38 PSYCHOBIOLOGY 



called summation of contractions. Again, stimuli so weak that singly 

 they produce no contraction, may, when given at a sufficiently rapid rate, 

 produce contraction. This condition is known as the summation of 

 stimuli. It is supposed that the ' voluntary ' contraction of muscle is 

 tetanic, due to a rapid succession of nervous discharges. There is some 

 evidence that these discharges to the muscle occur at a rate of 40 per second. 

 The only continuous stimuli are those of the normal nerve activity, heat, 

 and chemical action. Electric currents are effective only at the moment 

 of making or breaking the current. Mechanical pressure is effective only 

 at the moment when the pressure is increased or decreased. 



TONUS AND EXCITABILITY. 



The constant stimulation which is supplied by the nerves in the normal 

 body keeps the muscles in a state of normal contraction (tonus), the de- 

 gree varying continuously with the changes in the flux of nervous current. 

 This continuous stimulation also heightens the sensitivity of the muscle to 

 the more pronounced and definitely directed currents which bring about 

 the coordinated contractions which produce movements; or, in other words, 

 increases the irritability or excitability of the muscle. If the efferent 

 nerves supplying any of the muscles be severed, the muscles relax com- 

 pletely and become motionless, except in so far as artificial stimulation 

 (<?. g., electrical) may be employed upon them. 



In the case of cardiac and smooth muscle, and probably also in the case 

 of striped muscle, certain nerve currents have an action which is inhibi- 

 tory, i. e., the reverse of excitatory. Certain nerve fibers, as for example 

 the fibers of the vagus which supply the heart, have inhibitory action only. 

 Whether in all cases inhibitory currents are carried by special inhibitory 

 fibers, or whether both excitatory and inhibitory currents may be carried by 

 certain fibers, is unknown. 



The factors upon which the irritability of muscle depends, are (in addi- 

 tion to stimulation and tonus of nervous origin) , temperature, condition of 

 rest or fatigue, and activity of various adventitious chemical substances. 

 The greater the fatigue, the less the excitability. Salts of sodium increase 

 the excitability, and calcium salts lower it. By immersing a thin striated 

 muscle (e. g., the sartorius of the frog), in a solution of NaCl 0.5%, 

 Na 2 HP0 4 0.2%, and Na 2 C0 3 0.04% ( " Biedermann's fluid"), 7 it is 

 thrown into a state of excitability which shows a certain likeness to that 

 of smooth and cardiac muscle. The muscle, in this solution, contracts re- 

 peatedly, and may ' beat ' rhythmically like heart muscle, but at a more 

 rapid rate. 



7 Formula given by Starling, Physiology, page 235. 



