IX 



CARDIAC MUSCLE AND NERVES 



325 



Besides the iiiodili cations of rhythm (Engeliuann's chronotropic 

 effects) it is important to consider the changes produced by 

 vagus excitation on the amplitude of the beats, or more exactly, 

 on the degree of systolic contraction and diastolic expansion 

 (ino tropic effect). 



Coats was the first (1869) to note that the excitation of the 

 vagus obstructed systole 

 and favoured diastole. 

 He found by exact ob- 

 servations, carried out in 

 Lud wig's laboratory on 

 the frog's heart in situ, 

 connected with a record- 

 ing manometer, that 

 vagus stimulation, with- 

 out moderating, the 

 rhythm, not infrequently 

 diminished systolic con- 

 traction and augmented 

 the diastolic expansion. 

 It was on these and other 

 phenomena observed in the dog, in relation to the aspiration of 

 the heart, ' which increases during vagus excitation, that we in 

 1871 based the first principles of our theory of active diastole (see 

 Chap. VII. 8). 



Coats's results were fully confirmed and better worked out in 

 1882 by Heidenhain and Gaskell. The former found, on stimu- 



FIG. L47.Myographic tracing of frog's heart : A, Auricles, 

 V, ventricle, with method of double suspension. (Gaskell.) 

 Electrical excitation of the vagus in section between 

 the two vertical lines. Shows arrest of beats, both in 

 auricle and ventricle, which continues after the close of 

 stimulation ; the beats subsequently recommence, and 

 become rapidly larger than they were before stimu- 

 lation. 



Fio. 148. Mypgraphic tracing of frog's heart as in preceding figure. (Gaskell.) In this case, vagus 

 stimulation does not arrest the beats, nor retard them, but diminishes their amplitude. 



lating the vagus with induction shocks thrown in at intervals of 

 two to five seconds, that the systoles almost) disappeared without 

 diminishing in frequency, 



Gaskell found that vagus arrest only occurs in the frog when 

 the heart is well nourished, and fails to come off when it is slightly 

 fatigued. Figs. 147 and 148 represent two curves obtained by 



