Some of the most important experiments relating to the action of the various cavities of the frog's heart were first performed by Stannius, and bear his name.

When the venous sinus is separated from the rest of the heart by cutting it off with a sharp razor, or by a ligature tightly drawn round it at its junction with the auricle, it continues to pulsate, but the auricle and ventricle stand perfectly still (a, Fig. 108). If now the auricle is separated from the ventricle by another cut (b, Fig. 108), or another ligature be applied (c, Fig. 108), at the auriculo-ventricular groove, the auricles remain motionless, but the ventricle begins to beat, so that the venous sinus and ventricle are both pulsating, while the auricles are at rest. The venous sinus and the ventricle, however, no longer beat with the same rhythm, and the rate of the ventricular beats is usually much slower (f, Fig. 109). In this remarkable experiment the complete stoppage of the auricles and ventricle which follows the removal of the venous sinus has been supposed to show that the motor centres for the entire heart reside in the sinus, and that from them the motor impulses originate which keep up the rhythmical pulsations of the organ. But the fact that the ventricles begin to pulsate on their own account when separated by another cut from the auricle seems to show that they also contain motor centres. The hypothesis has therefore been advanced that both venous sinus and ventricles contain motor centres, while the auricles contain inhibitory centres.

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Fig. 108. - a, diagram of frog's heart ligatured at the junction of the venous sinus with the auricles. The venae cavse and sinus are represented with a crenated outline resembling the tracing which their beats might give if recorded on a revolving cylinder. The auricle and ventricle being motionless would only trace a straight line if connected with a recording apparatus. Their outline is therefore represented by a straight line. 6, diagram of a frog's heart in which sections have been made at the junction of the sinus with the auricles, and at the auriculo-ventricular groove. The sinus and ventricles pulsate, whilst the auricles remain motionless. The beats of the ventricle should have been represented as slower than those of the auricle, as in /, Fig. 109. c, the same as b, but with the parts of the heart separated by ligature instead of section.

So long as the auricles are in connection both with the venous sinus and the ventricle, the motor centres in the.latter two cavities are supposed to be sufficiently powerful to overcome the resistance offered by the inhibitory centres, and thus the cardiac rhythm is maintained. When the motor centres of the sinus are removed, the inhibitory centres of the auricle are supposed to be so powerful as to keep both it and the ventricle in a state of rest.

When the ventricle is separated from the auricles and their inhibitory influence removed, it again begins to pulsate rhythmically. In order to obtain a clearer idea of the mechanism of the heart, many variations of the above fundamental experiments have been made.

The chief results of these are the following : First, section or ligature of the venae cavae or of the venous sinus at any point before its junction with the ventricle does not affect the action of the heart (d, Fig. 109).

Second, section or ligature of the auricles at any point above the auriculo -ventricular groove arrests the movements of the part below them, while that connected with the venous sinus still continues to pulsate (e, Fig. 109).

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Fig. 109. - d, diagram of heart with ligature round the venous sinus. e, diagram of heart with ligature round middle of auricles. f, diagram of heart with ligature in the auriculo-ventricular groove. The pulsations of the ventricle are much slower than those of the auricle and venous sinus. This is indicated by the larger dentation of the outline of the ventricle.

Third, irritation of the vagus nerves usually produces stoppage of the heart-beats.

Fourth, ligature or section of the vagi before their entrance into the heart prevents their having any action upon it when they are stimulated.

Fifth, ligature or section of the venous sinus or auricles prevents any action of the vagi upon the part of the heart below the ligature or section.

It is evident that section or ligature of the heart at any point between the junction of the sinus and auricles and the auriculo-ventricular groove has the same action on the movements of the part below it as irritation of the vagus.

But more than this; although, as we have seen, the motor ganglia of the heart appear to be situated chiefly in the venous sinus, yet irritation of the sinus produces complete still-stand of the heart, even more perfect and prolonged than irritation of the vagus. Strong stimulation of the venous sinus has therefore the same effect as its removal. The parts whose motions have been arrested by section or by irritation, in the experiment just described, are not paralysed: this is shown by the effect of stimulation upon them.

When the auricles and ventricle are standing still after section or ligature of the venous sinus, irritation of the outside of the ventricle with a needle has no action (g, Fig. 110); but if its interior be irritated by a needle (h, Fig. 110) the auricle contracts first, then the ventricle, then the auricle again two or three times, but the ventricle does not respond. When the auricle is irritated by a needle applied to its outside, contraction both of the auricle and ventricle ensues (k, Fig. 110). When the auriculo-ventricular groove is irritated by a needle there are usually eight or ten contractions in response. When the outside of the auricle is irritated by an interrupted current, numerous and rhythmical contractions both of auricle and ventricle ensue.

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Fig. 110. - g, diagram of heart stopped by a ligature at the junction of the sinus and auricles. The outside of the ventricle is irritated by a needle, and the even outline indicates that no contraction occurs. h, diagram similar to g, but with the inside of the ventricle irritated by a needle. The projections on the outline of the heart indicate that one contraction of the ventricle and three or four of the auricles occur. k, diagram similar to g and h, but with the outside of the auricle stimulated by a needle. The projections indicate that one contraction of the auricle and one of the ventricle occur.

To sum up these results shortly, we find that either removal of the normal stimuli which pass in the direction of the circulation from the venous sinus to the auricle and then to the ventricle, or abnormally strong- stimulation, produces arrest of the rhythmical movements of the heart, or, as it is usually termed, inhibition.

Some exceedingly instructive experiments have been made by Gaskell, who, instead of separating the cavities of the frog's heart from each other by sections or by a ligature, compresses more or less completely the point of junction, so as to impede or block (as it is termed) to a certain extent the transmission of stimuli from one cavity to another (Fig. 111).

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Fig. 111. - Diagram to illustrate Gaskell's experiment. At a the jaws of the clamp hold the heart without compressing it, and each beat of the auricle is succeeded by one of the ventricle as shown by the figure 1/1 . At b the heart is compressed, and its rhythm disturbed, so that one beat of the ventricle only occurs for several of the auricles.

He does this by a clamp the two limbs of which are placed one on each side of the heart. By means of a micrometer screw their edges can be approximated so as either simply to hold the heart without pressure or to compress it to any desired extent. When the clamp is placed in the auriculo-ventricular groove, the beats of the auricles and ventricle are registered separately by levers above and below the clamp with which the auricles and ventricle are connected by threads.

When the heart is simply held by the clamp without compression, each beat of the auricle is followed by one of the ventricle; but when the auriculo-ventricular groove is compressed the transmission of stimuli from the auricle to the ventricle appears to be blocked in somewhat the same way as it is by compression in the contractile tissue of medusae, and one beat of the ventricle then occurs with every second, third, fourth, or more auricular beats, according to the degree of pressure, and if this be very great the ventricle will cease beating altogether.

The beats of the ventricle are shown in this experiment to be diminished or arrested by hindering or blocking the transmission of stimuli to it from the venous sinus and auricle. But, as one might expect, a diminution of the stimuli themselves has a similar effect as a block to their passage. Thus, if the auricle and sinus are heated, but not the ventricle, their rhythm is markedly quickened, but the ventricle now beats only once for every two or even more pulsations of the auricle, the heat appearing to render the impulses proceeding from the auricle and sinus more rapid but more weak. If the ventricle be heated as well, it will respond to each beat of the auricle, so that the whole heart beats more quickly, but if the ventricle alone be heated its rhythm remains unchanged.

Experiments which are likely to give useful information in regard to the action of various drugs on the cardiac muscle and nerves have been made by Gaskell by the aid of the clamp already described.