EFFERENT FIBERS OF THE OPTIC NERVE 217 



The following paper presents one phase of an investigation 

 upon the visual cells and retinal pigment, pursued at Harvard 

 University under the super\-ision of Prof. G. H. Parker. To Pro- 

 fessor Parker I am greatly indebted for the continued interest 

 and kindly criticism that characterizes his instruction. 



MATERIAL AND TECHNICAL METHODS 



The fishes used in this in\'ostigation were as follows : the com- 

 mon horned pout, Ameiurus nebulosus Lesueur; the shiner, 

 Abramis crysoleucas Mitchill; and the coinmon kilUfish, Fundu- 

 lus heterocUtus Linn. A greater part of the experimentation 

 was done upon .Ameiurus, since in this animal the activities of 

 the retinal pigment and visual cells proved to be especially 

 favorable imder the experimental conditions imposed upon thorn. 

 Furthennore, the occurrence of responses in the excised eyes of 

 Ameiurus was a happy concomitant circumstance, since the 

 results were thereby controlled at several important points. 



The technical methods used in preparhig retinas for micro- 

 scopical examiniition were sun]:)lo. The eyes of Ameiurus were 

 excised directly, for the skiji of this annual is soft and the eyes 

 are prominent. In the two other fishes, es])ecially when rapidity 

 of o]:)eration was desirable, the following procedure was obser\'ed. 

 With hea\'y scissors the cranium was bisected in the sagittal 

 plane; following this, a transverse cut just posterior to the orbit 

 freed the two halves of the cranium, with the contained eyes, 

 from the rest of the body. In either case the operation was per- 

 formed in a few seconds, and the o>-e, without being handled, was 

 allowed to drop into the fixhig fluitl. 



Perenyi's fluid gave good fixation and was used exclusively. 

 Fixatives containing nitric acid have long been recognized as 

 favorable in the obtainment of faithful presentation of the retina. 

 The preparatoiy steps prior to embedding in paraffine demand 

 generous allowances of time, yet the processes of dehydration 

 and clearing should progress as rapidly as possible, since other- 

 wise the sclera becomes extremely hard. 



Two methods were used in removing the lens, one of which, 

 although longer, gave much more satisfactory results. The first, 



