The credit of propounding a diminished frequency of the pulse towards the afterpart of the day, as the true theory, is due to Dr. Knox*, who made several series of experi ments, in order to establish it. The general results of these experiments with those of the writer, and some facts gleaned from other sources, are thrown together in a table.t As the true state of the case did not seem to be made out even by this balance of autho rity, it was thought desirable to add to the number of observations. Accordingly several averages of the number of the pulse.the first thin.. in the morning and the last at night, fount'ded on from two to ten observations at each period, in healthy young persons of both sexes, were obtained, with what result will be seen in the following tables.
The facts contained in these tables and in the previous table, together with the two series of experiments performed by Nick, may be taken to establish the general law first set forth by Dr. Knox, that the pulse is less fre quent in the evening than in the morning; but it is obviously subject to numerous ex ceptions.
This law derives some confirmation from the fact that the only series of experiments on females which the writer has met with (those of Friedrich Hohl on pregnant women I-) yield averages in conformity with it ; for on comparing the mean of 25 observations made on the pulses of pregnant women in the morn ing with a mean of the same number made on the same women in the evening, the pulse in the morning was 83'28, and in the evening 80.88, being a difference of 2.40. The same author also states that the pulse of the new born infant,and of the fcetus in utero are more frequent in the morning than in the even ing.* The interval between the morning. and evening is filled up by pulses of very variable frequency, where the experiments are not made with due precaution ; but where, as in the second series of experiments by Nick, and in those of Knox, and of the present writer, the body remains in the same posture, in a state of rest, and unexcited by stimu lating food, the fall in the frequency of the pulse is for the most part progressive, and free from those accelerations at noon and in the evening of which Double and Cullen have made mention.
The diminished frequency of the pulse to wards the afterpart of the day seems to de pend altogether on the exhaustion of the strength, and is a less degree of that marked diminution of frequency which often accom panies a convalescence from severe disease.
That it is not dependent merely on the ab sence of exertion ; in other words, that it is not the effect of continued rest, is proved by the facts now to be mentioned.
It has been experimentally proved, both by Dr. Knox and by the writer of this article, that the pulse is not only less frequent in the evening than in the morning, but that it is also less excitable. So marked is the dif ference in this respect, that in some experi ments recorded in the Guy's Hospital Re portst, the very- same food, which in the morning increased the frequency of the pulse from five to twelve beats, and kept it raised above its natural number from one to two hours, produced no effect whatever in the evening. This fact is in strict keeping with the well known effect of spirituous liquors 'in the early part of the day, as compared with their action on the system in the evening.
The pulse in males appears to follow the same rule in disease as in health. The rule is inverted in females ; but in both sexes the exceptions are very numerous. t 1?est.—From what has already been stated it will be inferred that the absence of exertion has the effect of diminishing the frequency of the pulse.
Steep.—The pulse falls during sleep, slightly in adults, but considerably in young children. In six observations made by Nick on as many young adults, the mean decrease was some what more than three beats. Quetelet, in a girl frotn three to four years of age, found a difference of 10 beats ; in a boy from four to five years old, 16 beats ; and in a female, in her 27th year, 10 beats. In two pregnant women Hohl observed a difference of 10 and 11 beats respectively ; and the same author reports a difference of from 20 to 10 beats in new-born infants. He also attributes a re markable decrease of frequency sometimes observed in the loatal pulse to the sleep of the embryo. Other authors have been cognisant of the effect of sleep, but have not made it the subject of experiment.
Food.—The general effect of food is to excite the pulse; this takes place to a very slight extent with vegetable food, but more with animal food. Some articles of diet, as warm drinks, alcoholic liquors, and tobacco have a very marked influence on the pulse. The effect of food is much more considerable in infancy than in after life.* Mental Emotions.— The effect of these on the pulse is too well known to require any comment.