THE PROGr, AND ON THE ACTION OE THE YAGTTS NERYE. 
1005 
directly with the strength of the current; also fluids containing different chemical 
substances, especially alkaline substances like sodium hydrate and sodium carbonate, 
which .according to Biedermann* increase most markedly the excitability of the 
striated muscular tissue, produce when supplied to the apex more or less rhythmical 
contractions. Further, nutritive fluids alone, such as the blood of the animal itself, 
cause the same effect, provided that in addition a certain pressure exists within the 
apex cavity; and the effect of this distension of the walls by pressure, which is the 
essential jpoint in this method of producing rhythmical apex contractions, may fairly 
be classed as a continuous mechanical stimulation. Here, also, according to Ludwig 
and Luchsinger,! the rapidity of the rhythm varies directly with the extent of this 
pressure. 
The facts which have just been urged show that the ventricular muscle has the 
power of summing up the effects of two, three, or more impulses so as to produce a 
contraction though each of those impulses is inefficient of itself to cause the muscle 
to contract. In the same way it is possible to conceive that the muscle should be able 
to sum up the effects of a series of stimuli which follow one another so closely as to 
deserve the title of a continuous rather than a discontinuous stimulation. If, then, we 
have reason to suppose that in the first case the discontinuous stimuli ultimately 
produce a contraction by increasing the excitability of the muscle, so in the second 
case we may suppose that a continuous stimulation causes a rhythmical series of con¬ 
tractions by acting in the same way. The difference between the two cases would 
consist in the extent to wdiich the excitability was increased; in the one, the muscle 
is not rendered sufficiently excitable to contract without the assistance of a distinct 
separate stimulus ; in the other, the excitability is increased so much that no separate 
stimulus is required : the muscle, in fa.ct, can be spoken of as capable of spontaneous 
contraction. 
PART II. 
On the Action of the Vagus Nerve. 
Ever since the brothers Weber discovered that stimulation of the vagus nerve 
caused the heart to stand still, physiologists have been incessantly endeavouring to 
discover the reason of this standstill, with the result that an enormous mass of facts 
in connexion with the action of the nerve has been accumulated, and a variety of 
hypotheses have been suggested to explain these facts. 
Thus it was found that the vagus at one time causes complete stoppage, at another 
only slowing of the heart’s rhythm; this fact has led to the view that the complete 
cessation of beats is simply a prolonged slowing, and therefore that the nerve by its 
direct action upon the motor ganglia causes the discharges from the ganglia to take 
place at a slower rate. 
tzungsber. d. k. Akad. d. Wiss. (Wien). Bd. lxxxii., 1880. 
t Pfluger’s Arcliiy, Bd. xxv., S. 211. 
