The cause of the different frequency of the pulse in different postures of the body is a question of some interest, in examining which it is necessary to premise that the increase or diminution of frequency attending the change from one posture to another, is not merely a transient effect, dependent upon the muscular effort involved in the act of change, but a permanent state, continuing as long as the respective postures are maintained. This was long since stated by Dr. Graves*, who proved experimentally that when the posture of the body was changed without any effort of its own mus'cles, " the difference between the frequency' in the horizontal and erect postures was not less than when muscular exertion was used." The mode in which Dr. Graves effected this change of posture is not stated ; but in experiments performed by the writer of the present article, by means of a revolving board ±, a difference amounting to less than a single beat was found to exist between the average of twenty experiments, in which the body -Was transferred from one posture to the other by the machine, and an average of twenty experiments, in which the change of posture was effected by the voluntary efforts of the same persons. The round numbers with the machine were, —standing 87, lying 74, difference 13 ; without the machine, — standing 89, lying 77, difference 12. This very ,slight difference is due to the effort of the muscles in effecting the change of posi tion. When this is subtracted there still remains a much more considerable difference attributable to some permanent cause, which may be either the continuance of muscular effort, or some other condition. The differ ence of opinion which has existed upon this subject, gives an interest to the following brief summary of the explanations advanced by the leading writers on the pulse.
Bryan Robinson t, Falconer §, and Knox, without attempting to submit the question to the test of experiment, attribute, directly or by inference, the different frequency of the pulse, in different postures, to muscular contraction. Dr. Graves II , how ever, confesses himself to -be altogether at a loss for an explanation; Dr, Arnott 11" seems to refer it to the more or less favourable position of the body, in respect to gravity, while other authors attribute it to the varying positions of the heart and its valves.** Very little consideration is required to show the futility of all the other causes, except that assumed by Robinson, Falconer, and Knox. The two postures between which there is the most marked difference in the frequency of the pulse, viz. the erect and sitting postures, are precisely those in which there is no dif ference in the position of the heart or its valves, and very little difference in the re sistance offered to the circulation ; while the sitting and recumbent postures, between which there is so slight a difference in the number of the pulse, are accompanied by a marked change In the position of the heart and its valves, and of the column of blood to be propelled. On the other hand, the dif ference in the amount of muscular contraction required to support the body in the erect and sitting postures, is much more considerable than that required to support the body in the sitting and recumbent positions —differences in strict conformity with the observed fre quencies of the pulse in the several postures. This simple process of reasoning, therefore, serves to show the fallacy of the explanations now alluded to, and the reasonableness of the remaining alternative— muscular contrac tion.
With this strong probability the authorities just cited seem to have been satisfied ; and as it did not occur to them to submit this very reasonable theory to the test of actual ex periment, it was reserved for the writer of this article to place this mooted question beyond the reach of doubt.
The 'experiments required for this purpose were of the very simplest kind. It was merely necessary in successive experiments to place the body in such circumstances as to exclude every other assigned cause but the contraction of the muscles; in other words, the position of the body, and consequently of the organs of tile circulation, remaining the same, first to support the body, and then to call its own muscles into action to maintain its position. ' The following are the results of a series of such experiments.
1. Difference between the pulse in the erect posture, without support, and leaning in the same posture, on an average of twelve experi ments on the writer, 12 beats; and on au average of eight experiments on other healthy males, 8 beats.
- 2. Difference in the frequency of the pulse in the recumbent posture, fully supported, and partially supported, 14 beats, on an average of five experiments.
3. Sitting posture (mean or ten experiments on the writer), back supported, 80 ; unsup ported, 87; difference, 7 beats.
4. Sitting posture with the legs raised at ' right angles to the body (average of twenty experiments on the writer), back unsup- . ported, 86; supported, 68 ; difference, 18 beats. An average of fifteen experiments of the same kind on other healthy males gave the follOwing numbers :—back unsupported, 80; supported, 68; being a difference of 12 beats.
These experiments, with the simple reason ings already advanced, serve to demonstrate. the true cause of the varying frequency of the pulse in different postures of the body to be muscular contraction.
The effect of an inverted position of the body on the frequency of the pulse has been made the subject of experiment by Dr. Graves, and subsequently by the writer of this essay. The reader is referred for an, exact account of the experiments to the Guy's Hospital Reports, No. VII. It will be sufficient to state, that in the inverted position of the body the pulse becomes less frequent, especially in persons accustomed to this unusual posture. In two such instances the difference between the erect and recumbent postures equalled that between the recumbent and the inverted postures, beins in both cases 15 beats.
The following is a short summary of the leading facts relating to the effect of posture on the pulse.
1. In the healthy adult male the mean numbers of the pulse are as follows:— Stand ing, 79 ; sitting, 70; lying, 67 ; including all exceptions to the rule. Standing, 81; sitting, 71; lying, 66; excluding all exceptions to the rule.
2. In the healthy adult female the numbers are :— Standing, 89 ; sitting, 82 ; lying, 80 ; including all exceptions to the rule. Standing, 91 ; sitting, 81 ; lying, 80; excluding all ex ceptions to the rule.
3. In both sexes the extremes are very remote from the mean results, and the ex ceptions to general rules Very numerous.
4. In both sexes, also, the effect of change of posture increases as the frequency of the pulse increases; but the exceptions to general rules are more numerous as the pulse is less frequent.
5. The effect of change of posture on any given frequency of the pulse is much greater in the male than in the female.
6. The effect of change of posture on the pulse is less in early youth than in the adult, and the modifying influence of age is greater in the female than in the male.
7. The exceptions to general rules are more numerous in early youth than at the adult age.
8. The exceptions to general rules are more numerous as the effect of change of posture is less.
9. The effect of change of posture on the pulse is greater in the forenoon than in the afterpart of the day.
10. The inverted posture of the body les '..sens the frequency of the pulse.
11. The varying frequency of the pulse in different postures of the body is due to mus cular contraction.