SURFACES. 155 



optic radiation and optic tract. The nucleus of the third nerve 

 (n. oculomotor ius), according to Perlia, is composed of seven dis- 

 tinct cell-groups, scattered from the posterior part of the lateral 

 wall of the third ventricle down to the level of the transverse 

 groove between the superior and inferior quadrigeminal collic- 

 uli. From these seven cell-groups the axones proceed forward 

 into the nerve of the same side; but there is, in addition, one 

 median group of cell-bodies which sends axones into both nerves. 

 The nuclei are also associated by decussating dendrites. The 

 root fibers run forward through the red nucleus and substantia 

 nigra and issue from the oculomotor groove. The nucleus of the 

 fourth nerve (n. trochlearis] is a single oval mass of cell-bodies 

 situated ventral to the inferior colliculus of the corpora quadri- 

 gemina. The root fibers of the fourth nerve, trochlear, proceed 

 dorsally and caudalward from the nucleus. They decussate 

 with the fibers from the opposite nucleus in the superior medullary 

 velum, from which they emerge on either side of the frenulum. 

 They then continue in the opposite nerve around the side and 

 over the anterior surface of the mid-brain. This is the only nerve 

 that decussates en masse between the genetic nucleus and the 

 apparent origin. 



The nucleus of the mesencephalic root of the trigeminal nerve 

 is composed of large cell-bodies scattered in the extreme ventro- 

 lateral part of the stratum griseum centrale, from the highest 

 level of the mid-brain down to the pons. There is no break be- 

 tween this nucleus and the chief motor nucleus of the fifth nerve 

 formed by the substantia ferruginea under the locus caeruleus. 

 The axones of these large cell-bodies run downward through 

 the nucleus, accumulating gradually until they form a distinct 

 crescentic strand, which joins the chief motor root of the same 

 side. 



The opposite pyramidal tracts and, probably, the three homo- 

 lateral cerebro-pontal tracts (fronto-pontal, temporo-pontal and 

 intermediate tracts) bring these nuclei into relation with the 

 cerebral cortex; and the anterior and the medial longitudinal 

 bundles establish their reflex relation. 



Formatio Reticularis (Fig. 47). Through the greater por- 



