306 H. W. NORRIS AND SALLY P. HUGHES 



THE OPTIC AND THE EYE-MUSCLE NERVES 



The thoroughgoing descriptions and discussions by Neal ('98, 

 '14, '18) of the origin, histogenesis, homologies, and general 

 topographical relations of the eye-muscle nerves, with especial 

 relation to the condition in Squalus acanthias, make any extended 

 treatment of the subject by the present writers quite superfluous. 

 For a review and list of the literature upon this subject the reader 

 is referred to the second ('14) of these papers by Neal. 



From the eyeball the optic nerve runs posteromesially into 

 the optic foramen, and thence anteriorly into the chiasma. The 

 anterior dorsal part of the chiasma is formed from fibers situated 

 dorsally in the nerve, and the ventral posterior part from the 

 ventral portion of the nerve. Near the brain a cross-section of 

 the nerve shows a deep notch on its posterior border; near the 

 eyeball a similar section reveals four such deep indentations (figs. 

 15 and 34). 



The nidulus of origin of the oculomotor nerve is situated im- 

 mediately dorsal to the ventral motor columns, slightly posterior 

 to a transverse level where the cavity of the midbrain communi- 

 cates ventrally with that of the inferior lobes. Numerous strands 

 of fibers, many of which decussate, pass posteroventrally from 

 the nidulus, emerging immediately posterior to the point where 

 in cross-section the inferior lobes separate externally from the 

 midbrain. After being collected into the external nerve, the 

 rootlets pass posteriorly in the angle between the midbrain and 

 the inferior lobes, the nerve lying at first at the dorsolateral 

 angle of the inferior lobes, farther posteriorly in the same relation 

 to the saccus vasculosus, finally passing laterally across the dor- 

 sal bolder of the latter to reach its exit from the skull (figs. 5, 6, 

 12 to 14, and 21). An intracranial ganglion on the oculomo- 

 torius, as reported by Nicholls ('15) in Scyllium, is not found by 

 the writers in Squalus. Entering the orbit the third nerve, turn- 

 ing sharply anteriorly, almost immediately divides into two 

 branches. An anterior dorsal division (Hid.) passes at once 

 into contact with the dorsal rectus muscle, and running along 

 its anteromesial bolder between it and the internal rectus muscle 



