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 subserve 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 /amina 
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 
