66 ELECTRO-PHYSIOLOGY CHAP. 



quick fibres are more easily exhausted than the sluggish, but 

 enduring, red fibres. 



Giiitzner finds the same reaction in the flexors and extensors 

 of the frog's foot. If in bloodless legs, the sluggish extensors 

 and quick, excitable flexors are made to serve up frequent 

 contractions, the initial difference in contraction process will 

 completely disappear, or even reverse itself. That is to say, the 

 flexors composed mainly of easily excitable, quick fibres are 

 more easily fatigued than the characteristically sluggish, resistant 

 extensors. This is demonstrated by the following experiment 

 (Grtitzner, I.e.} If the iliac artery of the frog is tied on one side, 

 the animal at first springs as if normal in the direction of its 

 long axis, since the extensors (gastrocnemius) of both sides are 

 able to function equally ; soon, however, the animal leaves the 

 leg of the tied side extended, and only draws it up later after 

 its spring to the body : the excitability of the flexors has 

 already been diminished by the short ana?mia. 



Where a single muscle is composed of two groups of fibres, 

 differing physiologically as above, and provided the one group 

 does not preponderate too much over the other, the contraction 

 curve (with sufficiently strong excitation) must obviously be 

 regarded as a combination of two curves, differing in form and 

 time -relations, as can even occasionally be detected in the 

 myogram. We have actually been familiar for a long time with 

 certain peculiar double-topped curves of contraction, and their 

 origin now becomes intelligible (13). In many cases, provided 

 the " sluggish " fibres do not lag too far behind the " quick " 

 fibres, they also come into play at the first excitation ; the curve 

 is double-topped from the beginning, as, e.g., in the gastrocnemius 

 group of the rat (14), and usually the frog's sartorius. In 

 other, and indeed most, cases, where the quick fibres are in the 

 ascendant, the mixed fresh muscle commonly contracts at the 

 first effort of artificial excitation as if composed of quick fibres 

 only the simultaneously excited, but slower, and more sluggish 

 portion being merely drawn along with the other. But if the 

 < | nick part be more and more fatigued, the sluggish fibres come 

 into action, and the curve becomes double- topped (15). 



The difference in excitability and contraction between 

 the quick and sluggish fibres is well exhibited in chemical 

 excitation of the sartorius (Griitzner, 16). If the upper surface 



