MUSCULAR CONTRACTION 427 



In a muscle-nerve preparation, the effect of a second stimulus 

 applied to the nerve before the first contraction is concluded is to 

 increase the extent of the first contraction. Indicating these contrac- 

 tions by curves, the curve of the second contraction is superimposed on 

 the first. When a number of successive stimuli are employed, the 

 curves are superimposed until a maximum is reached. Muscular con- 

 traction produced in this way is called incomplete tetanus. The rate of 

 the stimuli required to produce this kind of tetanus is fifteen to twenty 

 per second. Incomplete tetanus may be produced by voluntary effort 

 as well as by repeated electric shocks ; and this explains ordinary 

 muscular action. In voluntary effort, the normal stimulus that excites 

 muscular contraction is sent from the nerve-centres along the nerves 

 in the form of what are called impulses. The rate at which these 

 impulses are sent out by the nerve-cells is ten to twelve in a second. 

 By the superimposition of the contractions, the force exerted by a 

 muscle may be regulated by the will. Were it not thus, prolonged 

 muscular effort would be impossible. During voluntary contraction a 

 muscle is in a condition of incomplete tetanus; but this must not be 

 confounded with the conditions in the disease known as tetanus or in 

 poisoning with strychnin. 



A muscle possesses extensibility as well as elasticity. After it has 

 been stretched to its limit, when the force is removed it returns to its 

 original length. When any elastic body that is stretched afterward 

 returns to exactly its original dimensions, it is said to be endowed with 

 perfect elasticity. Elastic tissue is an example of this. Muscles, also, 

 are perfectly elastic structures. 



In a state of rest there is a feeble electric current flowing from the 

 general surface to the cut surface. This is called the current of rest 

 (Du Bois Reymond). When, however, the muscle contracts, the galva- 

 nometer, which has been deviated by the current of rest, shows a diminu- 

 tion in this current, and it may return to zero under the influence of a 

 current in the opposite direction. This second current is called the 

 current of action. This diminution or extinction of the currents of rest 

 is known as negative variation of muscle-current. 



Delicate apparatus for measuring changes in temperature show that 

 muscular action is attended with elevation of temperature in the muscle 

 itself. In large animals this has been found to amount to several 

 degrees ; but a rise in temperature may be noted even in a muscle-nerve 

 preparation from a frog. By the thermopile, variations of f^-^ of a 

 degree C. may be noted (Helmholtz). 



Muscles in contraction undergo certain important chemical changes. 

 The reaction of a resting muscle is alkaline. Muscular action results in 



