232 



ADAPTATIONS TO NOCTURNAL ACTIVITY 



fibrous tissue, with the pigment cells and large blood vessels proper to 

 the chorioid cut down locally to a minimum or eliminated. Of necessity, 

 chorioidal tapeta are perforated at intervals by capillaries running ver- 

 tically through their thickness to supply the choriocapillaris (Fig. 92b, 

 c). The arborizations of these into the choriocapillaris are visible with 

 the ophthalmoscope as stellate black dots on the bright background of 

 the tapetum — the so-called stellulae Winslowi. 



The tapetalized area of the chorioid, in most ungulates and carnivores 

 (which together include a great majority of all tapetum-bearing animals) 

 is roughly a triangle with its base horizontal, and either including or 

 avoiding the disc. The rounded apex, in the superior part of the fundus 



.>S:5--- 



s#=- 



Fig. 92 — Mammalian tapeta lucida, histological. From Franz, after Murr (ms 



a, bundle of fibers from tapetum fibrosum of the ox, Bos taurus. b, chorioid of 

 vulpes, showing modification of inner layers to form a tapetum cellulosum. 



p- pigmented portion of chorioid; /- tapetum cellulosum; c- capillary supplying 



capillaris; cc- choriocapillaris. 



)• 

 Vulpes 



chorio- 



of the retina, makes about a right angle and the other two corners are 

 not much less broad — being a spherical triangle, the tapetum can of 

 course have angles totalling much more than 180°. The fibers of the 

 tapetum are arranged in close-set concentric rings so that the entire 

 tapetum is a single many-layered whorl of spindle-shaped fibers. Over 

 the region occupied by the tapetum, the retinal pigment epithelial cells 

 are devoid of pigment granules, thus interfering minimally with the 

 passage of light back and forth through them. 



A tapetum assignable to the fibrosum category, though of course 

 independently evolved, is known in a few fishes and will no doubt 

 eventually be found in many others. Our American fishes are most im- 



