THE SIMPLE MUSCLE CURVE 33 



(i.) From the point of stimulation, a, to the point of commencing 

 contraction, b. This is the LATENT PERIOD. During this time 

 there is no change of length. The chronograph tracing a'U shows 

 that this occupied ^fu ths of a second, i.e. 01 sec. This method does 

 not give accurately the true measurement of muscle latency. It is too 

 high. More accurate measurements by specially designed methods 

 show it to vary from 003 to -008 sec. for frog's muscle. There are 

 several reasons why a measurement by the above method cannot give 

 the true result. In the first place, a muscle does not contract simul- 

 taneously all over, but the contraction starts at some one spot and 

 then spreads in a wave-like manner over the rest of the muscle. 

 Following an excitation at one spot, the fibres in that position contract, 

 but do not at first lead to a movement of the recording lever, but 

 rather to a stretching of the remainder of the fibre both above and 

 below the point contracting. This is because the parts which have to 

 be moved possess some inertia, and the part whose earliest movement we 

 wish to record is not united to the lever by a rigid connection, nor is its 

 upper end rigidly fixed, but at both ends muscle tissue is interposed. 



As muscle is elastic the first result of the contraction of a part of 

 a fibre is a stretching of the neighbouring parts, and movement will 

 only be communicated to the lowest extremity when either the 

 whole of the fibre has commenced to contract, or when the increase of 

 tension by the stretching has been transmitted through the elastic 

 fibre to that extremity. 



(ii.) From the point of commencing contraction b to the highest 

 point of the curve c. This is termed the PERIOD OF CONTRACTION. 

 The curve is for about the first third convex to the abscissa line, which 

 means that the rate of the contraction is gradually increasing. This rate 

 of contraction is at first very slow, as seen by the acute angle which the 

 first part of the curve makes with the abscissa ; it then rapidly increases, 

 as shown by the increasing inclination to the abscissa, and very soon 

 reaches a maximum rapidity. From this, again, there is another change 

 in rate, this time in the reverse direction, for the curve becomes con- 

 cave to the abscissa line, and gradually, shortening becomes slower 

 until at last it ceases, when the tangent to the curve becomes parallel 

 to the abscissa line. The time occupied by the writing point in 

 travelling from b toe, as shown by the piece of time tracing V c', was 

 ^fyths of a second, i.e. -075 sec. 



(iii.) The third portion of the tracing is from the highest point c 

 to the point d, at which the lever again reaches the abscissa line. This 

 part is called the PERIOD OF RELAXATION. The terminal point, 

 d, is often a difficult one to determine with any accuracy, because the 

 lever does not come instantly to rest ; but, as it always possesses some 



