ACTION OF THE HEART. 



371 



[Fig. 92. 



give the first indications of their operation in the same period, after they have 

 been introduced into the Venous circulation. Thus, in the Horse, the time 

 that is required for the blood to pass from the Jugular vein into the capillary 

 terminations of the Coronary arteries, is 16 seconds ; as is shown by the 

 power of Nitrate of Potass to arrest the Heart's action within that time ; and 

 Nitrate of Strychnia, injected into a vein, gave the first manifestation of its 

 action on the Spinal Cord in precisely the same number of seconds. In the 

 Dog, the Heart's action was arrested by the Nitrate of Potass in 11 or 12 

 seconds ; and the tetanic convulsions occasioned by Strychnia also commenced 

 in 12 seconds. In the Fowl, the former period was 6 seconds, and the latter 

 6 ; in the Rabbit, the first was 4, and the other 4| seconds. From these 

 experiments, it seems difficult to resist the conclusion, that the rapidity of the 

 Circulation is very much underrated in any 

 estimate that we found upon the capacity of 

 the Heart, and its number of pulsations in a 

 given time ; and that some other force than its 

 contractions, must have a share in producing 

 the movement of the blood through the vessels. 

 492. The force with which the heart propels 

 the blood, may be estimated in two ways ; 

 either by ascertaining the height of the column 

 of that fluid, which its contractile action will 

 support ; or by causing the blood to act upon a 

 shorter column of mercury. The former me- 

 thod was the one adopted by Hales, who intro- 

 duced a long pipe into the carotid artery of a 

 horse, and found that the blood would some- 

 times rise in it to the height of ten feet. From 

 parallel experiments upon sheep, oxen, dogs, 

 and other animals, and by comparing the cali- 

 bre of their respective vessels with that of the 

 human aorta, Hales concluded that the usual 

 force of the heart in man would sustain a column 

 of blood 7| feet high, the weight of which 

 would be about 4 Ibs. 6 oz. The second me- 

 thod is that more recently adopted by Pois- 

 seuille ; and the instrument which he contrived 

 for carrying it into practice (termed by him the 

 Hsemadynamometer) has been the means of 

 aiding many valuable inquiries on the physio- 

 logy of the circulation. The result of his ex- 

 periments is Very nearly the Same as that Of bent glass tube, filled with mercury in 

 Hales; his estimate of the force With which the lower part, a d e. The horizontal part 



the 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 propelling the blood to prevent its coagulation, when 



blood, is properly estimated by multiplying the the blood presses on the fluid in the 



pressure of blood in the aorta into the surface > tal limb ' the rise of the mercury 



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 ap- 



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

 a 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. 



HoemadynamometerofPoisseuille. A 



&, is provided with a brass head, which 

 fits into the artery. A small quantity of 

 a solution of the carbonate of soda is in- 

 terposed between the mercury and the 



towards e. measured from the level to 



which h has fa]|en towards fl> gives the 



pressure under which the blood moves.] 



