THE PHYSIOLOGY OF MUSCLE 483 



function, nutrition, irritability, etc. As to the frog's gastrocnemius, 

 the electrical latent period (the interval between stimulation and the 

 beginning of negative variation in the electrical state of the muscle) is 

 not over 0.001 second. The mechanical latent period is the interval 

 between stimulation at the end of the electrical latent period and the 

 beginning of actual movement by the muscle. This period is about 

 0.005 second long. Thus the "combined latent period" in the frog's 

 gastrocnemius in summer is not over 0.006 second normally. In addition 

 to this about 0.004 second is taken up by the passage of the nervous 

 impulses, making altogether 0.010 second. This combined quantity is 

 what is measured in the laboratory as the " latent period." In winter 

 it may be somewhat longer, and when the muscle still has blood circu- 

 lating in it, shorter. 



Study of the uprise of the lever (requiring about 0.05 second on the 

 average in the gastrocnemius) indicates that the contraction begins 

 slowly and ends slowly, the slow beginning occupying about 0.005 second 

 and the slow ending say 0.015 second. The shape of this part of the 

 myogram depends on many different conditions. The downfall of 

 the lever (relaxation of the muscle) occupies about 0.075 second; it 

 begins slowly (0.015 second) and ends slowly (0.01 second). 



Expt. 38. Galvanic Electricity as a Stimulus. (Apparatus : Muscle, 

 myograph or graphic-record apparatus, rheocord, tuning-fork.) Set up 

 the mechanism for making graphic records of the frog's gastrocnemius, 

 connecting the muscle through a key and rheocord with one dry cell. 

 Raise the drum from the friction-bearing and spin it slowly. Apply the 

 tuning-fork and close the key, holding the lever down and using a current 

 only just strong enough to produce contraction. Now open the key and 

 observe that no contraction then occurs : the galvanic make is a stronger 

 stimulus than is the break. Again, with full strength of current, make 

 records of make and of break (here, as always, separated by holding the 

 key closed while muscle contracts and relaxes from the make), and observe 

 that the make-contraction is the more vigorous, the lever rising higher 

 than from the break-shock. 



Observe also that in case of skeletal muscle no general contraction 

 occurs during the passage of the constant current, but only at its make 

 and break. The muscle, however, is meanwhile in the interesting electro- 

 tonic condition. 



That the make or application of the constant galvanic current is a 

 stronger and more effective stimulus than is the break or withdrawal of 

 the current needs no special explanation. The addition of energy would 

 naturally result in more activity than the withdrawal of energy already 

 present. It is owing in part to the sudden change in the strength of 

 the stimulus (Du Bois Reymond) that any contraction takes place at the 

 break of the galvanism. (See the next experiment.) There has arisen, 

 however, an elaborate system of explanation based on phenomena whose 

 meaning is obscure: In the case of galvanic stimulation, the excitation 

 on making or closing the constant current begins at the cathode (C.C.), and 



