Comparative Anatomy of the Mammalian Eye, 477 



In all the Mammals below the Simiag which have no macula we find a 

 larger sensitive area. Sensitive areas of restricted dimensions, omitting 

 those cases in which the area is limited to a macula, exist in the 

 Carnivora, in which order the divergence is not great. In the Ungu- 

 lates, Eodents, Edentates, and Marsupials, where we find great diver- 

 gence of the axes, large corneae, and nearly spherical lenses, the 

 sensitive areas are larger, and probably the degree of diff'erence in 

 perception over such areas, compared Avith the more peripheral parts, is 

 but little. 



The great transparency of the retina and the extreme brilliancy of 

 the reflecting surface of the choroid in the vast majority of Mammals, 

 and an extraordinary prevalence of colours of every hue, lead one 

 irresistibly to the conclusion that the rays of light do not form an 

 image on the retina as usually taught, but that the image is formed 

 behind the retina on the brilliant surface of the Tapetum or fusca 

 pigment layer of the choroid, and is then reflected back on to the 

 terminals of the bacillary layer. This arrangement for vision certainly 

 bears a close resemblance to Lippmann's method of obtaining coloured 

 negatives. He obtained negatives in natural colours by placing a 

 reflecting mercury surface in direct contact with the sensitive film, thus 

 reflecting the light which had traversed the film on to the particles of 

 sensitised silver. In the eye the light passes through the nearly 

 transparent retina (which is analogous to the photographic film) to be 

 reflected from the Tapetum, or choroidal pigment, on to the terminals 

 of the retinal elements (which may be compared to the particles of 

 silver haloid). In Lippmann's device the colours are produced by 

 interference. If we venture to carry our analogy still further, we 

 may presume the same occurs in the eye. One diff'erence between 

 the two methods is that in nature the reflecting surface is always 

 coloured, and only reflects a portion of the incident light. The colour 

 of the fundus, however, is remarkable for the absence of blues and 

 violets and the great prominence of red, yellow, and green colours. 

 Yellow and orange are the prevailing colours in nocturnal animals. 

 The peripheral area, which is characteristic of animals possessing a 

 Tapetum, is usually dark brown, and reflects but feebly. It is probably 

 nearly insensible to light, as it never occurs in animals having great 

 divergence of the optic axes. 



The eye is no exception to the rule that domestication greatly 

 increases variability. The colour of the Fundus oculi of domesticated 

 races differs not only from that of the wild species from which the 

 races are supposed to be derived, but the colour varies also individually, 

 an occurrence almost unknown in wild species. The influence of 

 domestication is also indicated by the frequent occurrence of myopia 

 and astigmatism. Myopia is almost unknown in wild animals, ])ut it 

 may occur in wild specimens which have long been kept in captivity. 



