ACTION OF THE HEART. 



555 



very nearly the same as that of Hales ; his estimate of the force, with which 



ihe blood is propelled into the Aorta, being 4 

 Ibs. 3 oz. The backward pressure upon the 

 walls of the Heart, or in other words the 

 force which they have to overcome in pro- 

 pelling the blood, is properly estimated, by 

 multiplying the pressure of blood in the aorta, 

 into the surface of a plane passing through 

 the base and apex of the left ventricle ; by 

 which calculation it is found to be about 13 

 Ibs.* The pressure appears, from the experi- 

 ments of Poisseuille, to be very nearly equal 

 for equal surfaces, throughout the larger arte- 

 rial branches ; since it diminishes regularly 

 in proportion to their calibre; in the radial 

 artery at the wrist, it was estimated by him 

 at 4 drachms. 



727. The number of contractions of the 

 Heart in a given time, is liable to great varia- 

 tion, within the limits of ordinary health, 

 from several causes ; the chief of these are, 

 diversities of Age, of Sex, of Muscular exer- 

 tion, of the condition of the Mind, of the state 

 of the Digestive system, and of the Period of 

 the day. 



a. Putting aside the other causes of uncertainty, the 

 following table may be regarded as an approximation 

 to the average frequency of the Pulse, at the several 

 ages specified in it. 



[Fig. 213. 



In the fetus in utero 

 Newly-born infant 

 During the First year 

 During the Second year 

 During the Third year , 

 About the Seventh year 

 Age of Puberty 

 Manhood . 



BEATS PER MINUTE. 



. 140 -- 150 



. . 130 140 



. 115130 



. . 100 115 



. 90 100 



8590 



80 85 



7080 



50 65 



Htemadynamometer of Poisseuille. A 

 bent glass tube, filled with mercury in 

 the lower part, a d e. The horizontal 

 part 6, is provided with a brass head, 

 which fits into the artery. A small 

 quantity of a solution of the carbonate 

 of soda is interposed between the mer- 

 cury and the blood, to prevent its coagu- 

 lation. When the blood presses on the 

 fluid in the horizontal limb, the rise of 

 the mercury towards P, measured from 

 the level to which it has fallen towards 

 a, gives the pressure under which the 

 blood moves.] 



Old age .... 



b. The difference caused by sex is very considerable, 

 especially in adult age; it appears from the inquiries 

 of Dr. Guy,"!' mat ^ ie pulse of the adult Female ex- 

 ceeds in frequency the pulse of the adult Male, at the 

 same mean age, by from 10 to 14 beats in a minute. 



c. The effect of muscular exertion in raising the pulse is well known; as is also the fact, 

 which is one exemplification of it, that the pulse varies considerably with the posture of the 

 body. The amount of this variation has been made the subject of extensive inquiry by Dr. 

 Guy ; and the following are his results. In 100 healthy Males, of the mean age of 27 years, 

 in a state of rest, the average frequency of the pulse was, when standing, 79, when sitting, 

 70, and when lying, 67 per minute. Several exceptions occurred, however, to the general 

 law; and when these were excluded, the average numbers were, standing, 81, sitting, 

 71, and lying, 66; so that the difference between standing and sitting was 10 beats, or 

 1 8th of the whole; the difference between sitting and lying was 5 beats, or 1 13th of the 

 whole; and the difference between standing and lying was 15 beats, or l-5th of the whole. 

 In 50 healthy Females, of the same mean age, the average pulse, when standing, was 89, 



* The extreme latitude of the estimates which have been made of this force, has been n. 

 subject of not undeserved ridicule. Borelli imagined it to be 180,000 Ibs.; whilst by Keill it 

 was supposed to be no more than from 5 to 8 ounces. 



f Guy's Hospital Reports, vol. iii. p. 312. 



