'26 



HANDBOOK OF PHYSIOLOGY. 



pupil is dilated. This will materially facilitate the examination ; but it is 

 quite possible to observe all the details to be presently described without the use 

 of this drug. The room being now darkened, the observer seats himself in front 

 of the person whose eye he is about to examine, placing himself upon a some- 

 what higher level. A brilliant and steady light is placed close to the left ear 

 of the patient. The atropia having been put into the right eye only of the pa- 

 tient, this eye is examined. Taking the mirror in his right hand, K and looking 

 through the central hole, the operator directs a beam of light into the eye of 



Fig. 429. Diagram to illustrate the action of the Ophthalmoscope, when a plane concave 

 glass is used. c. observer's eye. The light reflected from any point, d, on retina of a, would 

 naturally be focussed at e; if the lens b is used it would be focussed at /, in other words, at 

 back of c. The image would be enlarged, as though of g, and would be inverted. (After Mc- 

 Gregor Robertson.) 



the patient. A red glare, known as the reflex, is seen ; it is due to the illumi- 

 nation of the retina. The patient is then told to look at the little finger of the 

 observer's right hand as he holds the mirror ; to effect this the eye is rotated 

 somewhat inward, and at the same time the reflex changes from red to a 

 lighter color, owing to the reflection from the optic disc. The observer now 

 approximates the mirror, and \vith it his eye to the eye of the patient, taking 

 care to keep the light fixed upon the pupil, so as not to lose the reflex. At a 

 certain point, which varies with different eyes, but is usually when there is an 

 interval of about two or three inches between the observed and the observing 

 eye, the vessels of the retina will become visible as lines running in different 

 directions. Distinguish the smaller and brighter red arteries from the larger 

 and darker colored veins. Examine carefully the fundus of the eye, i.e., the 

 red surface until the optic disc is seen ; trace its circular outline, and observe 



a 



Fig. 430. Diagram to illustrate action of ophthalmoscope when a bi -convex glass is used. 

 The fig. d on retina of a is under ordinary conditions focussed at / and inverted. If the lens 6 

 be placed between eyes, the image h is seen by the eye c as an enlarged image. (After McGregor 

 Robertson. ) 



the small central white spot, the porus opticus, physiological pit : near the 

 centre is the central artery of the retina breaking up upon the disc into branches ; 

 veins also are present, and correspond roughly to the course of the arteries. 

 Trace the vessels over the disc on to the retina. The optic disc is bounded by 

 two delicate rings, the more external being the choroidal. while the more in- 

 ternal is the sclerotic opening. Somewhat to the outer side, and only visible 



