TOUCH, TEMPERATURE, MUSCULAR SENSE. 487 



nerves distributed among the ligaments and muscles of the 

 same part. In fact, for the proper manipulation of the 

 muscles we are largely dependent upon the impressions 

 which accompany the use of them. An individual with 

 sensation lost in the arm is unable to control the move- 

 ments of that arm. It not infrequently happens that when 

 an arm goes to sleep, the power to move it returns before 

 sensation returns, but the motions so accomplished are 

 of the most clumsy and inaccurate kind. Experiments 

 have shown that horses whose trigeminal nerve (the sensory 

 nerve to the head) had been cut, found it perfectly impos- 

 sible to perform even such simple muscular actions as the 

 chewing of their food. A frog whose spinal sensory nerves 

 are cut finds the greatest difficulty in the performance of 

 his otherwise simplest motions. That these guiding sensa- 

 tions do not come entirely from the skin is shown by the 

 fact that the skin may be entirely removed from a portion 

 of a frog's body without interfering with the accuracy of 

 the movement of his muscles, provided the sensory nerves 

 going to these muscles are left intact. 



In the chapter on the histology of the muscles it was 

 pointed out that muscles themselves are not sensitive in any 

 way, but that the sensations which seem to come from the 

 muscles really come from sensory nerves which are distrib- 

 uted in among such muscles. Even the sense of fatigue 

 of muscles has such an origin. 



But not only are we able to determine the amount of 

 motion, but it is possible actually to measure the intensity 

 of the muscle contraction. In this way we are enabled to 

 form judgments as to the weight of things. A weight must 

 change from at least one-fortieth to one-tenth of its entire 

 amount in order to perceive a difference. No doubt, how- 

 ever, the ability to make these distinctions varies within wide 

 limits and is largely perfected by practice. Individuals who 

 are in the habit of weighing things become finally so pro- 

 ficient as to be almost trustworthy in that matter. There 

 are many illusions in connection with the matter of the 



