844 



THE SENSE OF SIGHT 



low intensity. This theory will be more fully discussed later on in 

 connection with color-vision. 



Phosphenes. — It has been emphasized repeatedly that the ade- 

 quate stimulus for the retina is the light ray, because this receptor 

 presents the most favorable conditions for the transformation of 

 this form of energy into nerve impulses.^ In a slight measure, 

 however, the retina is also accessible to inadequate stimuli in the 

 form of mechanical and electrical impacts, but the visual impres- 

 sions then obtained retain the character of very general sensations 

 of light. These sensations are of course subjective in their quality, 

 because they are not caused by homologous stimuli of light, but by 

 stimuli of a heterologous kind. Thus, if the eyelids are closed and 



the eyes are turned inward, any pressure 

 upon the external part of the eyeball, such 

 as may be exerted with the blunt end of a 

 pencil, gives rise to luminous sensations, 

 known as *' phosphenes '^ (Fig. 445). In 

 this particular case, they appear in the form 

 of bright yellowish rings, each surrounding 

 a dark center. It is to be noted especially 

 that this sensation, although evoked at the 

 point of pressure, is referred to the opposite 

 visual field. In other words, any pressure 

 exerted upon the outer zone of the eyeball 

 gives rise to a sensation which is projected 

 into the nasal area of the visual field, be- 

 cause under ordinary conditions the outer 

 retina is stimulated by objects situated in 

 the nasal field. A phosphene of similar 

 character may be produced by gazing into 

 a bright light while the eyes are rapidly 

 Inasmuch as the eyeball is relatively fixed 

 at the point where the optic nerve leaves it, this abrupt lateral devia- 

 tion gives rise to a mechanical stimulation of the retinal elements situ- 

 ated around the edge of the optic disc. In this case, the visual sen- 

 sation is projected directly outward into the central visual field. 

 Phosphenes also result in consequence of stagnation at the points of 

 exit of the vense vorticosae and in consequence of the pulsations of 

 the retinal arteries. They are most intense in conditions of hyper- 

 excitability of the general nervous system. 



Electrical Variations in the Eye on Vision. — The retina shows a 

 current of rest or injury as well as a current of action. If an excised eye 

 is connected with a galvanometer by two non-polarizable electrodes, 

 one of which is adjusted to the cornea and the other to the end of 

 the optic nerve, the latter is galvanometrically negative to the former. 



1 Klein, Archiv fiir Physiol., 1910, 531. The phenomenon of the phosphenes 

 has been known since the time of Aristotle. 



Fig. 445. — Diagram to Il- 

 lustrate THE Phenomenon of 

 Phosphenes. 



S, The mechanical stimula- 

 tion of the coats of the eye ball 

 at s gives rise to a sensation of 

 light which is projected to i in 

 the opposite visual field. 



moved from side to side. 



