48 EVOLUTION OF THE HUMAN EYE 



carry in front of them an apparatus capable of emitting a 

 phosphorescent light (Fig. 8). In the eyes of pelagic fishes 

 reflecting membranes are met with; on the surface of the 

 iris is the membrana argentina, which gives it great brilliancy, 

 and acts by reflecting light on objects looked at in dark 

 places, much in the same way as that in which we employ 

 our ophthalmoscopic mirrors. On the inner surface of the 

 choroid they have another reflecting membrane, the tapetum 

 lucidum, which intensifies the power of seeing in dim lights 

 by reflecting rays back on the retina. The same device is 

 met with in several classes of mammals; in the carnivorous 

 Cetacese the reflecting membrane, as in fishes, covers the 

 whole fundus; in the terrestrial mammals, where a tapetum 

 is found, it covers only a sector of the fundus external to 

 the optic disk. In the terrestrial Carnivora the tapetum is 

 most brilliant and is composed of cells arranged in elongated 

 plates together with hexagonal crystals. In the Ungulata 

 the tapetum has an entirely different structure, being 

 composed of layers of fibrous tissue and not cells. Among 

 Primates a tapetum is met with in some lemurs with nocturnal 

 habits, and is similar in construction to that of the Carnivora. 

 Man and monkeys have no supplementary arrangement 

 for seeing in dim lights such as that afforded by a tapetum 

 lucidum. Monkeys, having resorted to arboreal life, where 

 they found their food by day, and where they were safe from 

 the attacks of carnivorous foes by night, have not had to 

 acquire nocturnal habits. Man, having inherited his visual 

 capacities from his arboreal ancestors, found himself, on his 

 descent from the trees, more than equal to his mammalian 

 foes by day, but inferior, and sadly handicapped, by night. 

 Not only had he not acquired the advantages of a tapetum, 

 but from prolonged life in the branches and reduction of 

 the snout he had lost much of the protection afforded by 

 acuteness of smell. 



