112 



AN AMERICAN TENT-BOOK OE PHYSIOLOGY. 



muscle is excited repeatedly, a certain interval of rest must be permitted 

 between the succeeding excitation- it* its normal condition is to be maintained, 

 and that the more extensive the chemical changes produced by the excita- 

 tions the longer must be the periods allowed for recovery. This being the 

 case, the rate of excitation and consequent length of the interval of rest will 

 have a great effect upon the condition of the muscle and its eapaeity for work. 



(1) Eti'cft of Frequent F.reitations on the Height of Separate Muscular 

 ( 'ontracUons. — Other things being equal, the height to which a muscle can con- 

 tract when excited by a given irritant can be taken as an index of its capacity 

 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 lesseu 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 moviug surface, the summits of the succes- 

 sive contractions were seeu to rise oue above the other like a flight of steps. 

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

 (see Fig. 1 1). 



A 



■*m 



1 1 1 j i 



Fig. 44 —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 Rossbach, 3 on the muscles of warm-blooded 

 animals, and by Romanes 4 on the contractile tissues of Medusae. 



The following .-erics of contractions (Fig. 45), 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. 



The contractions in Figure 45 did not begin to increase in height imme- 

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

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

 tour contraction- is the rule when a normal resting muscle is called into action 



1 BerichU der koniglichen sach&ischen Gesettsehqft der Wissenschaft, 1871. 



9 Pfliiger>8' Archie, 1882, 1884, Bd. xiii.. xv. 



* Romanes: Jelly-fish und Star-fish, International Science Series, p. 54. 



2 Ibid.. 1875. 



