94 PRACTICAL PHYSIOLOGY 



subsequent contractions become of shorter and shorter duration 

 until they reach the normal. If the muscle be allowed to rest, 

 the veratrine effect returns again. The absence, in the case of 

 the hyoglossus, of the sharp initial twitch seen in the gastrocnemius 

 contraction, is probably due to more complete poisoning of all the 

 muscle-fibres. The gastrocnemius is more bulky, some of its fibres 



FIG. 97. Contraction of the gastrocnemius muscle of a frog. The 



effect of veratrine. 



The first two contractions show the characteristic effect of the dmg ; further stimulation pro- 

 duced twitches without the prolonged contraction. The curve has been reduced to one -half 

 the actual size. The time is marked in seconds. (Pembrey and Phillips.) 



remain unpoisoned and respond with a normally rapid contraction, 

 followed by the slower and more prolonged contraction of the 

 poisoned fibres. 



Glycerine will produce a similar effect. The prolonged relaxa- 

 tion produced by veratrine should be compared with that seen in 

 a muscle after repeated stimulation (see Fig. 43). 



CHAPTER III 



STRENGTH OF STIMULUS AS AFFECTING THE EXCITA- 

 TION OF NERVE AND THE CONTRACTION OF VOLUN- 

 TARY MUSCLE 



In previous chapters minimal and maximal stimuli were 

 mentioned in relation to the differences in the extent of the 

 muscular contraction, and it might appear that the response was 

 due to a similar gradation in the stimulation of the nerve and 

 the contraction of the muscle. There is evidence, however, 

 that the rule of " all or none," which has been demonstrated in 

 the case of cardiac muscle (see p. 52) applies also to nerve and 

 voluntary muscle. Gotch and Keith Lucas have shown that 

 the gradation in the intensity of the electrical response of the 

 nerve and the extent of the contraction of the voluntary muscle 

 are due to these tissues being composed of a large number of separate 

 fibres, whereas in cardiac muscle there is continuity between the 

 fibres. The submaximal excitation of nerve or voluntary muscle 

 is the maximal excitation of some only of its fibres. 



The following experiment should be performed. Make a prepara- 

 tion of the sciatic nerve of the frog, with the nerves of the plexus 

 and a portion of the vertebral column. Two branches of the sciatic 



