SENSITIVENESS OF NERVES. 213 



delicately sensitive character. Were they not 

 carefully covered up within the interior of the 

 body, every contact with metals and other sub- 

 stances, and every change of temperature would 

 induce muscular contractions and convulsions. 

 The sensations of a bared tooth-nerve are too 

 familiar to require description. 



The proper electric current of the frog bears 

 this curious analogy to the electric discharges of 

 fishes, that it is not manifest if the connection be 

 made between corresponding points of the opposite 

 sides ; but that it shows itself when the communi- 

 cation is made between points higher or lower in 

 the body, whether on the same or on opposite sides. 



A PLANT is a mechanism immediately excited by 

 solar rays to produce groupings of atoms into 

 FOOD and FUEL. An ANIMAL, on the contrary, is 

 a mechanism excited indirectly by the combustion 

 of this fuel. A vigorous man breathes forth from 

 his lungs each day a volume of carbonic-acid gas, 

 which by analysis is found to contain about thirteen 

 ounces of pure carbon. This quantity of carbon 

 is equivalent to an equal weight of pure charcoal 

 burned daily in his lungs, to develop electric exci- 

 tation constituting animal heat and animal motive- 

 power. 1 



1 For a minute description of the organs of the eye and of the ear see 

 Helmholtz's profound work on " Sensations of Tone " and " Scientific 

 Lectures," and Leidy's and Sharpey's editions of Quain's Anatomy, with 

 illustrations ; also Carpenter's " Comparative Physiology." 



