1. Observations on tissue cultures of heart muscle from chicken embryos indicate that in general the growth rate of the individual cells decreases with age. The negative acceleration of growth is greatest at the beginning of life as in the intact embryo. 2. The latent period before growth commences increases with the age of the implanted tissue, but in a different fashion. The acceleration of the latent period is greatest near the end of the incubation period, thus demonstrating that the factors determining the initiation of growth and those determining the extent of growth are not similar.

These experiments show, then, that there are changes in rate which take place during the incubation period and that the direction of the curve expressing them changes in an orderly fashion. No attempt has yet been made to ascertain the influences operative in bringing about these alterations. It is our intention, however, to attempt to correlate them with such processes as changes in the rate of the general metabolism of the body, changes in structure of the heart muscle, and if possible with changes in the inherent dynamics of the muscle. It will be seen that the number of experiments at a given age is relatively speaking small. The temperature in spite of the precaution mentioned has likewise not been uniform, the range in the greatest number (38) of experiments having been 38°C. ± 1°. A few (four) counts have been made when the temperature was as low as 33.6°C. and a few (two) also at a temperature as high as 40°C. Although a curve as smooth as the one which has been drawn in all probability represents the general course of heart rate change, an explanation of the deviations which have been found is desirable. At least two influences which may be operative suggest themselves. First may be considered the effect of the unavoidable injury incident to cutting a window in the shell and removing so much of the shell membrane as to make the object visible. It has been a uniform practise to discard those eggs in which hemorrhage took place, so that if opening the egg has a share in the difficulty, this operation may be thought of as exerting a mechanical stimulus on a nervous or other controlling mechanism. But whether, especially at the early ages, a mechanism of such a nature exists within the embryo, we do not know. The search for one lies for the present beyond the range of our interest. A second influence responsible for the relatively large deviation in rate from the average may be that of fluctuations in temperature. The air of the room in which the observations were made was, in a given situation, fairly uniform at 38°C. ± 1.0°. as the temperature records show. In the earlier experiments the temperature was read at a distance about 1 foot away from the egg. Between this point and the egg a difference unknown to us may well have existed. It is improbable, however, that it can have been more than 2°C., that is to say, 38° ± 2°. Roughly if the change in rate is approximately 15 beats per degree a maximum deviation of 30 beats may be expected. In point of fact a deviation as great as this did not take place. In later experiments the temperature was read in the immediate vicinity of the eggs, so that a correction can be made more accurately. There is in all probability a high degree of uniformity in the rate of the heart of the chicken embryo at each stage of development, although variation is also found. What the extent of the deviation is that may properly be encountered we do not yet know. It is desirable to pursue this subject further. Observations will accordingly be made and will be reported later.

In these experiments we have shown that, with the technique adopted, differences in behavior are exhibited by fragments of the heart taken from different localities. The different localities behave in a more or less uniform manner. The pace-making function, for instance, is found at first throughout the cardiac tube but later it is restricted and comes to reside in a special small area at the back of the right auricle near the center. The pace-making system is able to develop a rate comparable to that shown by the whole intact heart, irrespective of the size of the fragment in which it is contained. Later, under the circumstances of the study, the ventricular structures lose the power of spontaneous contraction, and later still, the auricular ones also. It need scarcely be pointed out, however, that this loss refers only to the function of pace making. In its place, the various localities of the heart undoubtedly take on other capabilities. This is what is meant after all by differentiation. The question whether the pace-making and conduction systems reside in the remains of primitive portions of the cardiac tube in an undifferentiated form, or whether on the other hand these primitive portions develop into differentiated structures which preside over these functions may be reviewed afresh. Obviously the tube in its early state possesses these functions; obviously also the major part of the heart loses them during the course of development. A knowledge of the changes in form paralleling changes in function would have great interest. On this phase of the problem we hope to report later. On the basis of these observations, differentiation from the point of view of stimulus production may be viewed perhaps in this manner. Pace making, the conduction of impulses, and contraction are the primitive functions of the tube. As the tube develops into the adult structure, pace making and conduction are supposedly served by tissues resembling in structure the original ones. Whether as a matter of fact a structural change takes place is an interesting and important problem. Those portions of the heart which require to develop greater degrees of energy lose the primitive functions of pace making and conduction, and, in the transformation, take on a differentiated structure. It is, then, not the structures in which the primitive functions of pace making and conduction reside which are differentiated, but the greater mass of ventricular muscle. These reflections have their origin not only from our own work but they grow out of observations to be found in the writings of those (A. Keith and I. Mc-Kenzie) who call the nodal and conduction tissues in the heart, embryonic. But whether from the point of view developed here the use of this term is completely descriptive remains an interesting problem.

In Vol. xxxix, No. 1, Plate 2, Columns 1 and 2, now headed by the captions Lead I and Lead II respectively, should be interchanged. The electrocardiograms in Column 1 are Lead II ; those in Column 1 are Lead I .

