V. 



508 THE ORGANS 



or less well-defined reticular nuclei (see Fig. 340). The fibres of the lateral 

 Deitero-spinal tract (not distinguishable) are here emerging from Deiters' 

 nucleus. The medial longitudinal fasciculus occupies the same position, but is 

 here well separated from the fillet. Some of the internal arcuate fibres in the 

 dorsal part of the reticular formation may be fibers from Deiters' nucleus to 

 the medial longitudinal fasciculus. They may be crossed or uncrossed, and may 

 descend in it as already mentioned (pp. 472 and 505, or ascend (see Fig. 339). '{ 

 Other internal arcuate fibres here, as elsewhere, pass from the various terminal 

 nuclei to form secondary tracts. Other transverse fibres are axones of cells of 7 

 reticular nuclei or collaterals and terminals ending in them. 7 



Efferent Suprasegmental Neurones. — The coUiculo-spinal tract lies ventral 

 to the medial longitudinal fasciculus. The pyramids are in the same position, 

 but are partly surrounded by pontile fibres and nuclei. (See also pons, above.) t 



The Cerebellum. — The gray matter consists of the external gray or cortex, 1 

 and inlernal nuclei forming interruptions or relays in paths from the cortex. 

 The white matter consists of the fibres of various afferent and efferent cerebellar 

 paths and possibly association fibres of the cerebellum. The cortex is studied 

 elsewhere. The internal nuclei can usually be distinguished. They are the 

 nucleus dentatus cerebelli {corpus dentatum), a convoluted mass of gray resembling 

 the inferior olives (and sometimes called the cerebellar olives), and mesial to 

 this the nucleus globosus, nucleus enibolijormis and the nucleus tecti or fastigii. 

 The nucleus fastigii receives fibres from various parts of the cerebellar cortex and 

 also vestibular root fibres (p. 503). Its axones, in part at least, pass to Deiters' 

 nucleus and other nuclei in the reticular formation {fastigio-bulbar tract). This 

 forms a link of the cortico-fastigio-Deitero-spinal path (p. 484, XVIII). The 

 nucleus dentatus, nucleus globosus, and nucleus emboliformis also receive fibres 

 from the cerebellar cortex. Their axones form the superior cerebellar peduncle 

 {brachium conjunctivum) , cross and pass to the red nucleus, reticular formation, 

 nucleus of nerve III, and thalamus. At the level of the section the superior pe- 

 duncle is not yet fully formed. This forms links in both the cortico-dentato-rubro- 

 spinal and the cerebello-pallial paths (p. 484, XVH and \TI. See also p. 513.) 



Note where possible the structure of the plexus chorioideus of the fourth 

 ventricle. It consists of a layer of cuboidal epithelial cells next the ventricle 

 which are ectodermic, and an outer mesodermic part consisting of connective 

 tissue and blood-vessels. 



7. Transverse Section of the Hindbrain through the Roots of Nerves VI (Ab- 

 ducens and VII (Facial) (Figs. 332 and 342) 



Efferent Peripheral Neurones. — The nucleus facialis is usually not present, 

 but various portions of the root fibres may be present (see preceding section), 

 especially the longitudinal part. 



The nucleus oj the VI or nucleus abduceniis is present in about the middle of 

 the floor of the ventricle and just beneath the central gray or partly within it. 

 Its fibres, the root fibres of the abducens, are seen passing ventrally. The nucleus 

 receives collaterals from the axones of Deiters' nucleus (secondary vestibular) 

 on the same and opposite sides and collaterals or terminals from the superior 





