EYESHINE; THE TAPETUM FIBROSUM 231 



Wherever special tapeta have been constructed for reflecting light 

 back through the visual cells, they are most often located in the chorioid 

 coat just behind the retina; but they may be retinal, placed in the pig- 

 ment epithelium of the retina itself. 



The light reflected from a chorioidal tapetum, of either the 'fibrosum' 

 or 'cellulosum' type (v, /".), is ordinarily seen only if the observer is 

 stationed beyond the animal's near point. With large animals which 

 have little or no accommodation, this means not closer than from eight 

 to twenty feet. The light is always colored though unsaturated, some- 

 times so greatly as to appear almost white; and the hue may be situated 

 practically anywhere in the spectrum except in the violet. The color may 

 vary within a species or even, from moment to moment or from day to 

 day, in the same individual. Such variations are unquestionably due to 

 fluctuations in the amount of blood in the choriocapillaris, in the amount 

 of rhodopsin present, etc., through which the light reflected by the tape- 

 tum must pass to escape again from the eye. The fundamental color 

 thrown back from a chorioidal tapetum owes its hue to the interference 

 of light, for it is a surface color like that of a beetle's wing-cover, a 

 parrot's feather, or a film of oil floating on water. The hue depends upon 

 the microscopic dimensions of the reflective elements and has no biolog- 

 ical significance as far as one can tell. 



Retinal tapeta usually appear pure white ophthalmoscopically, though 

 the eyeshine of crocodiUans is said to be pinkish-orange (and extremely 

 brilliant in Caiman sclerops) . Didelphis virginiana is also described as 

 having a tinted (orange) reflex. With retinal tapeta, the glow can still 

 be seen when one is very close to the animal — less than a foot, in croco- 

 diUans, if one cares to go that close. The whiteness of retinal tapeta 

 makes it possible to see, ophthalmoscopically, the red shimmer of rho- 

 dopsin against the background of the tapetum in a dark-adapted speci- 

 men. Rhodopsin was first seen in this way, in the living eye, in crocodiles 

 and in a freshwater fish, the European bream (Abramis bratna) . 



The Tapetum Fibrosum — The simplest kind of tapetum lucidum is 

 the fibrosum type. Nearly all hoofed animals have this kind, most 

 tapeta fibrosa are in such animals, and none of them has any other kind. 

 A portion of the thickness of the chorioid, just outside of the chorio- 

 capillaris layer, has simply been converted from an areolar type of 

 connective tissue to a tendinous sort, and glistens just as does a piece 

 of fresh tendon. The tapetum fibrosum is composed of dense, regular. 



