VETEK1NAKY PHYSIOLOGY 



In this way such a tracing as is shown in fig. 23 is pro- 

 duced. 



From this it is evident that the muscle does not contract 

 the very moment it is stimulated, but that a short latent 

 period supervenes between the stimulation and the contrac- 

 tion. In the muscle of the frog attached to a lever this 

 usually occupies about y^th second ; but if the change in the 

 muscle is directly photographed without any lever being 

 attached to it, this period is found to be very much shorter. 

 The latent period is followed by the period of contraction. 



At first it is sudden, but it 

 becomes slower, and finally 

 stops. Its average duration 

 in the frog's muscle is about 

 T^-jyth second. 



The period of relaxation 

 follows that of contraction, 

 and it depends essentially 



Fro. 23. Trace of Simple Muscle Twitch 

 (1) showing periods of latency, contrac- 

 tion, and relaxation ; record of moment 

 of stimulation (2) ; and a time record 

 made with a tuning fork vibrating 100 

 times per second (3). 



on the elasticity of the 

 muscle, whereby it tends to 

 recover its shape when the 

 distorting force is removed. 

 The recovery is therefore at first fast and then slow, and it 

 lasts in the frog's muscle about roVth second. 



The whole contraction thus lasts only about T Vth second 

 in the frog's muscle. In mammalian muscle it is much 

 shorter, and in the muscle of insects shorter still. 

 2nd. Extent of Contraction. 



While, as will be afterwards considered, the extent of con- 

 traction is modified by the strength of stimulus and the state 

 of the muscle, the total extent of contraction is primarily de- 

 termined by the length of the muscle. If a muscle of two 

 inches contracts to one-half its length, the amount of con- 

 traction is one inch, but if a muscle of four inches contracts 

 to the same amount, it shortens by two inches. 



3rd. Force of Contraction is measured by finding what 

 weight the muscle can lift, and the absolute force of a muscle 

 may be expressed by the weight which is just too great to be 

 lifted. The lifting power of a muscle depends primarily upon 

 its thickness or sectional area. The absolute force of a muscle 



