Frequency of the heart's action.—The fre quency of the heart's action is considerably modified by age, condition of the other functions of the body at the time, by mental emotions, and by the original constitution of the individual. Its movements are influenced by very slight muscular exertion, and the extent of this appears to vary at different times of the day. In the fwtus its movements are rapid, being about 140 in the minute. At birth it is from 130 to 140 ; at one year 115 to 130 ; second year 100 to 115; third 90 to 100; seventh 85 to 90; fourteenth 80 to 85; middle age 70 to 75; in very old age 50 to 65. The heart's action generally sympathises powerfully with the other organs of the body, and this has always been regarded as a most important and necessary guide in the detection and cure of diseases.
It becomes strong and rapid in some cases of inflammation, while in others it becomes rapid and feeble. It becomes quicker after eating and slower during sleep. It is much increased in frequency during bodily exertion. In cases of great general debility it becomes very quick and feeble. It becomes more rapid and weaker during inspiration, slower and stronger during expiration.
It is an important fact that when the con tractility of the heart is much enfeebled by extensive injuries of the central organs of the nervous system or of the other parts of the body, (as when a limb is extensively crushed,) Its contractions are not only much weaker, but are also greatly increased in frequency. It is also worthy of remark that such injuries do not pro duce convulsive movements in this organ. The effect which severe injuries and certain inflam matory affections have in greatly debilitating or even destroying the contractility of the heart is a fact of great practical importance, as it not only explains the cause of the most alarming symptoms in such cases, but also points out the most appropriate remedies to avoid the chief tendency to death. To this cause, for example, we arc to attribute the rapid and feeble pulse, in concussion of the brain, in extensive mecha nical injuries, the shock after operations, exten sive burns, peritonitis, &e. It is very fortunate that the contractions of the heart become more frequent when its contractility becomes en feebled. I f the heart under these circumstances had required, as we would a priori expect, the presence of a greater quantity of blood to stimulate it to contraction, instead of a smaller quantity, as is actually the case, what would have been the consequence ? It is evident that since the resistance, under ordinary cir cumstances, which the heart has to overcome in contracting, is, according to a well-known hy drostatic law, in proportion to the extent of the area of the inner surface of the cavities of the heart at the commencement of their contraction, (each square inch of surface, according to the experiments of Hales, having a pressure upon it nearly equal to four pounds,) the more fre quent contractions, where there is a smaller quantity of blood present in the heart at the commencement of each contraction, will not demand the same degree of muscular force for their performance, as if these had been less frequent. If, when the contractility of the
heart became debilitated, the presence of a greater quantity of blood than usual in its interior had been necessary to stimulate it to contraction, and if the area of the inner surface of the cavities of the heart be in proportion to the quantity of blood contained there, it is apparent that the movements of the heart would have been much more rapidly and frequently arrested when its contractility became enfeebled, than they are under the actual arrangement.
The influence of mental emotions upon the movements of the heart requires no illustration, for this is so universally experienced that in common language the heart is considered to be the seat of the affections and passions, and this has had a powerful influence upon the phrase ology of all languages.
In sanguine temperaments the heart gene rally contracts more frequently than in phlegmatic temperaments. In women it is also generally a little quicker than in men.
It varies very much in different classes of animals.
Burdach• has given the following table col lected from numerous sources, as an approxi mative valuation of the frequency of the heart's action in various animals.
Number of pulsations in a minutc.t In the Shark .. 7 M ussel . . 15 Carp 20 Eel .... 24 Snake 34 I I orse 36 Caterpillar 36 Bullock 38 Ass 50 Crab 50 Butterfly 60 Goat 74 Sheep 75 Hedgehog 75 Frog 77 Marmot 90 Locust 90 Ape 90 Dormouse 105 Cat 110 Duck 110 Rabbit 120 Menoculus Caster 120 Pigeon 130 Guinea-pig 140 lien 140 Bremus terrestris 140 heron 200 Menoculus puler 200 For the effects of the respiration upon the contractions of the heart, and the influence of the circulation of dark blood upon its irrita bilitp see ASPHYXIA.