148 THE HUMAN BODY. 



effect of the prick gets to the centers. When that road is 

 cut off, we feel nothing. We infer, also, that the muscle 

 does not commonly move of itself. 

 It is caused to move by something 

 in the nerve centers, and the nerve 

 is the road by which the influence 

 from the center that makes it move 

 gets to it. 



3. The nervous system may be 

 compared to the telegraphic sys- 

 tem of a railroad. The nerves are 

 the wires. The gray matter of the 

 spinal cord contains the offices of 

 the district superintendents. In 

 the cerebrum is the office of the 

 superintendent of the road. Sup- 

 pose a mosquito lights on your face, 

 and puts in his bill. He can not 

 put it in, small as it is, without hit- 

 ting one or more little nerve fibers. 

 Instantly a message goes along 

 those nerve fibers, through the 

 nerve-trunks, through the spinal 

 cord, and finally to the general 

 superintendent's office. The mes- 

 NERVES OF THE FORE-ARM AND ga g e j s " Something wrong here." 



HAND. 1. Artery. 2. Nerves. _ 



Immediately an order is sent out, 



along other nerve trunks and fibers, to 'the muscles of the 

 shoulder and arm ; and they contract, so as to strike the 

 mosquito. 



4. This is done very quickly, and yet it takes some time. 

 The message goes in at the rate of one hundred and forty 



