MIDBRAIN AND THALAMUS OF NECTURUS 235 



root fibers arise in two or three fascicles, of which the larger 

 comes from the neurons of the central gray at about the same 

 dorso-ventral level as the superficial origin of the root (fig. 11). 

 These neurons are somewhat larger than the surrounding cells 

 of the central gray, though the limits of the nucleus are not 

 sharply defined. There is a more ventral fascicle of root fibers 

 which connects with the neurons of the floor of the fovea isthmi. 

 Some of these fibers appear to come from the nucleus of the 

 opposite side. This medial ventral nucleus is probably the 

 nucleus of Edinger-Westphal, giving rise to visceral efferent 

 fibers. 



2. The trochlear nerve 



The IV nerve immediately distally of its crossing in the de- 

 cussatio veli contains about 20 very coarse and heavily myeli- 

 nated fibers. Their peripheral relations have been described by 

 McKibben ('13, p. 158), who also describes the relations of 

 the IV nerve near its decussation with coarse myelinated fibers 

 of the tectum mesencephali and also with myelinated fibers 

 distributed peripherally to the choroid plexus of the fourth 

 ventricle. The latter have been previously noted by Kings- 

 bury ('95, p. 146). 



The internal course of the IV nerve is very difficult to follow, 

 for its fibers on the proximal side of their decussation are scat- 

 tered and mingled with those of the mesencephalic root of the 

 V nerve, which are also coarse and heavily myelinated. In Wei- 

 gert preparations cut in the sagittal plane they can be followed 

 from the decussation downward and forward in the deepest 

 layer of the stratum album, accompanying the finer fibers of 

 the tecto-peduncular system, to a region caudad of the nucleus 

 of the III nerve, but the exact location of their nucleus of origin 

 has not been demonstrated. 



3. The mesencephalic root of the V nerve 



The cells of the nucleus of this root (nucleus magnocellularis 

 tecti) are widely distributed throughout the entire tectum 

 mesencephali, especially caudad, and their axons, which are 



