PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE 25 



then an increase in the stimulus produces no increase in the 

 contraction. 



(b) As the weight of the load is increased the contraction 

 becomes less until a weight is reached which the muscle is 

 unable to raise. Also, the latent period is longer with a heavy 

 than with a light load. 



(c) If we apply a series of successive stimuli to a muscle we 

 notice that at first the contractions improve with each successive 

 stimulus which is due to the beneficial effect of contraction. 

 Later the contractions get less and less. As the contractions 

 get less, the period of contraction becomes longer, the latent 

 period is increased and the period of relaxation becomes very 

 much longer. As the period of relaxation becomes longer, the 

 muscle fails to return to its normal length before a second 

 stimulus arrives, so that the original base line is not reached at 

 all. This condition is known as contracture. 



(d) By varying the temperature of a muscle we find that it 

 causes a variation in the extent and duration of its contractions. 

 Thus, by beginning at o C. and increasing the temperature, 

 we find that the contractions increase up to 5-9 C. and then 

 decrease up to i5-i8 C. After this point is reached they 

 again increase reaching their maximum at 26-3o C. This 

 maximum is much greater than the first maximum which was 

 reached at 5-9 C. As the temperature is still increased, the 

 contractions decrease rapidly until at about 37 C. irritability 

 is entirely lost. If the temperature is increased to about 42 C. 

 heat rigor makes its appearance due to the coagulation of the 

 muscle plasma. 



(e) Veratrin is an alkaloid which exerts a peculiar effect 

 upon the contraction of muscle. By injecting it into an animal 

 before the muscle is removed the following effects are noted: 

 (i) The phase of shortening is not altered, but the period of 

 relaxation is very much prolonged, and (2) there is a secondary 

 rise in the curve of relaxation. 



