THE 



BRITISH NATURALIST. 



NEW SERIES, 



SENSE ORGANS. 



BY HENRY LANGDALE, M.D. 



In order that an animal may survive and reproduce its species it is 

 necessary that its actions shall be properly adjusted to the conditions 

 in the world around it. It must avoid danger, be attracted by food, 

 be differently affected by light and darkness, and by heat and 

 cold, and by various other conditions in its surroundings. That 

 this adjustment shall be more perfect animals are provided with 

 specially modified parts, known as sense organs. These organs 

 receive and modify stimuli from without, and transmit them along 

 nerves to the central nervous system; for example, certain minute 

 areas on the skin of man are specially sensitive to heat, and the 

 action of heat on these sensitive areas sets up a nervous current, which 

 travels along a nerve until it reaches the spinal cord, or other por- 

 tion of the central nervous system, and there encounters nerve 

 cells. In these cells the nerve current undergoes certain modifications 

 which result in its being transmitted, perhaps in an augmented form, 

 to other nerves, and along them to certain muscles, causing them to 

 contract. We have here, in fact, an example of reflex action, and it is 



