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HANDBOOK OF PHYSIOLOGY. 



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 ciicular outline, and observe the 

 small central white spot, the porus opticus, or 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, whilst the more internal is the sclerotic opening. Some- 

 what to the outer side, and only visible after some practice, is the yellow 

 spot, with the small lighter-colored fovea centralis in its centre. This 

 constitutes the direct method of examination; 

 by it the various details of the fundus are seen 

 as they really exist, and it is this method which 

 should be adopted for ordinary use. 



If the observer is ametropic, i. e , is myopic 

 or hypermetropic, he will be unable to employ 

 the direct method of examination until he has 

 remedied his defective vision by the use of pro- 

 per glasses. 



In the indirect method the patient is placed 

 as before, and the operator holds the mirror in 

 his right hand at a distance of twelve to eighteen 

 inches from the patient's right eye. At the same 

 time he rests his little finger lightly upon the 

 temple, and holding the lens between his thumb 

 and forefinger, two or three inches in front of 

 the patient's eye, directs the light through the 

 lens into the eye. The red reflex, and subse- 

 quently the white one, having been gained, the 

 operator slowly moves his mirror, and with it his 

 eye, towards or away from the face of the pa- 

 tient, until the outline of one of the retinal ves- 

 sels becomes visible, when very slight movements 

 on the part of the operator will suffice to bring 

 into view the details of the fundus above de- 

 scribed, but the image will be an inverted one. 

 The lens should be kept fixed at a distance of two 

 or three inches, the mirror being alone moved 

 until the disc becomes visible: should the image 

 of the mirror, however, obscure the disc, the lens may be slightly tilted. 



FIG. 410. The ophthalmo- 

 scope. The small upper mir- 

 ror is for direct, the larger for 

 indirect illumination. 



Visual Purple. The method by which a ray of light is able to 

 stimulate the endings of the optic nerve in the retina in such a manner 

 that a visual sensation is perceived by the cerebrum is not yet under- 

 stood. It is supposed that the change effected by the agency of the 

 light which falls upon the retina is in fact a chemical alteration in the 

 protoplasm, and that this change stimulates the optic nerve-endings. 

 The discovery of a certain temporary reddish-purple pigmentation of the 

 outer limbs of the retinal rods in certain animals (e. g., frogs) which 

 have been killed in the dark, forming the so-called visual purple, ap- 



