32 



THE FUNDUS OCULI OF BIRDS 



and not the vessel-wall, which is visible. In 

 the larger retinal vessels the blood-column in 

 the arteries is brighter than that in the veins. 

 In the smaller branches this difference is less 

 marked. The brighter color of the arteries 

 is due to the presence of a central streak of 

 light, which is less marked in the veins. The 

 cause of this light-streak is not definitely 

 known. The retinal artery, under normal 

 conditions, rarely pulsates, but venous pulsa- 

 tion occurs spontaneously in from 60 to 75 

 per cent of normal eyes. 



Besides the blood-vessels enumerated 

 above, it is necessary to mention the cilio- 

 retinal vessels. These are commonly small, 

 solitary vessels which arise from the circle 

 of Haller, and emerge at the temporal border 

 of the disc. Such a vessel may come from the 

 central vessel in the substance of the nerve, 

 and may be of larger size. Generally it sup- 

 plies blood to a small area between the disc 

 and macula. Cilio-retinal vessels are present 

 in from 10 to 16 per cent of normal eyes. 



The macula lutea ("yellow spot" of Soem- 

 mering) is situated about 3 mm. to the outer 

 side of the optic-nerve head, and slightly be- 

 low the horizontal meridian. It is a spot 

 darker than the surrounding retina and 

 apparently devoid of blood-vessels. It is the 

 area of greatest visual acuity. The centre 

 of the macula presents the foveal reflex, while 

 the periphery shows a whitish, glistening 

 ring, or halo, known as the macular reflex. 

 Differences in the appearance of the macula 

 in individuals are doubtless due to several 

 (mostly domestic) causes : to the difference in 

 methods of examination; to differences in the 

 age, complexion, race, and refraction, as well 

 as to variations in the distribution of pigment. 



There are several forms of (ophthalmoscop- 

 ically visible) macular rings. Lindsay Johnson 

 states that the most common is a bright, 

 scintillating reflex resembling shot-silk, very 

 marked in dark eyes, scarcely visible in fair 

 ones, and best seen with feeble illumination. 

 This ring is supposed to be due partly to 

 reflection from Miiller's fibres, where they 

 expand into the internal limiting membrane, 

 and partly to the fibrous sheaths of the vessels 

 which lift up the retina over-lying them. 



The foveal reflex is found in the centre of the 

 macula as a very small ring, or as a circular or 

 horseshoe-shaped spot of light, or as a "comet- 

 flare." It is due to reflection of the edge of 

 the fovea. 



The choroid. While each ocular tunic con- 

 tributes something to the highly colored oph- 

 thalmoscopic picture, the chief part must be 

 credited to the choroid. Light reflected from 

 the mirror of the ophthalmoscope passes 

 through the transparent part of the retina 

 to the pigment epithelium, and is partly 

 absorbed, partly reflected. Although the 

 pigment layer belongs embryologically to the 

 retina, it generally adheres to the retinal 

 surface of the choroid, and is accredited oph- 

 thalmoscopically to the latter tunic. The 

 brightness of the fundus picture depends on 

 the amount of pigment. The greater the 

 pigment, the greater the absorption of light 

 and the darker the fundus picture. In the 

 negro and the native of India the fundus is 

 of a brownish, brown-red, or slate color, while 

 in the Anglo-Saxon, and particularly in 

 blondes, it is of a bright-red color. If the 

 pigment layer is very thin, the choroidal ves- 

 sels are correspondingly exposed and are seen 

 as a network of large, flat vessels, without a 

 light-streak, between which are spaces of light 

 or dark color. They are seen best in albinos. 

 It is generally impossible to differentiate 

 between the choroidal arteries and veins, 

 although at the equatorial region the latter 

 converge to form the venae vorticosse. In 

 brunettes the vessels appear as " light streams 

 separated by dark islands," because the spaces 

 are more deeply colored than the vessels. 



The sclera, which may be spoken of as the 

 panel on which the fundus picture is painted, 

 is commonly invisible, being covered by the 

 nearly opaque choroid. Yet it is probable 

 that in all eyes some light passes through the 

 choroid, and thus the sclera has some in- 

 fluence on the ophthalmoscopic picture, serv- 

 ing to make it lighter. In albinos the sclera 

 appears as a white surface between the cho- 

 roidal vessels. It is best seen where the 

 choroid is absent, as in coloboma, or patho- 

 logically as a result of destruction of the 

 retina and choroid. 



