7 62 ENTOPTICAL PHENOMENA. 



apertures seen on the scale outside indicates the diameter of the pupil in lOOths of an inch. 

 Thenu8ir g Ua!saMo^^, it is assumed that the size of the pupil gives an index of the 

 intensity of the amount of light which influences the diameter of the pupil.] 



^aicular Pressure. -The movements of the iris are always accompanied by variations of 

 the intraocular pressure. The muscles of the iris affect the intraocular pressure in that the 

 dilatation of the pupil increases it, while contraction of the pupil diminishes it. The increased 

 or din nished tension can be felt when two fingers are pressed on the eyeball Stimulation ot 

 the m bathetic increases, while its section diminishes the pressure. Action of Drugs. -Atropin 

 dropped into the eye, after producing a short temporary diminution of the tension^ increases it; 

 eserin, after a primary increase, causes a diminution of the pressure (Graser and Holzke). 



393. ENTOPTICAL PHENOMENA. Entoptical phenomena depend upon 

 the perception of objects present within the eyeball itself. 



1 Shadows are formed upon the retina by different opaque bodies. In order to see them in 

 one's own eve, proceed thus : By means of a strong convex lens project a small image of a flame 

 npon a paper screen, prick a small opening through the image of the flame, and place one eye 

 a the other side of the screen, so that the illuminated puncture lies in the anterior focus of the 

 eye i t , about 13 mm. in front of the cornea. As the rays proceeding from this point .pass 

 liraliel through the media of the eye, a diffuse bright field of vision, surrounded by the black 

 margins of the iris, is obtained. All dark bodies which lie in the course of the rays of light 



Fig. 547. 

 Entoptical Shadows. 



throw a shadow upon the retina, and appear as specks. There are various forms of these shadows 

 (fig. 547) :- 



(a) The spectrum mucro-lacrimale, especially upon the margin of the eyelids, depending 

 ui>on particles of mucus, fat globules from the Meibomian glands, dust mixed with tears, causing 

 cloudy or drop-like retinal shadows, which are removed by winking. 



(b) Folds in the cornea. If the cornea be pressed laterally with the finger, wrinkled shadows, 

 due to temporary wrinkles in the cornea, are produced. 



(c) Lens' shadows. Bead-like or dark specks, bright and star-like figures, the former due to 

 de|>osits on and in the lens, the latter to the radiate structure of the lens. 



(d) Muse volitantes (DccJiales, 1690), like strings of beads, circles, groups of balls or pale 

 stripes, depend upon opaque particles (cells, disintegrating cells, granular fibres) in the vitreous 

 humour. They move about when the eye is moved rapidly. Listing (1845) showed that one may 

 determine pretty accurately the position of these objects. Whilst making the observation upon 

 one's own eyes, raise or depress the source of light ; those shadows which are caused by bodies 

 on a level with the pupil retain their relative positions in the bright fields of vision. Shadows 

 which appear to move in the same direction as the source of light are caused by bodies which 

 lie in front of the plane of the pupil those, however, which appear to move in the opposite 

 direction depend upon objects behind the plane of the pupil. 



2. Purkinje's figure (1819) depends upon the blood-vessels within the retina, which cast a 

 shadow upon the most external layer of the retina, viz., upon the rods and cones, these being 

 the parts acted upon by light. In ordinary vision we do not observe these shadows. Accord- 

 ing to v. Helmholtz, this is due to the fact that the sensibility of the shaded parts of the retina 

 is greater, and their excitability is less exhausted, than all the other parts of the retina. As 

 soon, however, as we change the position of the shadow of the blood-vessels, instead of being 

 directly behind, so that the blood-vessels come to lie more laterally and behind them, i.e., upon 

 places which do not receive shadows from the blood-vessels when the rays of light pass through 

 the eye in the ordinary way, then the figure of the blood-vessels becomes apparent at once. All 

 that is necessary is to cause the light to enter the eyeball obliquely. Method. (1) This may 

 be done by passing an intense light through the sclerotic, e.g., by throwing upon the sclerotic 

 a small, bright, luminous image from a source of light. On moving the source of light, the 



