554 OF THE CIRCULATION OF BLOOD. 



those of Mr. Blake;* who varied them by employing different substances, 

 and took other precautions against sources of fallacy. Ten seconds after 

 having injected a solution of Nitrate of Baryta into the Jugular vein of ahorse, 

 he drew blood from the Carotid artery of the opposite side ; after allowing 

 this to flow for five seconds, he substituted another vessel, which received the 

 blood that flowed during the five ensuing seconds ; and the blood that flowed 

 after the twentieth second, by which time the action of the Heart had stopped, 

 was received into a third vessel. These different specimens were carefully 

 analyzed. No trace of Baryta could be detected in the blood, which had 

 escaped from the artery between the tenth and fifteenth second after the in- 

 jection of the poison ; but in that which was drawn between the fifteenth and 

 the twentieth second, the salt was found to be present, and in greater abun- 

 dance than in the blood which had subsequently flowed. Moreover, the 

 coincidence between the cessation of the Heart's action, and the diffusion of 

 the salt through the arterial blood, bear a striking correspondence ; and it may 

 be hence inferred, that the arrestment of its muscular movement is due to the 

 effect of this agent upon its tissue, when immediately operating upon it, 

 through the capillaries of the coronary artery. This conclusion is borne out 

 by a variety of other experiments ; which show that the time of the agency 

 of other poisons, that suddenly check the Heart's action (which is the espe- 

 cial property of mineral poisons), nearly coincides, in different animals, with 

 that which is required to convey them into the Arterial capillaries. And it 

 seems to derive full confirmation from the fact, that poisons, which act locally 

 on other parts, 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 mani- 

 festation 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 63 ; in the Rabbit, the first was 4, and the other 4k 

 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. 



726. The/ore^ 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 method was the one adopted 

 by Hales, who introduced a long pipe into the Carotid artery of a Horse, and 

 found that the blood would sometimes rise in it to the height of 10 feet. From 

 parallel experiments upon Sheep, Oxen, Dogs, and other animals, and by 

 comparing the calibre 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 A feet high, the weight of which would be about 4 Ibs. 6 

 oz. The second method is that more recently adopted by Poisseuille ; and 

 the instrument which ho contrived for carrying it into practice (termed by him 

 the Htemadynamometer) has been the means of aiding many valuable inqui- 

 ries on the physiology of the Circulation. The result of his experiments is 



Edinb. Mecl. and Surg. Journal, Oct. 1841. 



