THE PHENOMENON OF CONTRACTION. 33 



nomenon is exhibited very clearly in the form in which it was first 

 described by Kronecker and Tiegel,* while in the following figure 

 (Fig. 15) the phenomenon is shown as it usually appears, that is, 

 after many contractions, and at a time when fatigue is beginning 

 to make itself felt. 



The Effect of Rapidly Repeated Contractions. When a 

 muscle is stimulated repeatedly by stimuli of equal strength that 

 fall into the muscle at equal intervals the contractions show certain 

 features that, in a general way, are constant, although the precise 

 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. 



3. Contracture. This phenomenon of retarded relaxation has 

 been described above. In frog's muscles stimulated repeatedly 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. Fatigue. 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. 



The Contraction Wave. Under ordinary conditions the fibers 

 of a muscle when stimulated contract simultaneously or nearly so, 

 and the whole extent of the muscle is practically in the same phase 

 of contraction at a given instant. It is comparatively easy to 



* Tiegel, "Pfliiger's Archiv fur die gesammte Physiologie," etc., 13, 71, 

 1876. 



