44 bulletin: museum of comparative zoology. 



fibers surround the vessel in a circular manner. For the remainder 

 of the distance the fibers are diagonal or longitudinal to the wall of the 

 vessel. Here the latter lies in immediate contact with the thymus 

 gland. 



The muscle is innervated by some very fine rami (not included in 

 plotting) from X and IX + X (Fig. K, rm. vn. j.), as was determined 

 by study of the sections. Some undetermined rami appearing in the 

 dissection (Plate 3, fig. 7) are probably of like function. 



J. OCULOMOTOR NERVE. 



The oculomotor nerve (///) arises from its nucleus of origin in the 

 floor of the aqueduct of Sylvius and emerges as one large root from the 

 ventral side of the mesencephalon (Plate 2, fig. 4). It swings laterad 

 and cephalad to pass out of the cranium through the membranous 

 wall, being covered laterally at this point by the bursalis muscle. 

 (Plate 6, fig. 16). Posterior to the origin of the recti muscles the nerve 

 divides into three large rami; the dorsal one goes to the dorsal rectus; 

 a large ventral one, which later divides (Plate 5, fig. 14, ///), supplies 

 the ventral rectus, the anterior rectus and the ventral oblique; be- 

 tween these two large rami is the short root of the ciliary nerve. The 

 latter is composed chiefly of fine neuraxons, which may be recognized 

 as a distinct bundle in the center of the main trunk from the point 

 where it emerges from the brain to its separation from the somatic 

 motor components as the ciliary root. Within the brain itself it could 

 not be independently traced in this series. 



The branch of nerve III to the dorsal rectus muscle (Plate 2, fig. 4, 

 rt. d.) is a large one. It follows the lateral face of this muscle near its 

 anterior ventral edge (Plate 5, fig. 14), several successive bundles of 

 fibers being given ofi^ to accomplish the innervation of the muscle. 



The large ventral ramus (Plate 5, fig. 15, ///) passes mesad of the 

 retractor oculi to attain a position on the ventral face of the ventral 

 rectus muscle. It is in this part of its course that a group of long rami 

 separate from it, and penetrate the ventral rectus from both the dorsal 

 and ventral surfaces (Plate 2, fig. 4). The remainder of the nerve 

 (Plate 5, fig. 14, ///) passes cephalad close to the median line, dividing 

 to send more than half of its fibers dorsally into the anterior rectus 

 (Plate 5, fig. 13, rt. a.), the remainder passing on to the ventral oblique 

 (Fig. 13, oh. v.). 



