Temperature of the Body.— Cold lowers the pulse, heat quickens it. Exposure to a very high temperature causes a marked accelera tion. Thus Sir C. Blagden, on exposing himself for 8 minutes to a temperature of about 260°, found his pulse rise to 144, or double its ordinary frequency.t Density of the Air.—In the observations hitherto made, it is very difficult to separate the influence of thiS agent from that of the exertion which acconipanied the change from one medium to another. There was a very considerable increase of frequency in the case of the men who accompanied Saussure in the ascent of Mont Blanc. The pulses that beat at Chamounix 49, 66, and 72, became, on the summit of the mountain, 98, 112, and 100 re spectively. Dr. Clark also found the pulses of his companions, in a state of rest on the summit of the mountain, 81, 81, 88, 92, 102, and 108 respectively, being a considerable increase above the probable frequency of the pulses of the same persons under ordinary eircumstances.t Miiller §, on the authority of Parrot, gives a table of the frequencies of the pulse corresponding to different elevations. They are as follows : — Level of tbe sea, 70 ; 1000 metres, 75; 1560 metres, 82 ; 2000 me tres, 90 ; 2500 metres, 95 ; 3000 metres, 100; 4000 metres, 110. These numbers are pro bably unauthorised by experiments.
The foregoing are some of the leading causes which affect the frequency of' the pulse in health. They may be thrown into two classes ; those which increase, and those which diminish its frequency.
I. The more common causes of increased frequency of pulse are : —Exercise, active and passive ; continued muscular effort ; a change from a posture requiring little, to o:ie re quiring more exertion ; food, especially warm drinks ; spirituous liquors and tobacco; a high temperature ; diminished pressure of the air; extreme debility ; sleeplessness ; the first degree of plethora; and exciting passions and emotions.
2. The common causes of diminished fre quency of the pulse are, — continued rest; sleep ; fatigue, when not carried to excess ; debility, when not extreme, and unaccompa nied by disease; cold ; increased atmospheric pressure ; a change from the erect to the sitting, and from the sitting to the recumbent posture, and the inverted position of the body; and depressing passions of tbe mind.
Hitherto we have been speaking solely of that character of the pulse which is most easily examined, — its frequency. To render the subject complete, it will be necessary to speak briefly of certain other characteristics of the healthy pulse.
The pulse of the healthy adult male may be described as regular equal, moderately full, compressible, and swelling slowly under the finger ; that of the female, and of the child of both sexes, is smaller, and quicker in the beat. The pulse of persolis of the sanguine temperament is full, hard, and quick ; that of persons of the lymphatic temperament is softer, and slower in the beat. In old age the pulse, in consequence of the increased firmness of the arteries, assumes a hardness which would not othervvise belong to it.
Exceptions to the general rule are not of very rare occurrence in persons who enjoy -good health. — There are some persons, for instance,. in whom every slight attack of indio.estion, especially when attended with flatulence, leads to a well marked intermission. Instances are also on record in which the pulse is uniformly irregular or even distinctly intermittent in health, becoming regular in disease, and resuming its irregularity on re covery.
One other subject connected with die phy siology of the pulse still remains to be exa mined, viz.