ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 93 



manner, and has been found excitable even more than 12 hours 

 after death. So, too, the ureter of the rabbit or guinea-pig will, 

 even after long immersion in cold, physiological salt solution, or 

 when taken from an animal some hours after death (no trace of 

 excitation being left under natural condition), become once more 

 fully excitable if warmed to the body temperature (I.e. 387). 

 A similar tenacity of life was found by Grlinhagen and his pupils 

 in the sphincter iridis of different mammals (53). Yet more 

 resistant, according to Sertoli (54), are the equally smooth 

 retractor penis muscles of certain mammals (horse, ass, dog), 

 in which excitability continues for as much as seven days 

 after extirpation. During the greater part of this time the 

 muscle was in a temperature of 5 8 C., and at the time of 

 the experiment was only warmed to 30 37 C. If the tempera- 

 ture remains at uniform height (39 40) the excitability dis- 

 appears in a short time. 



The rapid fatigue of certain smooth muscles under perfectly 

 normal conditions is in striking contrast with this great capacity 

 of resistance to ordinary nutritive influences. Engelmann 

 (Pflugers Arch. vol. ii. p. 263 f.) pointed out the effect of 

 fatigue in the rabbit's ureter after every individual contraction, 

 mechanical excitability being nil immediately after each twitch 

 has completed itself. During the subsequent pause it is gradu- 

 ally recovered. In a warm, fresh rabbit's ureter, where the 

 blood is still normally circulating, the initial height of excita- 

 bility is recovered after a few seconds. In the rat even one 

 second is not required under the same favourable conditions. 

 In cooled ureter, withdrawn from circulation, excitability returns 

 much more slowly and imperfectly after contraction (5, 10, or 

 more sees.) 



Thus we see that excised muscles of cold-blooded (inverte- 

 brate and poikilothermic) animals usually become fatigued, and 

 die, more slowly than those of warm-blooded animals ; yet this is 

 by no means an invariable rule, for, on the one hand, there are 

 muscles of cold-blooded animals which lose their excitability 

 quickly even at a low temperature (fishes, insects), while, on the 

 other, certain smooth muscles of the warm-blooded animals re- 

 main excitable at low temperature for an extraordinary length of 

 time, even when fully deprived of circulation. 



The muscular fatigue consequent upon excitation is, as already 



