For the purpose of observing alterations in the strength of the cardiac pulsations as well as their rhythm, a convenient piece of apparatus is the one devised by Ludwig and used under his directions by Coats (Fig. 98).

One objection to this apparatus as shown in the engraving is, that the blood does not circulate freely through the heart, but this can be overcome by closing the tube at f only partially instead of completely, and according to the amount of closure the pressure under which the heart works may be regulated. Or the tube f may be lengthened and made to empty itself into the reservoir a. The pressure under which the heart works may be regulated by the height at which the tube is allowed to discharge.

Another apparatus is that used by Williams in his researches on digitalin (Fig. 99).1 It consists of a Y-shaped cannula whose stem is divided by a longitudinal septum into two halves, each of which is continuous with the fork on its own side. The stem is inserted through the aorta into the ventricle of the heart, which is kept moist by being dipped in a vessel containing serum or a dilute saline solution. One fork of the Y is connected with a flask containing blood-serum or other nutritive fluid, and the other with a manometer. By means of valves these fluids are made to flow only in one direction. These valves consist of a piece of glass tubing with a slit on one side; over this slit is loosely tied a piece of thin membrane (gold-beater's skin) which covers about three-quarters of the circumference of the tube. This membrane allows fluid to pass readily out of the tube from within outwards, but not from without inwards, any external pressure causing the membrane to become tightly applied to the slit and to close it.

Fig. 99.   Diagram of Williams's apparatus for investigating the action of drugs on the heart of the frog.

Fig. 99. - Diagram of Williams's apparatus for investigating the action of drugs on the heart of the frog.

1 Arch. f. exp. Path. u. Pharm., Bd. xiii. p. 1.

A very useful form of apparatus for investigating the action of drugs on the frog's heart and on the effect of the vagus upon it is made by combining the valves in Williams's apparatus with the apparatus of Ludwig and Coats.1

The apex (as the lower two-thirds of the ventricle is commonly called) contains, as has been mentioned, no nerves, and when separated from the rest, either by cutting or by tight ligature, usually lies perfectly quiet without contracting. When irritated by a single induced shock, it answers by a single contraction, just like any other muscular fibre.

But though the muscular fibres contained in the apex cease to contract rhythmically, when the nervous stimulus usually supplied by Bidder's ganglia is removed, they still retain a tendency to rhythmical contraction; and when subjected to a constant stimulus of another kind they again commence to pulsate. This is seen when the apex is stimulated by supplying it with oxygenated blood through a cannula under pressure (the pressure supplying the necessary stimulus), or by passing through it a constant or interrupted current, or by adding a trace of del-phinine to the nutritive fluid with which it is supplied. This phenomenon is similar to that which occurs in the bells of medusae already described (p. 110), which cease to contract rhythmically when their marginal ganglia are removed, but recommence when an additional stimulus is applied to the bell itself, by putting it into acidulated water.

A curious point has been made out by Bowditch regarding the excitability of the heart-apex. It has already been mentioned that the amount of contraction of voluntary muscle varies with the intensity of the stimulus, and that this is also the case with the reflex contraction produced by irritation of sensory nerves. The apex when fed with serum usually stands still for a long time before it begins to beat, but when in this condition may be made to contract by the application of an induction shock. The difference between the reaction of an ordinary striated muscle and of the apex to such a shock is, that the heart, instead of responding by a strong or weak contraction to a strong or weak stimulus, either does not contract at all or contracts with as much force as it can exert. The weakest stimulus which will act at all and the strongest have thus exactly the same action, or, in other words, a minimum is also a maximum stimulus. This condition does not correspond to that which obtains in the normal striated muscle when stimulated either directly or reflexly. We find, however, a corresponding condition in the reflex contraction of the muscle produced by stimulation of sensory nerves in an animal poisoned by strychnine (p. 181). We noted, however, in discussing the action of strychnine on the spinal cord, that, just after exhaustion had occurred from a spasm, strong and weak stimuli produced strong and weak contractions in the muscle. A somewhat similar condition appears to occur in the heart, for Mays has noticed that, when the apex is supplied with blood which has stood three or four days instead of with fresh blood, strong and weak stimuli produce strong and weak contractions.1

1 Harnack and Hoffmann, Arch. f. exp. Path. u. Pharm., Bd. xvii. p. 159.

It is obvious that, although the contractions of voluntary muscle on reflex stimulation may be analogous to the contractions of the apex, yet, in the former case, the alterations occur in the nervous centres, while in the apex the changes occur in the muscular substance.