THE PHENOMENON OF CONTRACTION. 35 



has been made by a number of authors* on various grounds. It has been 

 suggested by some that there are two different contractile substances in mus- 

 cle, one giving the usual quick contraction, known as a "twitch," the other 

 the slower contraction, which exhibits itself as tone or contracture.f 



The Effect of Rapidly Repeated Contractions. When a mus- 

 cle is stimulated repeatedly by stimuli of equal strength that 

 fall into the muscle at equal intervals the contractions show cer- 

 tain features that, in a general way, are constant, although the pre- 

 cise degree in which they are exhibited varies curiously in different 

 animals. Such curves are exhibited in Figs. 14, 15, and 16, and 

 the features worthy of note may be specified briefly as follows: 



1. The Introductory Contractions. The first three or four con- 

 tractions decrease slightly in extent, showing that the muscle at 

 first loses a little in irritability on account of previous contractions. 

 This phenomenon is frequently absent. 



2. The Staircase or " Treppe." After the first slight fall in 

 height has passed off the contractions increase in extent with great 

 regularity and often for a surprisingly large number of contractions. 

 This gradual increase in extent of shortening, with a constant 

 stimulus, was first noticed by Bowditch upon the heart muscle, 

 and was by him named the phenomenon of "treppe," the 

 German word for staircase. It indicates that the effect of activity 

 is in the beginning beneficial to the muscle in that its irritability 

 steadily increases, and the fact that the same result has been ob- 

 tained from heart muscle, plain muscle, and nerve fibers indicates 

 that it may be a general physiological law that functional activity 

 leads at first to a heightened irritability^. According to Lee,J 

 the "treppe" in muscle is due to an initial increase of irritability 

 set up by the chemical products formed during contraction. 



3. Contracture. This phenomenon of maintained contraction 

 has b^en described above. In frogs' muscles stimulated repeat- 

 edly it makes its appearance, as a rule, sooner or later in the 

 series of contractions; but there is a curious amount of variation 

 in the muscles of different individuals in this respect. 



4. Fatigw. After the period of the " treppe " has passed, the 

 contractions diminish steadily in height, until at last the muscle 

 fails entirely to respond to the stimulus. This progressive loss of 

 irritability in the muscle caused by repeated activity is designated 

 as fatigue. It will be considered more in detail under the head of 

 Compound Muscular Contractions and in Chapter II. The 

 curve obtained in an experiment of this kind illustrates in a 



* See especially Uexkull, "Zentralblatt f. Physiologic," 1908, 22, 33; also 

 Guenther, ''American Journal of Physiology," 1905, 14, 73. 



t loteyko, "Travaux du labaratoire de Physiologie," Institut Solvay, 1902, 

 5, 229. 



J See "American Journal of Physiology," 1907, 18, 267. 



