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PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



already refer it to a distant body, because the condition 

 of sensation is air-borne particles. Finally, in hearing, 

 and especially in sight, we lose entirely the sense of con- 

 tact or local impression, and refer it wholly into space. 



Illustrations of the Law.— We shall now try to 

 make the law clear by many illustrative experiments : 



Experiment i. — If we could bare the retina and touch • 

 its surface we would not feel it, but would see a flash of 

 light — Where ? In space and in a direction exactly oppo- 

 site, or at right angles to, the touched surface. If the 

 optic nerve be laid bare and pinched we would feel noth- 

 ing, but would see a flash of light in space opposite that 

 part of the retina where the pinched fibers are distributed. 

 Of course we can not deliberately make this experiment, 

 but the flash of light is observed in passing electricity 

 through the nerve, and also in cases of extirpation of 

 the eye at the moment of rupture of the optic nerve. 



Experiment 2. Phosphenes. — Close the eyes, and 

 then press the finger into a corner of one of them. A 

 brilliant-colored circle is seen in the field of darkness 

 opposite the point pressed. These are called phos- 

 phenes. In my own case they are brilliant golden rings, 

 with steel-blue centers. They are caused by the inden- 

 tation of the sclerotic and, through it, the retina. But 

 any change whatever in the retina shows itself as an 

 appearance in space. 



Experiment 3. Muscce Volitantes. — Look at a white 

 wall, or better, abright sky. Nearly all observers will see 

 specks or clouds or tangled threads in the bright field, 

 slowly gravitating downward. They are called " muscse 

 volitantes," or flying gnats. What are they ? They are 

 slight imperfections in the vitreous body. These less 

 transparent spots cast their shadows on the retinal bot- 

 tom. But any variation of the retinal surface shows itself 

 as an appearance in the field of view directly opposite. 



