PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE 25 



stimulus, then an increase in the stimulus produces no in- 

 crease in the contraction. 



(b) As the weight of the load is increased the contrac- 

 tion 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 re- 

 laxation becomes very much longer. As the period of relax- 

 ation becomes longer, the muscle fails to return to its nor- 

 mal length before a second stimulus arrives, so that the orig- 

 inal 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 contrac- 

 tions. Thus, by beginning at o C.. and increasing the tem- 

 perature, 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 tempera- 

 ture is still increased, the contractions decrease rapidly until 

 at about 37 C. irritability is entirely lost. If the tempera- 

 ture is increased to about 42 C. heat rigor makes its appear- 

 ance due to the coagulation of the muscle plasma. 



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

 upon the contraction of muscle. By injecting it into an ani- 

 mal 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. 



