32 PEACTICAL PHYSIOLOGY 



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 gastroc- 

 nemius is more bulky, some of its fibres remain unpoisoned and respond 

 with a normally rapid contraction, followed by the slower and more 

 prolonged contraction of the poisoned fibres. 



CHAPTER V. 



THE CONDITIONS WHICH AFFECT SINGLE MUSCULAR 

 CONTRACTIONS-CONTINUED. 



(c) Temperature Since the shortening of muscle during its contrac- 

 tion is but the outward and visible sign of chemical changes taking 

 place in the muscle, it is not surprising that changes in temperature 

 should greatly affect the single muscle-twitch. 



In warm-blooded animals whose bodily temperature does not undergo 

 a greater variation than about 2 C., the effect of different temperatures 

 on muscular activity is unimportant. But it is quite otherwise in cold- 

 blooded animals whose range of bodily temperature is that of their 

 external medium. In them, the muscular activity of which they are 

 capable at any moment is determined largely by the temperature of 

 their muscles. Again, the subject becomes important for warm-blooded 

 animals when, from any cause, their bodily temperature is materially 

 altered, as it may be by disease. These abnormal variations in their 

 temperature may be sufficiently great to affect the muscular activity of 

 which the animal is capable. More frequently, however, they are 

 important because of the effect which an abnormally high bodily tem- 

 perature, especially when long continued, may have upon the actual 

 chemical constituents of muscle, and especially upon its proteids. 



In order to study these effects, the apparatus is arranged to stimulate 

 a muscle with single maximal induction shocks, using the " striker " 

 of the drum, in the primary circuit. Either a hyoglossus or gastroc- 

 nemius preparation may be used ; if the latter, it must be prepared 

 without a covering of skin, in order that its temperature may be more 

 readily altered. Also, the muscle must be stimulated directly and not 

 through its nerve, since changes of temperature affect nerve (see p. 316). 



It is important to use maximal stimuli, for cold increases the 

 excitability of muscle, and a stimulus which is minimal at 5 C. will 

 be sub-minimal at 25. The lever should be weighted near its axis 

 and the drum should revolve at a rate of about 20 cm. per sec. 



