402 MABEL BISHOP 



optic nerve (fig. 11). I was unable to trace fibers across the 

 chiasma ridge, and as yet the external optic chiasma of adult 

 brain is undeveloped. 



III. Nervus oculomotorius. From its superficial origin at 

 the concavity of the cephalic flexure the third nerve pursues a 

 ventral and slightly rostral course medial to the fourth nerve, 

 and at the level of the eye turns outward to supply the superior, 



Fig. 15 Section through the two outer (normal) eyes of the embiyo, and the 

 median (conjoined) internal ear capsules. The ciliary ganglion of the left side 

 of head B has been added from section 243. S. 242. 



inferior, and medial recti muscles, and the inferior oblique. 

 The superior palpebral branch is absent, as would be expected 

 from the absence of the superior palpebral muscle. 



IV. Nervus trochlearis. Spinalward of the oculomotor, the 

 trochlearis emerges from the dorsal surface of the brain at the 

 isthmus, swings around the cerebral peduncle, and courses 

 external to the third nerve in a ventrorostral direction to its 

 termination in the superior oblique muscle of the eye. 



