BOWERS. CRANIAL NERVES OF SPELERPES BILINEATUS. 181 



C. Eye-muscle Nerves, III., IV., and VI. (PI. 2, Figs. 3, 4). 



It was my intention to confine my work to the fifth, seventh, ninth, 

 and tenth nerves, but in my examination of the ophthalmic branch of the 

 trigeminus it became necessary to make a study of the eye-muscle nerves, 

 from which some facts of interest were established. I therefore give 

 a brief account of these nerves, — the third, fourth, and sixth cranial 

 nerves. 



The oculomotor (III.) arises, as usual, from the floor of the mesen- 

 cephalon, passes through the brain wall, and then, lying between that 

 wall and r. ophthalmicus V. ( V. opt.), immediately gives off its r. supe- 

 rior (PI. 2, Figs. 3, 4, III. rt. sit.) to m. rectus superior. It then passes 

 under r. ophthalmicus V., — which takes a more median position (Fig. 4), 

 — and here lies for a short distance in close contact with a branch of the 

 abducens (VI.) In some series of sections the two nerves, while indis- 

 tinguishable on one side of the head, were clearly separable on the other. 

 There is, then, no real fusion of the two. 



A cluster of ganglion cells (cL gn.) is constantly found enveloping that 

 portion of the oculomotorius that lies directly ventrad to the optic nerve 

 (see Figs. 3, 4). In the 23 mm. stage these ganglionic cells are grouped 

 into a compact mass, but in older stages (40 mm.) they are somewhat 

 scattered along the nerve. The oculomotorius follows closely m. inferior 

 rectus, giving fibres to it, aud ends in m. inferior obliquus. 



The trochlearis (IV., PI. 2, Figs. 3, 4) is very minute, and its whole 

 course was not traceable in the 23 mm. stage ; but in older larvae an 

 interesting condition was observed. At the posterior margin of the eye- 

 ball this nerve joins a dorsal branch of r. ophthalmicus ( Va), and the 

 two run along the median dorsal surface of the eyeball to near its ante- 

 rior margin as one nerve. Here the fibres of the trochlearis (IV. ob. su.) 

 pass ventrad and are distributed to the superior oblique muscle ; this 

 ophthalmic branch of the fifth ( Va) then passes forward and dorsad to 

 innervate the skin in the regions in front of the eye. Since IV. joins the 

 dorsal branch of V. on its dorsal side and separates from it on the ventral 

 side (Fig. 3), there must be a crossing of fibres; this crossing I found 

 clearly shown in sagittal sections of one of my older embryos. Gaupp 

 ('97, p. 136) speaks of this relation of IV. and V. in the frog, and pre- 

 sumes that the branch given off to m. superior obliquus is composed of 

 fibres from IV., but he did not actually observe the crossing of fibres. 



For the study of the abducens (VI.), Spelerpes proved to be especially 

 advantageous. In this species the abducens does not enter the Gas- 



