34 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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. 



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

 a rule, fatigue is accompanied by a prolongation, 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 

 with a definite velocity. In a long muscle with parallel fibers one 

 may prove, by proper recording apparatus, that if the muscle is 

 stimulated at one end a point near this end enters into contraction 

 before a point farther off. Knowing the difference in time between 

 the appearance of the contraction at the two points and their 

 distance apart, we have the data for determining the velocity of its 

 propagation. In frog's muscles this velocity is found to be equal 

 to 3 to 4 meters per second, while in human muscle, at the body 

 temperature, it is estimated at 10 to 13 meters per second. Know- 

 ing the time it takes this wave to pass a given point (d) and its 

 velocity (v), its entire length is given by the formula 1= vd. In the 

 frog's muscle, therefore, with a velocity of 3000 mm. per second, 

 and a duration of, say, 0.1 second, the product 3000 X 0.1 = 300 

 mms. gives the length of the wave or the length of muscle which 

 is in some phase of contraction at any given instant. Under 

 normal conditions the muscle fibers are stimulated through their 

 motor plates, which are situated toward the middle of the fiber, 

 or perhaps one muscle fiber may have two or more motor plates, 

 giving two or more points of stimulation. It follows, therefore, 

 from this anatomical arrangement and the great velocity of the 

 wave, that all parts of the fibers are in contraction at the same 

 instant and, indeed, in nearly the same phase of contraction. Under 

 abnormal conditions muscles may exhibit fibrillar contractions ; that 

 is, separate fibrils or bundles of fibrils contract and relax at different 

 times, giving a flickering, trembling movement to the muscle. 



Idipmuscular Contractions. In a fatigued or moribund muscle mechan- 

 ical stimulation may give a localized contraction which does not spread or 

 spreads very slowly, showing that the abnormal changes in the muscle prevent 

 the excitation from traveling at its normal velocity. A localized contraction 

 of this kind was designated by Schiff as an idiomuscular contraction. It may 



