MUSCLE 279 



is directly measured. After the time that the impulse took in 

 reaching the muscle has been allowed for, there still seems to 

 be an interval before the muscle begins to shorten. This was 

 termed the " latent period," under the impression that some 

 time is actually lost in turning the nerve-impulse into a muscle 

 impulse. The impulse was supposed to be latent in the end- 

 plates of the nerve. Various hypotheses were formulated as 

 to the nature of the transformation. The progressive im- 

 provements in apparatus and methods is testified by the 

 diminution in this latent period as given in the textbooks of 

 successive decades. It is now put at ^1^ second, and is re- 

 garded by most physiologists as a delay due to the inertia of 

 the muscle. Owing to its elasticity, the molecular change 

 in muscle does not immediately affect its shape. When the 

 latent period appears to be longer say T l^. second the 

 balance is due to the inertia of the recording apparatus. 

 Usually the curve shows a rise lasting T ^^ second and a fall 

 occupying y^p due to the fact that inertia of muscle and 

 apparatus delays the commencement of the rise, but does not 

 hasten the termination of the fall. 



When an impulse is generated artificially by an induction 

 shock, a single spasm or twitch is the result ; but in Nature 

 contraction is never limited to a single twitch. Impulses 

 descending from a motor nerve-cell to a muscle are always 

 rhythmic. They follow at the rate of eight or ten a second 

 in human nerves ; and since in our muscles contraction and 

 relaxation take longer than in a frog, a second impulse reaches 

 the muscle before the effect of the first has passed away. The 

 muscle has not had time to relax, when it is again called 

 upon to contract. Hence a summation of contractions. The 

 muscle continues to shorten until the maximum of contraction 

 is reached. This condition is termed " tetanus," to distinguish 

 it from a single spasm. In fullest contraction the length of 

 a muscle may be diminished by one-half, or even by two-thirds. 



It would be impossible to treat of the electrical phenomena 

 displayed by nerves and muscles without presupposing some 

 acquaintance with the methods and laws of physics. As this 

 is contrary to our understanding with our readers, we must be 

 content with the statement of a few salient facts. At the 

 moment when an impulse is passing along a nerve, or a wave 



