402 EXPERIMENTAL PHARMACOLOGY 



tliis is to give the student a chance to know when he has his 

 eyes focused for a long distance (with which point most 

 students are unfamiliar). 



The subject looks out into the clear sky (avoid sunlight) 

 and holds the card in his right hand in front of and near 

 to his right eye. The left eye is closed. The right eye looks 

 through the pin-hole in the card and is focused for a long 

 distance, i.e., the ciliary muscle is completely relaxed and 

 the pupil is dilated. The card is now moved in a rotary 

 manner in such a way that the pin-hole will describe a small 

 circle in front of the pupil. The diameter of this circle 

 should be about one-eighth of an inch or less and the circu- 

 lar movement should be repeated at the rate of about two 

 or three times per second. Thus a small ray of light will 

 pass from the window through the pin-hole in the card, 

 through the pupil and back on to the retina. Since the pin- 

 hole is Being constantly moved in a circle this ray of light 

 will also l)e continually traveling round and round in a 

 circle over the region of the fovca in the retina. This is a 

 very unusual manner for light to enter the eye. As a result 

 the blood vessels which are located in the anterior layers of 

 the retina cast back their shadows on to the rods and cones 

 in the posterior layers of the retina in such an unusual man- 

 ner that these shadows are recognized by the subject. Their 

 form and general distribution can be easily made out. It 

 usually takes a long time for a student to learn to recognize 

 these shadows correctly. He should focus his eyes for a 

 long distance and keep moving the pin-hole round and 

 round as he gazes through it up into the clear sky. The 

 vessels, when finally recognized, will appear very much like 

 the leafless tops of a number of trees ranged in a circle in 

 such a manner that the tops all point toward the center but 

 do not quite reach it, so that a small clear space is left en- 

 tirely free from vessels in the center. This clear space cor- 

 responds to the macula lutea or yellow spot. What do you 

 know about this area? 



