IN HUMAN PHYSIOLOGY. 1 97 



municates. Thus, an electric shock sent through the eye 

 gives rise to the appearance of a flash of light ; and pres- 

 sure on any part of the retina produces a luminous image, 

 which lasts as long as the pressure, and is called -a.phos~ 

 phene. If the point of the finger be pressed upon the 

 outer side of the ball of the eye, a luminous image 

 which, in my own case, is dark in the centre, with a bright 

 ring at the circumference (or, as Newton described it, like 

 the "eye" in a peacock's tail) is seen; and this image 

 lasts as long as the pressure is continued. HUXLEY. 



31. Why can we not see with the nose, or smell with the 

 eyes ? 



Each set of nerves is adapted to transmit to the brain a 

 peculiar class of sensations alone. 



32. What causes the roughness of a cat's tongue ? 



The sharpness and strength of the papillae upon its 

 tongue. This is a peculiarity of the lion tribe. 



33. Is the cuticle essential to touch ? 



Yes. If the cuticle be removed, as in case of a blister, 

 contact with the exposed surface produces pain rather 

 than a sense of touch. 



34. Can one tickle himself 1 



It is said not ; but the author has found persons who 

 averred that they could produce this sensation upon them- 

 selves. The sense, it is noticeable, is present only in those 

 parts where that of touch is feeble. 



35. Why does a bitter taste of ten produce vomiting? 

 The 5th pair of nerves, which supplies the lip and sides 



