2l8 



BIOLOGY AND HUMAN LIFE 



Fig. 109. Reflex arc 



Stimulation of the re- 

 ceiving end a of an 

 afferent nerve A leads 

 to a discharge of en- 

 ergy to all parts of 

 the neuron, including 

 the fine terminals, or 

 dendrites, d. The dis- 

 charge passes over to 

 connected nerves, as 

 the efferent nerve £, by 

 way of the interlacing 

 dendrites, or synapse, 

 s: The discharge in E 

 leads to stimulation of 

 the organ with which 

 it is connected, as a 

 muscle M. Starting 

 from a, in the spinal 

 cord the disturbance is 

 reflected by one of the 

 side branches, or collat- 

 erals, c, oi A, through 

 the synapse 5 into E, 

 leading to a movement 

 by the contraction of M 



working. A lowering of the amount of sugar 

 in the blood starts the liver discharging more 

 glycogen. The sight of a ghost increases the 

 flow of sweat from the skin glands. 



3. Nerve. Sometimes a stimulus fails to 

 bring about an immediate reaction, but pro- 

 duces instead some change in the nerve cells 

 of the brain. You hear a word, one of many- 

 used in sentences — a lecture, a scolding, the 

 rules of a game. You do nothing about it, 

 apparently, at the time, but later you give 

 evidence that the word had an effect : you 

 recall the word when needed, you do what 

 you were told to, and so on. 



The whole behavior of a man or an ameba 

 could be described as a system of reactions 

 to stimuli. We cannot always recognize the 

 stimulus ; we cannot always discover the 

 connection between the stimulus and the re- 

 action ; nevertheless both the single cell of 

 the ameba and the nervous system of man, 

 made up of many millions of cells, can best 

 be understood in this way. 



179. Reflexes. When you are tickled, you 

 draw away the touched part. When some- 

 thing gets close to your eye, you wink. When 

 the illumination is suddenly increased, the 

 pupil of your eye contracts ; when it is di- 

 minished, the pupil expands. When some- 

 thing tickles the inside of your nose, you 

 sneeze. When a solid particle touches the 

 lining of your windpipe, you cough. When 

 you chew tasty food, the glands of the stom- 

 ach secrete the gastric juice. When some- 

 thing touches the lining of the pharynx, you 

 swallow. 



