Chap. XII.] THE CRAYFISH. 251 



and cells of the optic bulb. The connective rods between the crys- 

 talline cones and striated spindles may after treatment with 

 osmic acid and teasing be resolved into four fibres. The inner 

 ends of striated spindles are probably continuous with the nerve- 

 fibrils of the optic bulb. Pigment is developed round the outer 

 ends of the crystalline cones, the striated spindles, and the outer 

 nerve-fibrils of the optic bulb. 



Two theories of arthropod vision have been advocated : (1) 

 that of compound vision ; (2) that of mosaic vision. According 

 to the former, a separate image of the object seen is formed at 

 the end of each cone and spindle, the multitude of images being 

 presumably co-ordinated by the arthropod cerebral ganglion, just 

 as two images are in us by the human brain. This view finds few 

 supporters to-day. The principle of the other view, that of 

 mosaic vision, will be understood with the aid of the diagram 

 78, F. At a. b. are a number of transparent rods separated by 

 pigmented material absorbent of light. At c. d. is an arrow placed 

 in front of them. At e. f. is a screen placed behind them. Eays 

 of light start in all directions from any point c. of the arrow ; but 

 of these only that which passes straight down one of the trans- 

 parent rods reaches the screen. Those which pass obliquely into 

 other rods are absorbed by the pigmented material. Similarly 

 with rays starting from other points of the arrow. Only those 

 which pass straight down one of the rods reach the screen. 

 Wherefore there is thrown on the screen a reduced image c'. d'. 

 of the arrow. The image is not continuous, but stippled. 



So much for the general principle of so-called mosaic vision. 

 According to this theory the several points of the object 

 which excites vision stimulate corresponding points in the re- 

 cipient organ. To the question, What are the true recipient 

 end organs of the optic nerve 1 several answers have been sug- 

 gested. 



1. The crystalline cones are the end organs, and are 

 continuous through the basilar membrane with the nerve- 

 fibrils. 



2. The cones and spindles are simply dioptric, and beneath 

 the basilar membrane is a retina which is the true recipient. 



