106 



THE FUNDUS OCULI OF BIRDS 



American Mockingbird. Mimus poly- 

 glottos. The ophthalmoscope shows the pre- 

 valent coloration of the fundus in this species 

 to be a clear, blue-gray. Thickly scattered 

 over the background are irregularly shaped, 

 whitish-gray dots or granules that in the 

 upper sectors of the field give the fundus an 

 unusually bright gray appearance. 



Near the upper extremity of the optic 

 papilla and towards the nasal side is the 

 macula. This area is darker blue than its 

 surroundings, while its centre (the fovea) is 

 marked by a bright white dot which, again, is 

 enclosed in a green reflex. This greenish 

 reflex-ring has a striated appearance, the 

 striae being quite translucent and convergent 

 towards the fovea centralis. 



The optic disc is long and narrow; the visible 

 portion is brilliant white and its margins are 

 traceable on either side the whole length of 

 the pecten. Radiating from the boundary 

 of the nervehead are many long, opaque 

 nerve fibres that extend over the eyeground 

 to its periphery. 



The pecten is, as usual, of a chocolate-brown 

 color; it is long, somewhat club-shaped and 

 extends well behind the margin of the iris, 

 so that its outlines are not entirely visible by 

 the mirror through the undilated or only 

 partially dilated pupil. 



The fundus of the Cuban Mocking Bird 

 (M. orpheus) is similar in every respect to 

 that of Mimus polyglottos. 



Bluebird. Sialia sialis. This was one 

 of the earliest species examined ophthalmo- 

 scopically by the writer, and he was so struck 

 by the wonderful blue coloration of the ocular 

 background that the question arose in his 

 mind as to whether there might not be some 

 connection between plumage tints and fundus 

 coloring. That query was soon answered in 

 the negative but the beautiful blue shades 

 in the eyeground of Sialis are still to be 

 admired. 



As seen in Plate LI, the ophthalmoscope 

 shows that the general blue-gray coloring of 

 the eyeground is modified by numerous, pin- 

 point dots. The punctate deposits near the 

 periphery assume a more or less dull-golden 

 or orange tint, a condition also noticed just 



below the pecten. The elongated, ovoid 

 optic disc is white (quite woolly at its margins) 

 and covered almost entirely by the convo- 

 luted pecten, which is very large when com- 

 pared with the size of the eye. A portion of 

 the unusually extensive pecten cannot be 

 seen through the small pupil. It is of the 

 usual dull-brown color, and has "frilled" 

 borders. On the inner or nasal side and near 

 the lower part of the optic disc is an orange- 

 red area, containing chiefly choroidal vessels 

 with black pigment grains between them. 

 This red area extends a little to the temporal 

 side of the disc, subsequently merging into 

 the dull orange color below. On either side 

 of the disc and radiating from it, are numer- 

 ous filiform nerve fibres which cross the back- 

 ground almost to its periphery. In an area 

 a disc-length to the nasal side of the papilla, 

 the white dots of the fundus are fainter and 

 much closer together than elsewhere; and in 

 this region, also, there is a reflex plainly 

 marked. Here, if in any part of the fundus, 

 is the fovea or macula, but it is not sufficiently 

 well defined to be certain of it. 



Figures 106 and 140 demonstrate quite con- 

 clusively that in the fundus of Sialia sialis 

 there are two macular areas, symmetrically 

 placed above and on either side of the pointed 

 end of the papilla. The area nasalis is removed 

 about half a disc length; the fovea temporalis is 

 somewhat more distant from the same point. 

 The nasal macular region is larger and more 

 plainly marked; it is surrounded by a dotted 

 ring, incomplete above; the corresponding 

 foveola is represented by a black spot crowned 

 by pigment grains. More than half the inter- 

 vening zone is lightly strewn with minute, 

 dark granules. 



The heavily -built pecten is composed of 19 

 thick, rounded, double folds — or 38 con- 

 volutions. A comparatively massive, paddle- 

 shaped crest surmounts these and extends 

 along the whole of the upper free border. 

 The major axis of the papilla makes with a 

 line joining the two foveolse an infulapapillary 

 angle of about 70°. There is nothing to be seen 

 of the nerve head from above. 



Wheatear. Saxicola cenanihe. The color 

 reproduction shown in Plate LII, page 171, 



