396 A MANUAL OF VETERINARY PHYSIOLOGY 



variation ; the tissues are immediately excited by a sudden 

 increase or sudden diminution of a current, though they may be 

 unresponsive when the current is gradually increased even to a 

 considerable strength. The resistance of animal tissues generally 

 to electrical currents is very considerable ; the body is a bad 

 conductor. 



Induced currents stimulate muscle less readily than nerve. 

 When a nerve has degenerated induced currents produce no 

 contraction of the muscle, while continuous currents are capable 

 of stimulating it into contracting activity. 



The most conveniently controllable stimulus for experimental 

 purposes is that obtained as single induction currents or the 

 interrupted current of an induction-coil. These have the 

 advantage of being extremely efficient as stimuli, and of giving 

 rise in the nerve to impulses which we may regard as the nearest 

 artificial approach to the impulses which in the body are dis- 

 charged along the nerves 

 by the cells of the cen- 

 tral nervous system. 



When a muscle is 

 stimulated to contract, 

 it becomes shorter and 

 thicker, but there is no 

 change in volume. If 



~ ~ c n made by means of a 



Fig. 114.— The Curve from a Single Con- J 



TRACTION OF THE GASTROCNEMIUS MUSCLE myograph [rig. 113) 10 



of the Frog (Waller). trace its contraction on 



From 1 to 2 is the latent period ; from 2 to 3 the a revolving drum, a 

 period of shortening ; from 3 to 4 the period of musc 1 e curve is pro- 

 relaxation. The sinuous line below the curve 1 



indicates periods of r|, second. duced. A muscle curve 



consists of three parts : 

 (1) A period following stimulation during which no contraction 

 occurs ; it is known as the latent period, and during this time 

 the muscle is preparing itself for work ; (2) a period of contraction 

 or shortening ; (3) a period of relaxation. 



The length of time involved in the various phases of a con- 

 traction may be measured by bringing a time-recorder, vibrating 

 fractions of a second, to bear on the smoked surface of the 

 revolving drum ; the sinuous line in Fig. 114 below the muscle 

 curve is produced in this way. Though the nerve was stimulated 

 at the point marked 1 on the tracing, it was not until 2 was reached 

 that the muscle responded, the time value for the latent period 

 being T ^ second. As a matter of fact, the latent period is less 

 than this, for a minute loss in time occurs in the impulse passing 

 along the nerve, and there are also instrumental defects, such as 

 friction of the lever. When these are deducted the latent period 



