EFFERENT FIBERS OF THE OPTIC NERVE 219 



The method of operation was as follows. With curved scissors 

 an incision was made in the skin ventral to the eye and the cut 

 carried upward around both sides of the ball until a semicircular 

 (or greater) incision resulted. The eyeball could now be rolled 

 back, and when thus displaced, no excessive strain was exerted 

 on the conjoined tissues. By dissecting through the aperture 

 thus exposed, the optic nerve was separated from the overlying 

 muscles and severed. If a delicate razor-edge scalpel, such as 

 ophthalmologists use in cataract operations, is employed, it is 

 comparatively easy to cut the ner\'-e without injury to the sur- 

 rounding parts. Ameiurus has no large central arter>' and vein 

 in the optic nerve, as is the case in mammals. A blood-vessel, 

 however, runs beside the optic ner\'e, but if care is taken it need 

 not be injured by the operation. 



1. Retinal piqmenl. My first effort was to discover the efTects 

 upon pigment migration of the severance of the optic wqtyq only, 

 A typical cx})oriment will illustrate the response of the pigment 

 when a previously dark-adapted fish was operated on and exi)osed 

 to light. 



Experiment 8.1.12. Tiie optic nerve of a dark-adapt od fish was 

 severed and the animal allowed to recover from any shock cfToct until 

 the next day, when it was exposed to diffuse daylight for 2\ hours. 

 At the expiration of this time both the operated ancl the normal eye 

 were excised. Subsequent examination showed that the retinal pig- 

 ment in the operated eye had retained the position typical of tlarkness 

 (fig. 4), while the pi^nnenl in the control evo had nuf>;rato(l norniallv 

 (%. 1). 



Thus it is seen that the integrity of the otitic ner\'e must be 

 maintained in order that the retinal pigment of an otherwise 

 intact animal may undergo a positional change when stimulated 

 by light. 



In successful ex|)erinients of this type the length of exposure 

 ^^aried between 45 minutes and 3 hours, yet uniform results 

 were obtained. ^ A possible influence of operative shock, on ani- 

 mals that were subjected to experimentation directly after the 



' Under normal conditions the pigment is fully extended by light in about 45 

 minutes (Arey, '16). 



