ITS NERVOUS SYSTEM AND SENSE ORGANS. 107 



result. It is not certain that the cones really assist in the 



M 



production of the image, which may be due to the corneal 

 facets alone, though modified by the decolorised cones. 



Grenacher has pointed out that the composition of the nerve- 

 rod furnishes a test of the mosaic theory. According as the 

 percipient rod is simple or complex, we may infer that its 

 physiological action will be simple or complex too. The 

 adequate perception of a continuous picture, though of small 

 extent, will require many retinal rods ; on the other hand, a 

 single rod will suffice for the discrimination of a bright point. 

 What then are the facts of structure ? Grenacher has ascer- 

 tained that the retinal rods in each element of the compound 

 eye rarely exceed seven, and often fall as low as four further, 

 that the rods in each group are often more or less completely 

 fused so as to resemble simple structures, and that this is 

 especially the case with Insects of keen sight.* 



Certain facts described by Schultze tell on the other side. 

 Coming to the Arthropod eye, fresh from his investigation of 

 the vertebrate retina, Schultze found in the retinal rods of 

 Insects the same lamellar structure which he had discovered in 

 Yertebrata. He found also that in certain Moths, Beetles, and 

 Crustacea, a bundle of extremelv fine fibrils formed the outer 



m) 



extremity of each retinal or nerve-rod. This led him to reject 

 the mosaic theory of vision, and to conclude that a partial image 

 was formed behind every crystalline cone, and projected upon a 

 multitude of fine nerve-endings. Such a retinula of delicate 

 fibrils has received no physiological explanation, but it is 

 now known to be of comparatively rare occurrence ; it has 

 no pigment to localise the stimulus of light ; and there is 

 no reason to suppose that an image can be formed within its 

 limits. 



The optical possibility of such an eye as that interpreted to 

 us by Miiller has been conceded by physicists and physiologists 

 so eminent as Helmholz and Du Bois Reymond. Nevertheless, 

 the competence of any sort of mosaic vision to explain the 

 precise and accurate perception of Insects comes again and 

 again into question whenever we watch the movements of a 



* Flies, whose eyes are in several respects exceptional, have almost completely 

 separated rods, notwithstanding their quick sight. 



