92 



THE FUNDUS OCULI OF BIRDS 



one-third the length of the disc above and two- 

 thirds to its outer side, is seen the second or 

 temporal macular region. At its center is a 

 light, gray spot surrounded by a dark brown 

 area. Both these arese are in their turn 

 encircled by a small, pale-green, reflex ring 

 about one-fourth the diameter of the corre- 

 sponding circle about the superior macula. 

 Slender, opaque nerve fibers run out at right 

 angles to the disc on both sides. 



Red-tailed Hawk. Buteo borealis. The 

 optic-nerve entrance in the individuals examined 

 by the writer is almost entirely covered by the 

 pecten, except at its superior end, which can be 

 outlined and examined by the mirror. It lies 

 (obliquely) at an angle of about 70 degrees with 

 the upper mandible, and can be seen through 

 the more or less translucent pecten. The back- 

 ground generally has a striated appearance 

 with the choroidal vessels showing through. 



Little Bustard. Tetrax tetrax. Plate 

 XXXVI, page 155, reproduces the left fundus 

 oculi, direct image, of this species. The 

 fundal coloration is mostly a warm gray, 

 covered with small, dull orange-red dots. In 

 the lower half of the field and on each side 

 of the optic nerve entrance is a number of 

 choroidal blood-vessels of a dull orange-red 

 color, running parallel with the disc. 



So far as seen, the papilla is a dead white ; 

 optic fibres form a fringe-like setting about it. 

 The central zone of the disc is covered by a 

 mass of minute, brown, pigment deposits of 

 irregular shape. Opaque nerve fibres extend 

 at right angles to the disc on either side. 



A little to the temporal side, about one 

 disc-length from the upper end of the optic 

 nerve, is a small, brown spot like a tiny hole. 

 This macular area is surrounded by a brilliant, 

 green reflex ring. 



The pecten, of a dark chocolate-brown color 

 and shaped like a corkscrew, comes well for- 

 ward towards the inferior border of the lens. 



The fundus of the Great Bustard — Otis 

 tarda — closely resembles that of 0. tetrax. 



Strigiformes 



Tawny Owl. Syrnium aluco. (Figs. 132 

 and 96). The ocular background of this 

 species shows, macroscopically, a solitary, 



temporal, macular region and fovea. This 

 single macular area is about a disc length and 

 a half above and to the temporal side of the 

 smaller or superior end of the pecten. 



The irregular shape of the eyeball and fun- 

 dus in the Owl must not, in this connection, 

 be forgotten. The comparatively undersized 

 pecten exhibits 14 convolutions (four large 

 and of equal size, ten unequally smaller) that 

 when viewed from above look like a small 

 bunch of raisins. Between the folds may 

 be seen the margins of the optic nerve en- 

 trance. The crest surmounting the upper free 

 border is very thin, inconspicuous and unde- 

 veloped. 



The ophthalmoscopic view of the normal 

 fundus (the result of examination of numer- 

 ous individuals) is shown in Plate XXXVII, 

 page 157. 



On the outer aspect of the papilla and 

 about one disc-length from it, on a level with 

 the upper margin of pecten, is a slight de- 

 pression {macula) almost devoid of cho- 

 roidal vessels. Its centre is a dark red color, 

 the whole macular region closely resembling 

 the same area in the (brunette) human eye. 

 On the margins of this depression and sur- 

 rounding it is a bright circular reflex. Head 

 found this reflex to be so bright and dense in 

 some individuals that it was difficult to see 

 the other macular details, although, in most 

 instances, by shifting slightly the reflected 

 rays from the mirror the macular margins 

 come out distinct and sharp. For some dis- 

 tance outside this foveal ring the eyeground 

 is seen as if one were viewing it through a 

 translucent, grayish mist or haze; and the 

 fundus thus seen generally appears to be 

 granular with a few, small choroidal capillaries 

 showing here and there. Inside the ring, 

 however, the details are strikingly clear, and 

 the coloration bright red. Above the macula, 

 on the outer side of the disc, is a quantity of 

 dark pigment deposited between the cho- 

 roidal vessels, by contrast making the latter 

 stand out as of a bright orange color. The 

 inner aspect of the fundus appears yellow; it 

 exhibits numerous choroidal blood-vessels, 

 whose yellow tint is distinctly brighter near 

 the inferior segment of the disc. 



