164 THE NERVOUS SYSTEM 



aspect of this nucleus, curve dorsally around the central gray matter, and decus- 

 sate in the anterior medullary velum (Fig. 112). 



The nucleus of the oculomotor nerve is composed of the cells of origin of 

 the motor fibers for all of the ocular muscles except the superior oblique and 

 lateral rectus. It lies in the ventral part of the central gray substance beneath 

 the superior colliculus (Fig. 116). This nucleus, a part of which occupies a 

 median position and supplies fibers to the nerves of both sides, is 6 or 7 mm. 

 long and extends from a little beyond the rostral limit of the mesencephalon to 

 the nucleus of the trochlear nerve, from which it is not sharply separated. From 

 the nucleus the fibers of the oculomotor nerve stream forward through the 

 tegmentum and red nucleus. They emerge through the oculomotor sulcus along 

 the ventromedial surface of the basis pedunculi. 



The interpeduncular ganglion is a median collection of nerve-cells in the 

 posterior perforated substance situated between the two cerebral peduncles near 

 the border of the pons (Fig. 1 14) . It receives fibers from the habenular nucleus 

 of the epithalamus by way of the fasciculus retroflexus of Meynert; and from 

 it spring fibers that run to the dorsal nucleus of the tegmentum (Fig. 211). 



The substantia nigra is a broad thick plate of pigmented gray matter, which 

 separates the basis pedunculi from the tegmentum and extends from the border 

 of the pons throughout the length of the mesencephalon into the hypothalamus. 

 In transverse section it presents a semilunar outline. Its medial border is super- 

 ficial in the oculomotor sulcus and is thicker than the lateral border, which 

 reaches the lateral sulcus of the mesencephalon. Its constituent nerve-cells, 

 irregular in shape and deeply pigmented, send their axons into the tegmentum. 

 But we are still ignorant as to the destination these may have; and the func- 

 tion of the substantia nigra is equally obscure. It receives collaterals from the 

 corticifugal fibers of the basis pedunculi. Furthermore, there terminates within 

 it a bundle, consisting of both direct and crossed fibers from the corpus striatum, 

 the strionigral tract (Fig. 117). 



The basis pedunculi is a broad compact strand, crescentic in transverse sec- 

 tion, which consists of longitudinal fibers of cortical origin. These are con- 

 tinued from the internal capsule into the longitudinal bundles of the pons 

 through the basis pedunculi. It consists of four tracts. The medial and lat- 

 eral fifths are occupied by fibers which terminate in the nuclei pontis. Those 

 of the medial one-fifth arise from the cortex of the frontal lobe of the cerebral 

 hemisphere and constitute the frontopontine tract. Other fibers, arising from 

 the temporal lobe, form the temporopontine tract and occupy the lateral one- 



