1885.] of the Heart in Fishes, especially that of the Eel. 115 



circulation is going on, a peculiar dark bulging of the ventricular wall 

 occurs at the stimulated part ; this bulging occurs towards the latter 

 part of the systole which results from the excitation. This phenome- 

 non occurs with great constancy and in well-marked form in all cases 

 where the circulation is intact and the ventricle is well filled with blood. 

 It is not seen in the bloodless ventricle. The momentary or continued 

 application of pressure may cause the same result, even though the 

 pressure be too weak to act as a direct stimulus leading to contrac- 

 tion of the part where it is applied ; the bulging may occur at the 

 spot where pressure has been applied, even though the contraction of 

 the heart occur in the normal fashion. The rationale of the phenome- 

 non in question seems to be that in the area where a direct stimulus or 

 even pressure insufficient to act as a direct stimulus has been applied, 

 the muscular contraction is much impaired in efficacy, and as a result 

 that area is unable to resist the high intra- ventricular pressure which 

 occurs during systole. The area in question, therefore, becomes 

 dilated with blood forced into it by the vigorous contraction of the 

 rest of the ventricle ; hence the dark bulging apparent on the sur- 

 face. A somewhat similar bulging is occasionally to be seen in the 

 ventricle of the frog as a result of strong direct stimulation. 



The application of a strong direct stimulus {e.g., an induction shock) 

 to the ventricle is able to elicit a forced beat at almost any phase of the 

 cardiac cycle. In a spontaneously- acting heart the occurrence of a 

 forced beat from ventricular stimulation is usually followed by a pro- 

 longed diastolic interval before the appearance of the next sponta- 

 neous beat. In an intact and normally acting heart this diastolic 

 prolongation observable on a ventricular tracing is not due directly to 

 the properties of the ventricle ; it is due to the conduct of the leading 

 part of the heart — the sinus. For the rate of contraction of the ven- 

 tricle of an intact heart is determined by the sinus — in which the 

 contraction of the heart originates. The property of exhibiting a 

 prolonged diastolic interval after a forced beat is not, however, pecu- 

 liar to the sinus. . It is equally well seen in the case of an isolated 

 automatically-acting auricle or ventricle. A similar phenomenon 

 has been described in the case of the entire frog-heart, and has there 

 been attributed by some observers to an influence of the nervous 

 mechanism. 



The contraction which occurs after the prolonged diastolic interval 

 is frequently of markedly increased size. 



When the heart is intact and the circulation going on, the form of 

 curve yielded by an artificially excited ventricular beat (resulting 

 from direct stimulation of the apex of the ventricle) is quite distinct 

 from that traced by a beat which occurs in normal sequence to an 

 auricular beat ; and the naked-eye characters of the beats obtained in 

 the two instances are quite different. 