These observations then show that in one way or the other the heart was affected in 35 of 37 cases of rheumatic fever. The evidence was of three sorts; first, the duration of auriculoventricular conduction was increased, though not always, and usually not to the degree of causing heart block; second, alteration in the ventricular complex of the electrocardiogram affecting in certain instances the Q R S complex and in others either the interval between R wave and T wave, or the T wave itself; and third, the occurrence of numerous irregularities in cardiac rhythm. These signs are characteristic of cardiac involvement, but are not specific for rheumatic fever; that is to say, from their presence rheumatic fever cannot be diagnosticated; all of them express merely derangement of the heart. They are signs which are, however, to be taken as evidence that the heart is affected by the rheumatic process even though an inference cannot be drawn as to the nature or the permanence of the injury. This may be slight and altogether transient even if the sign appears to be of advanced degree. The facts brought forward show that it is possible during the course of the disease to know whether the heart is involved in the general process. When sufficient data have been accumulated over a sufficiently long period of time, the usefulness for prognosis of observations like these will become established.

1. Slowing of the heart occurred in monkeys and guinea pigs during the febrile period of the experimental infection due to Leptospira icteroides . A similar reaction took place in animals inoculated with Leptospira icterohœmorrhagiœ . 2. The mechanism of slowing was usually due to slowing of the whole heart. 3. Once incomplete heart block was seen. Changes in the ventricular complex occurred four times.

It has been shown that enzymes in serum will change maltose, dextrin, and starch so that they will react as dextrose in media. These enzymes are destroyed by heating to 60°C. for 15 minutes, but they are present in sera that have been refrigerated for as long as 18 months. The practice of using carbohydrate media containing unheated serum should be discouraged, and if it is used the possibility that the carbohydrate may be changed by the enzymes present must be considered.

The function of the arteries as an elastic reservoir between the heart and the capillaries is reviewed. The appropriateness of selecting the exit from this reservoir as the point for estimating its effective pressure is shown. The technique for taking the pressure here by the method of Gaertner is described and its advantage and certain apparent disadvantages are indicated. The technique of Gaertner is shown to be especially applicable to the study of the blood pressure in fibrillation of the auricles. The use of this technique has brought out a defect in the so called fractional method of taking the pressure in this condition; the brachial and digital curves cross. Taking pressure of both brachial and digital arteries has shown that certain different types exist; first, that in which both central and peripheral pressures are stable; second, that in which the more central pressure is stable and the peripheral pressure fluctuates; and third, that in which both pressures fluctuate together.

1. Experiments on dogs show that whereas the pulse pressure in the larger arteries (femoral) varies extensively, it varies within narrow limits only in the small ones (dorsal artery of the foot). These results supply an experimental explanation for the fact that in man uniform pressure readings were obtained by Gaertner's method at the digital artery. 2. The experiments show likewise that in certain instances the level of pressure is maintained when fibrillation of the auricles sets in. It is therefore clear that when the mechanism of the heart beat in man changes to fibrillation of the auricles, a change, that is a fall, in pressure need not necessarily develop.

We have shown in a series of 105 cases of pneumonia, 95 of which we have selected as available for statistical study, that digitalis given by mouth has an action on the heart. We have judged this action to be present because changes occurred in the auriculoventricular conduction time and in the form of the T wave of the electrocardiogram, just as they do in the non-febrile heart. This conclusion is strengthened by finding that the pulse rate in fibrillating and fluttering cases fell in the presence of fever, exactly as it does in non-febrile cases. The dose and the time required to produce these effects are given and are the same as in the non-febrile cases. When there was a difference in the amount necessary to produce one or the other of the changes, it was found that the T wave is more often and more readily affected than the conduction interval. We have shown that the intoxication due to pneumonia is probably not responsible for the changes found, both from a study of the statistics and because in the control cases reverse tendencies were often found (that is, decrease in conduction time and increase in the size of the T wave). We have shown that the method of selection in consequence of which we treated a large number of severe cases did not prejudice our results, because it could be demonstrated that the proximity of death, whether in control or treated cases, was not necessarily associated with the changes we are describing. We have also, by referring to the literature of the subject, brought evidence to show that heart muscle does not undergo those changes in pneumonia, as it does in other infectious diseases, which would lead one to expect changes in conduction found in other diseases. The changes in conduction which have been reported by others were almost entirely associated with the giving of digitalis.

It has been shown in this investigation that digitalis, administered orally to patients, can modify the T wave in the electrocardiogram. When the T wave in the initial curve is directed upward, the first change noticed is a lowering, and the final change is an inversion of the wave. It is not only the wave itself, but that portion of the curve between the end of R and the end of T which is involved. Instances in which the initial T waves have other than upright forms are described and their behavior under the influence of digitalis has been indicated. This influence of digitalis on the T wave may be detected in thirty-six to forty-eight hours after the administration of digitalis has commenced; it may persist as long as twenty-two days after the administration has been stopped. Instances where it persisted only five days have been encountered. The unexpected length of duration of the sign probably explains why a second treatment with digitalis requires a smaller amount of the drug to produce the same effect, than the first.

