THE PRO a, AND ON THE ACTION OP THE VAGUS NERVE. 
1031 
energy, are less excitable, and less compact than the combinations which occur in the 
higher part of the series. Finally, I cannot help being struck with the fact that 
alkaline substances such as sodium hydrate or sodium carbonate increase the excita¬ 
bility, increase the tonicity and increase the force of the contractions in weakly-beating 
hearts ; while, on the other hand, such substances as lactic acid diminish the excitability, 
diminish the tonicity and diminish the force of the contractions of the cardiac muscle. 
Whether the resemblance in these two cases is purely fictitious, or whether the trophic 
action of the vagus nerve is connected with some such action of acids and alkalies I 
cannot say, and only put forward the resemblance in the hope that a possible clue 
may thus be found to the chemical action of the vagus upon the muscular substance of 
the heart. 
Postscript. 
(Added July 3rd, 1882.) 
Throughout this paper I have spoken of impulses proceeding from the sinus to the 
ventricle without attempting to define the nature of the stimulus which causes the 
ventricle to contract, or the path along which the impulse travels. Subsequent 
investigations which I have made upon the hearts of Tortoises have thrown new light 
upon the cause of the sequence of the ventricular upon the auricular beat, and point 
to a possible explanation of the want of sequence observed under different circumstances 
which is somewhat different to that suggested above. 
The heart of the Tortoise is removed from the body and suspended between two 
levers in the manner above described ; no clamp is used, but the aortic trunk is held 
firmly in a suitable holder. By means of section of the auricle in different directions 
and to different extents I find that— 
1. The sequence of the ventricular upon the auricular contractions is associated with 
the passage of a wave of contraction along the auricular muscular fibres to the auriculo- 
ventricular groove, and is not dependent upon the integrity of the large nerve trunks 
between the sinus and the ventricle. 
2. By means of section of the auricle the passage of this wave can be blocked and 
the ventricle in consequence remains quiescent. 
3. If the section be severe the block will be complete ; no contractions will pass. If 
less severe, the block will be partial and then every second contraction will pass, and as 
soon as it reaches the auriculo-ventricular groove will cause a contraction of the 
ventricle. 
In this way by hindering the passage of a contraction wave to the ventricle, the 
ventricle can be made to beat with every second contraction of the sinus and that 
part of the auricle in connexion with the sinus, in precisely the same way as by 
tightening the clamp between the sinus and ventricle of the Frog. The probability 
therefore is strong that the clamp produces the half-rhythm observed because it blocks 
MDCCCLX XXII. 6 Q 
