448 MUSCLE-NERVE PHYSIOLOGY 



The rate at which conduction takes place when a contraction accompanies 

 it has been carefully measured by numerous observers. It varies greatly 

 in the different kinds of muscle, from two-tenths of a meter per second in the 

 rabbits' ureter (Engelmann ) to ten meters per second in the voluntary muscles 

 of man. 



SINGLE MUSCLE CONTRACTIONS. 



Characteristics of a Single Contraction. The Myogram. The con- 

 traction of a muscle in response to a single effective stimulus of short 

 duration is called a simple muscle contraction. A record of such a contraction 

 is called a myogram. The character of the myogram, and therefore the facts 

 revealed by it, are dependent on whether or not the record is made on a rapidly 

 moving recording surface. If the myogram is made on a recording surface 

 that is standing still, then it shows merely the extent of shortening of the 

 muscle. The amount of shortening for a given muscle will depend on a series 

 of conditions, such as nutrition, load, temperature, etc., all of which will 

 be discussed presently. 



When the record is made on a rapidly moving drum or on the pendulum 

 myograph, it is revealed that the simple contraction occupies a definite period 

 >f time with well-marked periods or phases. Although the stimulus may be 



FIG. 320. Record of a Simple Contraction of the Gastrocnemius of the Frog. Time in .01 

 seconds. St, Moment of stimulation. Record taken on a rapid drum that was provided with 

 an automatic key. - 



practically instantaneous, the contraction lasts a considerable fraction of a 

 second, in the frog's gastrocnemius about o.i of a second. 



It will be observed that after the stimulus has been applied, as indicated 

 by the vertical line St, there is an interval before contraction commences. 

 This interval, termed the latent period, when measured by the number of vi- 

 brations of the tuning-fork directly beneath, is found to be about o.oi of a 

 second. The latent period is longer in some muscles than in others, and 

 differs also according to the condition of the muscle and the kind of stimulus 

 employed. During the latent period there is no apparent change in the 



