ELEVENTH ANNUAL REPORT. 163 



strikes the retina, the iris contracts, only to enlarge a moment 

 later, and the whole optical fundus, with the delicately pigmented 

 choroidal coat, veined with its myriad of blood vessels, becomes 

 plainly visible. Below the level of the pupil and to the nasal side 

 may be seen the curious fan-like pecten, throbbing rhythmically, 

 its black tissue crumpled into a half dozen folds. We know 

 nothing positively concerning its function, but judging from its 

 elastic nature, and the frequent enlargement and contraction as the 

 flow of blood fills it or flows back, considerable weight is lent to 

 the theory that it functions in accommodation — in pressing for- 

 ward toward the lens and perhaps aiding in changing the angle of 

 refraction of the vitreous humor, thus altering the shape and 

 consequently the focus of the eye. 



In the snowy owl at least, there seems to be a certain amount of 

 regular pulsation — about twenty-four to the minute. A few 

 seconds after the ophthalmoscopic beam of light is first flashed 

 into the eye, a sudden tremor or twitching may be observed 

 throughout the entire structure of the pecten — as of a spasmodic 

 effort to readjust the eye to the suddenly altered condition of 

 illumination. If this be its most important function there is no 

 reason why the pecten may not also aid in supplying the retina 

 and vitreous humor with nutriment. Judging by analogy with 

 other organs of the body, such a specialized structure as this may 

 well svibserve one or more secondary as well as a primary function. 



It is interesting to notice what a pronounced difference exists in 

 the pigmentation of the choroid of a light and a dark plumaged 

 owl. In the snowy owl the choroidal vessels are so conspicuous 

 that the scanty pigmentation of the background is subordinated. 

 In the great horned owl the pigmentation is much more abundant 

 and darker, while the dusky horned owl shows the ramifying lines 

 of scarlet choroidal vessels sharply silhouetted against the dark 

 background of pigment. 



The view which we get of the pecten in an ophthalmoscopic 

 examination of the eye of a living owl, is directly anterior and 

 shows no trace of the basal attachment except at the ends. At the 

 upper end is plainly visible the edge of the disk of the optic nerve 

 — a white plate-like structure clearly differentiated from the sur- 

 rounding pigment and choroidal vessels. Upon the white sur- 

 face of the nerve-disk, a myriad of tiny dots represents the laiiiina 

 cribrosa: otherwise it is immaculate. Radiating out from the 

 optic disk, like slender teased threads, and especially conspicuous 

 in the snowy owl, are many opaque nerve fibres. These nerve 

 fibres are visible for only a short distance, becoming transparent 



