580 THE NERVOUS SYSTEM. 



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cernible. The velocity of the flow of blood in the capillaries of 

 the retina is from 0.5 mm. to 0.9 mm. per second. 



Intra-ocular Images. In addition to the blood-vessels and 

 blood-corpuscles, other objects within the eye may throw shadows 

 upon the retina ; indeed, any opacity iii the media of the eye 

 through which the rays of light pass would do this, as, for in- 

 stance, the muscce volitantes. These are little bodies floating in 

 the vitreous, which are supposed to be the remains of cells or 

 fibers which exist during fetal life, and which have not become 

 converted into the vitreous humor, as have most of the cells and 

 fibers. They assume various shapes in different individuals, but 

 the shape is invariable in the same person. They may appear 

 as a string of beads, or in the form of streaks or granules. 



The Ophthalmoscope. This is an instrument by means of which 

 one person can examine the eye of another and obtain a view of 

 the retina. Inasmuch as some of the rays of light which enter 

 the eye and fall upon the retina are reflected from the surface and 

 are brought to a focus again at the source of illumination, it is mani- 

 fest that without some special device it would be impossible to see 

 the image which these reflected rays make, for to have the eye of 

 the observer in the path of these reflected rays would cut off the 

 light which caused them. To overcome this obstacle, Helmholtz 

 devised the ophthalmoscope, which consisted of several plates of 

 glass, one upon another (Fig. 357), that reflect rays of light from 

 a lamp or other source of illumination into an eye to be examined ; 

 these rays illuminate the retina, and those that are reflected issue 

 from the observed eye and are brought to a focus on the retina of 

 the observer. The observed eye and that of the observer are con- 

 sidered to be emmetropic and in a condition of negative accommo- 

 dation or accommodative rest. A reference to Fig. 357 will render 

 this explanation clearer. At the present time glass plates are not 

 used, but in their place a concave mirror, with an opening in it, 

 through which the observer can look. There are two methods of 

 using the ophthalmoscope : the direct (Fig. 358) and the indirect 

 (Fig. 359). These will be readily understood after an examination 

 of the illustrations and their respective legends. 



It is customary, though not absolutely necessary, before making 

 an ophthalmoscopic examination to drop into the eye a solution 

 of atropin of a strength of two grains to the ounce. This paralyzes 

 the accommodation and dilates the pupil. The examination is 

 conducted in a dark room. 



The illuminated retina produces a red glare, the reflex, which, 

 as the observed turns his eye slightly inward, becomes lighter 

 in color, because of the white surface, the optic disc, from which 

 the light is reflected when the eye is in this position ; in its center 

 is the porus opticus, with the arteria centralis retince, and radiating 

 from this are its branches ; veins also are seen. The macula lutea 

 and thefovea centralis may likewise be discerned. 



