120 Dr. J. A. Mc William. The Structure and Rhythm [Jan. 29, 



end of a cut vagus nerve) to cause contraction of the oesophagus, 

 leads readily to cardiac standstill. 



Single induction shocks to the vagus nerve have no distinct effect 

 on the heart's action unless the shocks are very powerful. 



When arrested by vagus stimulation, the heart stands still in a 

 state of diastole, and — when the circulation is going on — the whole 

 heart, but more especially the auricle and sinus, become greatly 

 distended with blood, if the inhibition be of any considerable dura- 

 tion. The great veins also become much gorged; they are very 

 capacious, whilst the presence of dilatations upon the jugular veins 

 (jugular sinuses) allows of the accommodation of a large amount of 

 blood. 



The effects of cardiac standstill on the circulation can readily be 

 observed by fixing the transparent part of the animal's tail under a 

 microscope, and then inducing inhibition of the heart by stimulation 

 of the vagus nerve, or simply by pressing on the gill — a proceeding 

 which causes reflex cardiac arrest. When the heart stands still, there 

 is a very gradual slowing of the blood-current which at length comes 

 to a state of complete stagnation ; this result is not arrived afc until 

 the cardiac action has been completely stopped for a period of from 

 one to two minutes. When inhibition has passed away, and the 

 heart's action has recommenced, the blood-flow is very speedily 

 restored ; the change is a much more rapid and abrupt one than the 

 change observed when the heart was brought to a standstill ; the first 

 recommencing beat causes a distinct movement of blood in the capil- 

 laries of the tail. 



High intra-ventricular systolic pressure and distension of the heart 

 with blood (caused by clamping the branchial artery) do not obviate 

 the inhibitory effects of vagus stimulation. 



The eel's heart presents some striking peculiarities as regards its 

 mode of recommencing action after it has been arrested through the 

 medium of the inhibitory nerves. 



It will be remembered that in the case of the frog, toad, rabbit, 

 dog, when the heart's action goes on after an inhibitory standstill, 

 the various parts of the heart recommence action in the fashion in 

 which they normally beat. Thus in the frog, when the cardiac 

 action recommences after a period of inhibition, the Heart beats 

 present their usual characters as regards the order of succession ; 

 the contraction first affects the sinus, and then passes over the auricles 

 and ventricle successively. In the eel the mode of recommencing is, 

 after slight inhibition, similar as a rule to that seen in the other 

 animals mentioned, — the contraction begins in the sinus (ostial part) 

 and passes over the rest of the heart in the normal fashion. But 

 after profound inhibition the renewal of cardiac action presents 

 features of an entirely different and very peculiar character. For 



