62 OPTIC NERVES TESTS OF HEAT. 



names. The gelatinous fibres of the nerves of 

 feeling in the hand would serve only once for 

 contact with an ignited body. They would by 

 that single contact be converted into vapor. To 

 hold communication with ignited bodies without 

 pain or injury, there is provided another arrange- 

 ment of telegraph nerves from the brain to the 

 retina of the eye ; which is studiously protected 

 from liability to injury by highly excited bodies. 

 The ball of the eye is sunk within a socket, cov- 

 ered by an external shutter adapted to close " as 

 quick as a wink." The ends of the optic nerves, 

 denoted the retina, are placed behind a watery 

 lens, with an aperture arranged to be automati- 

 cally closed by too intense excitation of light. The 

 exterior of the eye-ball is also kept constantly 

 cooled by a trickling fountain of tears. 



By these ingenious arrangements, the optic 

 nerve holds communication with intensely heated 

 bodies without injury. It may be convenient to 

 give different names to the sensations produced 

 by the transmission of electric action to the brain 

 through different nerves ; but this does not war- 

 rant us in ascribing the ignition of a fine wire, ex- 

 cited by the discharge of an electrical jar, to three 

 different causes, named Heat, Light, and Elec- 

 tricity. As well might the transmission of electric 

 action through three different telegraph wires to 

 ;a telegraph office, be ascribed to three different 

 agents of Nature. 



