110 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



to do work, and if a muscle be excited many times in succession, the effect of 

 action upon the strength of the contraction process, the endurance, and the 

 coming on of fatigue can be estimated from the height of the succeeding con- 

 tractions. One might expect that every contraction would tend to fatigue and 

 to lessen the power of the muscle, but almost the first effect of action is to 

 increase the irritability and mobility of muscle protoplasm. 



Introductory and Staircase Contractions. The peculiar effect of action to 

 increase muscular activity was first observed by Bowditch, 1 when studying 

 the effect of excitations upon the heart. He found that repeated excitations 

 of equal strength applied to the ventricle of a frog's heart caused a series of 

 contractions each of which was greater than the preceding. If the contrac- 

 tions were recorded on a regularly moving surface, the summits of the succes- 

 sive contractions were seen to rise one above the other like a flight of steps. 

 This peculiar phenomenon received the name of the " staircase contractions " 

 (see Fig. 41). 



FIG. 41. Staircase contractions of a frog's ventricle in response to a series of like stimuli, written on 

 a regularly revolving drum -by the float of a water manometer connected with the chamber of the 

 ventricle (after Bowditch). The record is to be read from right to left. 



This effect of repeated excitations was later observed by Tiegel, 2 on the 

 skeletal muscles of frogs; by Eossbach, 3 on the muscles of warm-blooded 

 animals, and by many others on various forms of contractile protoplasm. 



The following series of contractions (Fig. 42), which closely resembles the 

 above, was obtained from the gastrocnemius muscle of a frog, excited at a 

 regular rate by a series of equal breaking induction shocks. 



FIG. 42,-Staircase contractions of gastrocnemius muscle of a frog, excited once every two seconds by 

 strong breaking induction shocks. 



The contractions in Figure 42 did not begin to increase in height imme- 

 diately ; on the contrary, each of the first four contractions was slightly lower 

 than the one which preceded it. A decline in the height of the first three or 



1 Serichte der koniglichen sachsischen Gesellschaft der Wissenschaft, 1871. 2 Ibid., 1875. 



3 Pfliiger>s Archiv, 1882, 1884, Bd. xiii., xv. 



