326 PHYSIOLOGY CHAP. 



Gaskell, from the frog's heart, in which the inotropic negative 

 effects of vagus stimulation on the auricles and the ventricle are 

 simultaneously recorded. The greater diastolic relaxation pro- 

 duced by the vagus is not necessarily associated with diminished 

 frequency and height of systole, since it occurs also when vagus 

 stimulation produces no change either in frequency or intensity of 

 the contractions. This fact, which has been substantially con- 

 firmed for mammalia by the researches of Mac William, Johannson 

 and Tigerstedt, Fram;ois-Franck, and especially by Stefani (Chap. 

 VII. 8) completely justifies us in applying the term of diastolic/^ 7 

 nerves to the vagi. We shall presently consider the nature of the 

 process by which the vagus actively incites the cardiac diastole. 



In addition to producing negative chronotropic and inotropic 

 effects, the stimulation of the vagus can also impede the conduc- 

 tion of the contraction wave, or, in Engelniann's nomenclature, can 

 produce negative dromotropic effects. 



Nue'l found in the frog, on recording the contractions of the 

 auricle and ventricle separately, by means of writing levers 

 connected to those parts of the heart by threads, that vagus 

 excitation acts more easily on the auricle than on the ventricle. 

 Gaskell, on the other hand, has frequently observed the opposite 

 effect, i.e. that the ventricular contractions almost disappeared, 

 while the auricular contractions increased. In the land tortoise, 

 he failed to establish any action of the vagus on the ventricle, 

 while the contractions of the auricles were much reduced, without 

 any slowing of rhythm. Wesley Mills again found in certain 

 amphibia, reptiles, and fishes that the effect of the vagus was 

 greater on the auricles than on the ventricle ; Mac William, how- 

 ever, found the opposite on other animals. We cannot at present 

 give any explanation of these phenomena. 



Other facts, on the contrary, show clearly that vagus excitation 

 diminishes conduction of excitation from one segment of the heart 

 to another. Gaskeli noted in tortoises that stimulation of the 

 right vagus had no effect on the beats of the sinus venosus, while 

 it brought the auricles and ventricle to a complete standstill. In 

 mammalia, Mac William observed cases in which the auricles beat 

 with a more frequent rhythm than the ventricles ; the excitation 

 was not propagated from the first to the second segment, although 

 the excitability of the ventricles was undiminished. Bayliss and 

 Starling finally discovered a method by which it is easy to show 

 that the stimulation of the vagus produces negative dromotropic 

 effects. They induced an artificial rhythm of the heart by direct 

 excitation of the auricles three to four times per second, and then 

 found that a gentle excitation of the vagus sufficed to reduce the 

 number of ventricle beats to half the number of those of the 

 auricle, or even stopped them for a short time, while the auricle 

 beats continued. 



