapezoid, lateral lemniscus nucleus and inferior coUiculus. It may be reducible 

 to three principal neurone groups: (i) the spinal ganglion and its cochlear nerve, 

 (2)the lateral lemniscus and (3) the geniculo-temporal system. 



The medial lemniscus is now a laterally placed, curved bundle, displaced 

 laterally by the red nucleus. According to some authorities, some of its fibres 

 terminate in the superior coUiculus. The spino-thalamic tract is plainly distin- 

 guishable as a bundle dorsal to the dorsal edge of the medial lemniscus. At this 

 level, then, the afferent paths from cord to pallium have practically united. 



In the superior coUiculus are terminations of the optic tract (the continuation 

 past the optic chiasma of the so-called optic nerve) (see below). 



The central tegmental tract is displaced dorsally by the red nucleus. 



Intersegmental Neurones. — The reticular formation is smaller. The rubro- 



