THE PERCEPTION OF LIGHT 



609 



(a) a GUANINE TAPETUM in certain fishes — Chondrosteans, the 

 coelacanth and certain bathypelagic Teleosts — wherein 

 compact layers of cells are packed with guanine crystals ; 



(6) a TAPETUM CELLULOSUM (Fig. 580), a closely packed series of 

 layers of endothelial cells filled with doubly refractive 

 (? lipoid) rodlets, found in a relatively small area of the 

 upper half of the fundus of all Carnivores except Cynictis 

 and Suricata, over the entire posterior part of the fundus of 

 Pinnipedes, and in the nocturnal lemuroids ; 

 and (c) a tapetum fibrosum (Fig. 579), formed by a tendon-hke 

 condensation of fibrous tissue. This last has a relatively 

 widespread distribution — in many pelagic Teleosts, in certain 

 Marsupials (the dasyure, Dasyurus, the Marsupial wolf, 

 Thylacinus, the Tasmanian devil, Sarcophihis, and the 

 flying phalanger, Petaurus), among the Rodents in two 

 species (the spotted cavy, Cu7iiculus, and the flying squirrel, 

 Pteromys), in all Ungulates (except the Suoidea and Tylo- 

 poda), in the elephant, the Cetaceans, and in the only noc- 

 turnal Simian, the night-monkey, Nyctipithecus, in which 

 the eye-shine from the tapetum is unusually brilliant. 



II. The organization of the retina of the nocturnal eye depends 

 essentially on two features — great sensitivity of the sentient elements 

 and a high degree of summation within the retinal structure so that a 

 large number of receptor elements can combirue to stimulate a single 

 optic nerve fibre (Figs. 756, 757). It has generally been accepted that 

 the rods were particularly sensitive to light, a property which was 

 considered due to the great sensitivity of rhodopsin or the closely 

 related pigments with which they are provided. It may be that this 

 assumption is incorrect, for the evidence now available points to the 

 possibility that the rods and cones, considered individually, are equally 

 sensitive to light and that the apparent superiority of the rods in this 

 respect may be entirely due to summation by which a ganglion cell 

 can be excited into activity by a comparatively large number of 

 stimuli each of which acting by itself would be subliminal. In the 

 present state of our knowledge it would be dangerous to dogmatize on 

 this problem ; it has most recently been discussed by Weale (1958). 



However that may be, the rods, either by reason of their own 

 properties or on account of their neural connections, are specifically 

 adapted to a high degree of sensitivity and therefore subserve scotopic 

 vision, while the cones are adapted to a high degree of analytical 

 acuity and therefore subserve photopic vision ; these structures are 

 therefore diff"erentiated sharply from the functional point of view. 

 Their structural difterentiation, however, is not always easy since 



S.O.— VOL. I. 39 



Suricata 



Sarcophilus 



