1885.] of the Heart in Fishes, especially that of the Eel. 113 



ence to blocking in the tortoise heart seem to obtain in the eel's heart 

 as well. If the means employed to bring about blocking have been 

 kept within certain limits — if the section has not been carried too far 

 or if the pressure applied has not been too great — the blocked con- 

 dition usually passes off after a time, and the normal propagation of 

 the contraction is again evident. The recovery of the tissue from the 

 blocked condition can be materially accelerated by the application of 

 the normal salt solution, and a similar beneficial effect is often 

 apparent after the cardiac action has been arrested for a time by 

 stimulation of the vagus nerve. 



The normal sequence of the events constituting a cardiac beat is 

 often in the course of prolonged experiments seen to become inter- 

 rupted. The change which presents itself with the greatest fre- 

 quency is a failure of the ventricle to respond to each beat of the 

 other parts of the heart. Such failure often occurs without any loss 

 of excitability in the ventricular tissue : the condition often seems to 

 be one in which the propagation of the contraction from the auricle 

 to the ventricle is interrupted. The ventricle then seems to remain 

 quiescent, not because it is incapable of contracting, but because the 

 contraction is not transmitted to it from the auricle and sinus. The 

 failure of transmission seems to occur at the junction of the ventricle 

 with the rest of the heart — at the mitral orifice. This fact is of 

 interest when considered in relation with the peculiar character of the 

 muscular connexion between the ventricle and the rest of the heart 

 — a connexion which is established by means of an exceedingly nar- 

 row and prolonged isthmus of muscle substance. Whether or not 

 this peculiarity in structural arrangement is the cause of the fre- 

 quent failure of conduction at this part is a question on which it 

 would be premature to make a decided statement. 



A similar failure of the ventricular sequence can usually be 

 brought about by repeatedly heating the whole heart or the sinus 

 and auricle alone. Heat causes a great acceleration of the beat 

 with a simultaneous enfeeblement. When the contractions of the 

 auricle and sinus have thus been rendered extremely weak, a com- 

 plete suspension of the ventricular action is commonly seen. When 

 the auricular beats begin to recover their strength, the ventricle 

 again begins to respond to each auricular beat, even though the 

 auricular rate be still very rapid ; the ventricle contracts in sequence 

 to each auricular beat if the auricular beats are tolerably strong. 

 The strength of the auricular beats seems to be a very important 

 factor in regard to the question of the transmission of the contraction 

 to the ventricle. And this statement is borne out by a number of 

 facts — among others by a result which is often seen when the 

 auricular beats are rendered excessively weak in consequence of 

 their being elicited in rapid succession by artificial stimulation. In 



vol. xxxviii. - I 



