844 ENTOPTIC PHENOMENA. 



illumination that stimulates the retina. In birds, irritation of the oculomotor 

 produces exceedingly rapid contraction ; in rabbits dilatation of the pupil does not 

 appear until 0.89 second after irritation of the sympathetic. 



In the enucleated eyes of amphibians and fishes, stimulation by light causes 

 contraction of the pupil. In fact the iris of the eel, when removed from the 

 eye and laid in salt-solution, contracts on stimulation by light, the green and 

 blue rays being the most active. In these animals the cells of the sphincter 

 muscle are pigmented; the contractile action of the light-rays seems to take 

 place through the intermediation of the pigment. 



Increase of temperature produces mydriasis of the enucleated eye of the frog 

 or eel, while decrease of temperature causes myosis. 



Grunhagen has disputed the existence of the dilator muscle. He explains 

 the dilating action of the sympathetic by the contraction of the vessels of the 

 iris, while Gaskell ascribes it to an inhibitory action on the sphincter. However, 

 the dilatation of the pupil is not synchronous with the vascular contraction. 

 Irritation near the center of the cornea causes contraction of the pupil. 



Pathological. Imperfect contraction of the pupil on illumination of the 

 eyes may be caused: (i) By a lowered sensibility of the retina (loss of sensory 

 reflex), or (2) by paralysis of the pupillary oculomotor fibers (loss of motor reflex), 

 or (3) both may be combined. Such conditions have also been designated by 

 the badly chosen term reflex immobility of the pupil. The remarkable cases of 

 so-called paradoxical light-reaction exhibit dilatation of the pupil upon stimula- 

 tion by light, perhaps as the result of profound exhaustion of the oculomotor, which 

 is soon paralyzed by the light-stimulation. 



ENTOPTIC PHENOMENA. 



SUBJECTIVE OPTICAL MANIFESTATIONS. 



The designation entoptic phenomena is applied to those that depend 

 on the perception of objects that are present in the eye itself Subjective 

 visual sensations are those that are not produced by the normal, homol- 

 ogous stimulation of the retina by light, but by internal, heterologous 

 (mechanical, electrical, somatic) stimuli, which act upon the eye, the 

 optic nerve, or parts of the central organs 



Among entoptic phenomena are: 



i. Shadows of various opaque bodies thrown on the retina. They may be 

 recognized by the following method : the small image of a light is thrown upon a 

 pasteboard screen by means of a convex lens ; a fine hole is pricked through the 

 image of the flame and the eye is placed on the other side of the screen, so that the 



.uminated point coincides with the anterior focal point of the eye (about 13 

 mm from the cornea). As the rays from this point pass parallel through the 



cular media, a diffusely illuminated field is formed, which is surrounded by the 

 dark outlines of the pupillary margin. All dark objects that intercept these 

 rays throw shadows on the retina and appear as spots (Fig. 291). Several varieties 

 of these shadows can be distinguished: (a) The muco-lacrimal spectrum, especi- 

 \l -u n lld - mar g ms > arising from particles of mucus, fat-globules from the 

 Meibomian glands, dust mixed with tears. These give rise to striated, nebulous 

 or drop-like shadows, which are dissipated by winking. (6) Pressure on the 

 cornea with the finger produces wrinkled shadows, due to temporary corneal 

 pressure-folds, (c) Bead-like or dark specks, light and dark stellate figures, 

 ormer arising from deposits on and within the lens, the latter from the stellate 

 structure of the lens, (d) The mouches volantes (muscae volitantes) , like strings of 

 beads, circles, groups of tiny balls, or pale threads, are images of small, opaque par- 

 tic j*f m the V1 treous cells, broken-down cells, granular fibers. They move about on 

 sudden movement of the eye. Listing showed that it is possible to determine the 

 approximate position of these objects. If the source of light (the illuminated open- 

 u 1 t! S aised and lowered, those shadows that retain their relative position in the 

 bright field of vision are due to objects at the plane of the pupillary orifice (2) ; 

 that move apparently in the same direction as the light are due to bodies in 

 the pupillary plane (i) ; those, however, that move in the opposite direc- 



