220 



THE CIRCULATION OF THE BLOOD 



auriculoventricular junction and on the ventricle remains. After the 

 atropinization, vagus stimulation delays the transmission of beat from 

 auricle to ventricle and shortens the time of each beat in the ventricle. 

 It was further found that by the local application of atropine various 

 parts of the ventricle can be rendered irresponsive to the influence of 

 the vagus and the effects of this nerve on the form of the cardiogram 

 modified at will. These results have an important bearing in the in- 

 terpretation of the cause of the T-wave of the electro-cardiogram 

 which will be referred to later. Mines' results show that the proba- 

 ble explanation is that the T-wave is due to the greater duration of the 

 excitatory state at the base than at the apex, for by altering the relative 

 duration of this state at. base and apex by the above methods, he could 

 cause the T-wave to appear or disappear. 



The direct excitability of the heart muscle to external stimuli is also 

 depressed during vagus stimulation. This effect is, however, not evi- 



Fig. 67. Tracing to show that vagus stimulation may facilitate transmission from auricles to 

 ventricles. It shows the effect of right vagus stimulation on heart-block produced in the turtle by 

 a clamp. Upper tracing records ventricle; lower tracing, auricles. Weak faradization of the right 

 vagus nerve beginning at A affected the degree of block only at 'f , when a lengthened period 

 between auricular contractions caused a single ventricular contraction. At B stronger faradiza- 

 tion of the same nerve produced marked slowing of the auricles, in consequence of which the block 

 disappeared and the ventricles contracted after each auricular contraction. Block reappeared when 

 the rate again became rapid. Initial auricular rate = 36 per minute. (From Garrey.) 



dent in the case of all hearts, but is seen in those of certain fishes (e. g., 

 the eel). 



The Mammalian Heart. The action of the vagus on the mammalian 

 heart may be investigated either by exposing the heart and connecting 

 the auricles and ventricles with specially designed recording levers 

 (myocardiograph), or if we desire to study the influence on the heart as 

 a whole, by taking a blood-pressure tracing from one of the large arteries 

 by means of a spring manometer. The results are in general similar to 

 those observed on the frog or turtle heart, the main effects being de- 

 veloped on the auricle. Considerable differences are found in the effect 

 on the heart as a whole in different animals, particularly with regard to 

 the facility with which escapement occurs. In the dog when the vagus 



