GENERAL STRUCTURE OF THE BODY. 25 



est contraction. The work done depends in like manner 

 upon the strength of the stimulus. During contraction 

 certain sounds are given off called muscle sounds, which 

 can be heard with the stethoscope but have no special 

 significance. 



The muscles which have the greatest power of rapid 

 contraction are generally attached to levers. Indeed, 

 striated muscle is characterized by the rapidity and 

 strength with which it works, though its rhythmic mo- 

 tion is slight. Smooth muscle, on the other hand, is 

 characterized by its great force, considerable rhythm, 

 considerable tone, and slight rapidity, that is, its con- 

 traction is slower and lasts longer than that of striated 

 muscle. Cardiac muscle is characterized by great rhythm 

 and force, fair rapidity, and slight tonicity, tonicity 

 being the amount of tone or readiness to work. For 

 even in sleep muscle is always in tone, that is, ready to 

 do its work. It is this that makes the difference in ap- 

 pearance between a living and a dead person and en- 

 ables one to spring to his feet at night if he hears a noise, 

 a thing he could not do if his muscles were wholly re- 

 laxed. Thus, rapidity is the great function of striated, 

 tonicity of smooth, and rhythm of cardiac muscle. In 

 paralysis the muscles droop and lose their tone. Mus- 

 cles are frequently the seat of rheumatic disorders. 



When set free, potential energy accomplishes work. 

 In muscle there is a good deal of potential energy, 

 which is set free as heat and as work accomplished. 

 Even when the muscles are at rest, chemical changes are 

 going on and heat is being produced, though more heat 

 is produced when they are functioning. If the body de- 

 pended upon its gross motions for all its heat it would 

 grow cold while a person rested. The respiratory or- 

 gans, however, and the heart are always working and 

 chemical changes are constantly taking place. 



Ordinarily a muscle has some object in contracting, 

 such as the raising of a load, and it contracts voluntarily 

 more or less according to the weight of the load. The 



