478 MUSCLE-NERVE PHYSIOLOGY 



muscle. A stimulus applied at any point of a muscle will quickly be propa- 

 gated through the mass as far as there is protoplasmic continuity. In cardiac 

 muscle and in smooth muscle there is uninterrupted conduction from cell 

 to cell. But in voluntary muscle each fiber is physiologically isolated from 

 its neighbors. When a voluntary muscle fiber is stimulated either at the ex- 

 tremities or at its middle, the effect of the stimulus quickly passes through 

 the entire fiber, whether it arouses a distinct act of contraction or not. 



The rate at which conduction takes place when a contraction accom- 

 panies 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 dura- 

 tion 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 



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

 in o.oi second. Si, Moment of stimulation. Record taken on a rapid drum that was 

 provided with an automatic key. 



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 per- 

 iod of time with well-marked periods or phases. Although the stimulus may 

 be practically instantaneous, the contraction lasts a considerable fraction of a 

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