The effect of injecting morphin in right vagus dogs is to slow or practically to stop auricular systoles, the circulation being maintained by ectopic, independent, ventricular contractions. The function of conduction is, relatively speaking, undisturbed. The fact that after morphin injections there is little or no disturbance of conduction in right vagus dogs and a profound one in left vagus dogs may be used as a factor in distinguishing between them, especially when there is a reduction in rate greater than usual in left vagus dogs. The effect on left vagus dogs is to slow the rate of the auricles moderately and to increase the length of conduction, so that partial auriculoventricular block or complete auriculoventricular dissociation results. These are precisely the results that have been obtained by faradic stimulation of one vagus nerve, the other being divided. There are slight differences in the results obtained between the two methods, but the explanation for these is probably to be found in the differences in the nature of the experiments. When faradic stimulation is employed, the stimulus is applied a short time only,—in our experiments for periods not longer than ten seconds. The occurrence of the maximum effect is sudden and does not provide for a gradual introduction of the ventricles to new conditions. In right vagus stimulation the ventricles usually stop beating. A gradual introduction is, however, not always necessary, as may be seen in figures 5 and 6 of the paper (4) already quoted, where idioventricular rhythms began without delay. Another difference between the two methods is found in the degree of effect produced on stimulating the left vagus; in the faradic method the change in conduction varies from mere lengthening to a condition so profound that complete dissociation results; these degrees have been described. The morphin method usually produces only incomplete dissociations. Twice only was complete dissociation produced. The similarity between the results of the two methods is sufficiently close to render it likely that in obtaining them an identical mechanism in the heart is involved. It may, therefore, be concluded that the inconstant action of the vagus resulting from morphin injections, called "ungleichartig" by Einthoven and Wieringa, appears so on account of the fact that the predominating effect of morphin may be exercised now on one and now on the opposite cardio-inhibitory system, and not, as was suggested, on account of a shifting of predominance from one to another of the fiber tracts in the vagi themselves. The explanation offered as the results of this series of experiments differs from theirs. The results obtained substantiate the conclusion reached in the former series, that the two vagi act differently. A fact relating to the mechanism of the right vagus nerve can now be added, from a consideration of the cardiac mechanism in right vagus dogs, namely, that derangements in conduction result, to a very slight extent only, as the effect of the influence of the right vagus nerve Although no solution of the production of the extreme grades of sinus irregularity seen in morphin intoxication is offered, the failure of this rhythm to be converted into a more profound irregularity may be explained. There seems little doubt that the occasion for the occurrence of an independent ventricular rhythm in these experiments was due to the slowness of the auricular rate resulting from morphin injection. But in the cases of sinus irregularity (Nos. 683, 685, and 700), the rate of the auricles was never sufficiently low for the ventricles to initiate an independent rhythm. The lowest rates recorded in them were 66.3 (No. 683), 85.7 (No. 685), and 91.4 (No. 700), and it appears that before an independent ventricular rhythm occurred in any of the experiments, the auricular rate had always fallen below 41 (Nos. 686, 688, 697, 698, 706, 715, and 721). On the other hand, the ventricular rates in the cases of sinus irregularity were higher than the highest idioventricular rates observed and no advantage could consequently have been gained by the onset of a new rhythm. No. 683 was an exception, but the rate in this case exceeded the idioventricular rates of all but Nos. 702 and 706 and was only 11.1 beats below that of the highest recorded. Whether sinus irregularity itself is a morphin irregularity the result of a sino-auricular block, in the sense of Eyster and Meek, has already been discussed.

It may be concluded from the results obtained in these experiments : 1. That stimulation of the right vagus nerve in the dog usually causes arrest of all the chambers of the heart. 2. That stimulation of the left vagus nerve exerts a moderate negative chronotropic effect on the auricles. 3. That stimulation of the left vagus nerve has a profound effect on the conduction of impulses over the auriculoventricular system. 4. That the degree of effect exercised on the auriculoventricular system by stimulation of the left vagus nerve varies. In some dogs conduction is depressed to an extent which causes only a delay in the conduction of impulses from auricles to ventricles (P-R time) ; in other dogs the conduction is depressed to a degree which results in incomplete heart-block; while in still other dogs conduction is so depressed that although the auricles continue to contract, no impulses pass from them to the ventricles. 5. That when stimulation of either the right or left vagus nerve causes asystole of nomotopic ventricular contractions, ectopic ventricular contractions may occur. 6. That the time which elapses before ectopic ventricular contractions occur depends upon the irritability of the ventricular muscle, and this may vary in different dogs. 7. That stimulation of the left vagus nerve may rarely cause sino-auricular block. Possibly stimulation of the right nerve may also produce this effect. 8. That there is consequently usually a great qualitative difference in the action of the two vagus nerves on the heart of the dog.