1 8 PERSONAL APPEARANCES IN HEALTH AND DISEASE. 



maintaining the body in the erect position. The whole 

 framework of the body is, as we have seen, composed of 

 a large number of separate parts. Each of these parts 

 has relations with others, and in many regions one can be 

 moved on the other by means of muscular action. 



There is moreover a state of equilibration between 

 the muscles performing opposite kinds of movement ; 

 and so long as the muscles are in a state of tonicity, 

 and so long as the one set bears its due amount of work 

 as compared with the other set, so long is this equi- 

 libration maintained. 



This may be readily illustrated by the part played 

 by the muscles placed before and behind the spine, in 

 maintaining the erect posture of the body. The position 

 is kept up without effort, without even consciousness, 

 by the healthy man whose muscles are well balanced and 

 in good "tone." It may be, however, that the same 

 man after a long day's work over a desk, in an, ill-venti- 

 lated city office, no longer presents that supreme un- 

 consciousness of his muscles and their action, and 

 the stoop of his shoulders and bent head demonstrate 

 to others that the balance is no longer kept, that the 

 tonicity of the morning has passed off, and the wearied 

 muscles are no longer on the watch. And so it is when 

 in sleep the muscles are relaxed and gravity asserts its 

 force, so that the head falls forward by its own weight, 



