94 



THE FUNDUS OCULI OF BIRDS 



Burrowing Owl. Spcotyto cunicularia 

 hypogoea. The ocular fundus of this species is 

 irregularly round, as in all the Owls, and 

 in prepared specimens the details show dis- 

 tinctly. 



The single, temporal, oval macular region 

 lies above and about a disc length and a half 

 from the upper end of the papilla. In the 

 center of the macula is the fovea — a dark 

 pigmented spot with fine granules arranged 

 cap-like above it. Outside this, again, is the 

 ovoid circumference of this region, incom- 

 pletely edged with fine dots. These are more 

 numerously distributed below the macula 

 than above it. Connected with the macular 

 region is a light colored and rather broad band 

 that extends horizontally to the center of the 

 visible background. It is unevenly divided 

 into two strips by a parallel arrangement of 

 minute pigment dots. 



Seen from above the relatively small pecten 

 closely resembles a disarticulated, acuminate 

 leaf, the stem representing the spinous pro- 

 jection immediately above the lowest terminal 

 convolution. The light double folds of the 

 marsupium slope backwards and cover most 

 of the optic entrance; they meet above in a 

 uniform, very narrow, slightly undulating 

 crest whose posterior end projects half the 

 height of the unlying coil well into the vitre- 

 ous cavity. An extension upwards of the 

 long axis of the disc cuts the retinal band at 

 the junction of the inner and second fourth, 

 making an infulapapillary angle of, perhaps, 

 40°. 



This interesting Owl is especially subject, 

 like other Strigiformes, to pathological varia- 

 tions in the fundus picture due to confinement 

 and domestication. Both Head and the 

 writer examined a number of individuals that 

 undoubtedly exhibited choroidal disease and 

 other pathological changes. Rejecting these, 

 the general color of the fundus of this species is 

 found to be dull-orange, mottled and blotched 

 in its upper half with orange-red. Choroidal 

 vessels are plainly seen, covering all the lower 

 part of the eyeground, just as in the Tawny 

 Owls. The well defined macular area is seen 

 within the outer half of the fundus, a little 

 above the upper extremity of the optic disc. 



It is distinguished from the surrounding cho- 

 roid by a collection of minute pigment 

 granules or dots with a bright, white spot in 

 their centre. 



The optic disc is white and of oblong shape, 

 slightly rounded at the ends. From its edges 

 run a few, short nerve fibres that form a com- 

 plete fringe about the visible papilla. 



The pecten is decidedly larger in proportion 

 to bodily measurements than one finds it in 

 most of the larger Owls, especially larger than 

 in the Tawny Owl. It extends well forward 

 into the vitreous, and its lower half appears 

 very massive and of a dark brown color. The 

 pectinate convolutions are plainly seen and 

 the anterior or upper half is more delicate in 

 structure, being perforated where it joins the 

 disc. Here it forms a dark network on the 

 surface of the nerve-head where, also, a few 

 red granules mingle with the chocolate-brown 

 texture of the pecten. 



Snowy Owl. Nyctea nyctea. It is a popu- 

 lar belief, and one held in many public 

 aviaries, that these owls are likely to become 

 blind if much exposed to daylight. Conse- 

 quently they are often caged in darkness or 

 semi-darkness, although their habits are really 

 diurnal, or at least they are as much Day 

 birds as Night birds. 



In any event the changes in habitat in- 

 volved in their captive surroundings are, in 

 the judgment of the writer, responsible for 

 certain alterations found in their fundi and 

 in those of other Bubonidae kept for more 

 than a year or two in public gardens. 



The normal, dominant coloration of the 

 fundus is very light yellow-red, but de- 

 cidedly less red than in Syrnium aluco. See 

 Plate XXXVII, page 157, of this Atlas. In 

 the first individual (two years in captivity) 

 examined there were throughout the left back- 

 ground extensive choroidal changes, especially 

 one cruciform, pigmented, partially atrophic 

 area running across the field towards the 

 nasal side. These long, striated alterations 

 were also seen in the temporal part of the 

 fundus. 



The large, club-shaped pecten springs from 

 a well defined whitish, ovoid disc. The mar- 

 gin of the papilla is occupied by numerous 



