38 



THE FUNDUS OCULI OF BIRDS 



Gardens of the London Zoological society the 

 ophthalmoscopic appearances of several hun- 

 dred avian and other eyes. The details of 

 these fundus views, as revealed by the oph- 

 thalmoscope in many species, are described 

 and depicted in Chapter X and elsewhere. 



A. The Avian Eyeground in General. 



The average eyeground or fundus oculi of 

 most Day Birds resembles, as much as any- 

 thing, the texture of the so-called "scotch 

 mixtures" in smooth finished cloth — usually 

 light brown, gray, gray-blue, blue mixed with 

 striate rays, or fine concentric marking of 

 lighter gray or white. Scattered over this 

 background are numerous yellowish, yellow- 

 white, brown or gray points of pigment. 

 Although this matter has not yet been satis- 

 factorily determined yet these punctate de- 

 posits are, in part at least, the colored oil 

 droplets described in Chapter IV. 



Nocturnal Birds have, almost invariably, 

 yellow-red, orange, orange-red or reddish 

 brown fundi, with the choroidal vessels plainly 

 visible through the semitransparent retina. 

 Some of the Owls present almost a scarlet 

 vermilion eyeground, and this intensity of 

 colors appears to be peculiar to Strigiformes. 

 For this reason alone might say with confi- 

 dence that the New Zealand Owl Parrot 

 (Stringops habroptilus) belongs to this order 

 and not to Psittaciformes or to Cuculiformes 

 in one of which he is commonly placed. 



Ophthalmoscopy of the areas of distinct 

 vision in Birds supplements the macroscopic 

 and microscopic examination. In many in- 

 stances it affords a better idea of the relations 

 of these arese than does any other means of 

 observation; at least this is true of most avian 

 fundi explored by the writer. As a rule the 

 deep, single fovea and its surrounding macular 

 region are, other conditions being favorable, 

 readily observed by means of the mirror, and 

 their locality, comparative size, component 

 parts, coloration, etc., easily depicted. Con- 

 trariwise, a shallow or organically ill-defined 

 "yellow spot" is not so readily made out. 

 Band-like areas are also better defined by the 

 aid of a magnifying glass and in prepared 

 half-eyes, although in the fundus of the 

 Secretary Bird (Plate XXXI), of the White- 



bellied Sea Eagle (Plate XXXIV) and of a 

 number of other bimacular species the ribbon 

 of tissue is well shown by the mirror. 



With, perhaps, this exception the macro- 

 scopic description of the arese centrales given 

 not only in the lists of Chievitz and Slonaker 

 but in Chapter VII corresponds in a remark- 

 able fashion with the ophthalmoscopic find- 

 ings described and depicted in this work. 

 Nothing has been discovered in this study of 

 the avian fundus by the aid of the mirror that 

 is likely to add to or subtract from the classi- 

 fication of the types of central vision sug- 

 gested by the writer in the same Chapter. 



The Pecten and Optic Disc 



Viewed ophthalmoscopically the avian mar- 

 supium or pecten exhibits three fairly definite 

 varieties which may be classified as follows: 



I. Those pectens whose mass uniformly 

 springs from and equally covers the optic disc. 

 Such pectinate bodies do not extend into the 

 vitreous cavity farther than the length (often 

 less) of their widest segment, and they are 

 sessile on the face of the papilla. Examples of 

 this form of pecten are seen in Nycticorax 

 nycticorax (Fig. 45), Haliaetus leucocephalus 

 (Fig. 56), Caprimulgus europeus (Fig. 65), 

 Strixflammea (Fig. 59), Serpentarius (Fig. 52), 

 Struthio camelus (Fig. 24), Cancroma cochle- 

 aria (Fig. 46), Casuarius occipitalis (Fig. 26), 

 Spheniscus demersus (Fig. 37), Tinnunculus 

 alaudarius (Fig. 55), Calodromas elegans (Fig. 

 28) and Syrnium aluco (Fig. 58). This method 

 of arranging the pectinate tissues makes pro- 

 vision for a blood reservoir large enough to 

 supply nutritional needs but so placed that it 

 does not materially obstruct visual or light 

 rays. Such a disposition and configuration 

 of the fundal organs meet the needs of many 

 species, among them Eagles and Owls, who 

 require (and have) very acute vision. 



II. Some pectens, while they originate from 

 the whole surface of the optic disc, immediately 

 slope away from the visual axis, approach the 

 bulbar wall and terminate without projecting 

 far into the vitreous. In this type there is 

 generally a disc-length or more between the 

 free terminal of the pecten and the posterior 

 surface of the crystalline lens. 



