THE PHENOMENON OF CONTRACTION. 35 



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 hi 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,* 

 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 been 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. 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 and in Chapter II. The 

 curve obtained in an experiment of this kind illustrates in a 

 striking way one of the general characteristics of living matter, 

 namely, that every effective stimulus applied to it leaves a record, 

 so to speak. The muscle in this case is in a changed condition 

 after each stimulus, as is indicated by the difference in its re- 

 sponse to the succeeding stimulus. While it cannot be said that 

 a similar effect has been shown in all tissues, still the evidence in 

 general points that way, and some of the complicated phenomena 

 exhibited by living matter, such as memory, habits, immunity, 

 etc., are referable in the long run to this underlying peculiarity. 



Lee has discovered the interesting fact that while in frog's muscle, as a 

 rule, fatigue is accompanied by a prolongation of the curve, especially of the 

 phase of relaxation, this does not hold for mammalian muscle. In the latter 

 muscle the successive contractions become smaller as fatigue sets in, but their 

 duration is not increased. 



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 

 show, however, that the process of contraction spreads over the 

 fibers, from the point stimulated, in the form of a wave which moves 

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



