VELOCITY OF THE CIRCULATION. 149 



a horse could be detected in blood drawn from the carotid ar- 

 tery of the opposite side in from fifteen to twenty seconds after 

 the injection. In sixteen seconds a solution of nitrate of 

 potash, injected into the jugular vein of a horse, caused com- 

 plete arrest of the heart's action, by entering and diffusing 

 itself through the coronary arteries. In a dog, the poisonous 

 effects of strychnia on the nervous system were manifested in 

 twelve seconds after injection into the jugular vein; in a fowl, 

 in six and a half seconds, and in a rabbit in four and a half 

 seconds. 



In all these experiments, it is assumed that the substance 

 injected moves with the blood, and at the same rate as it, and 

 does not move from one part of the organs of circulation to 

 another by diffusing itself through the blood or tissues more 

 quickly than the blood moves. The assumption is sufficiently 

 probable, to be considered nearly certain, that the times above- 

 mentioned, as occupied in the passage of the injected sub- 

 stances, are those in which the portion of blood, into which 

 each was injected, was carried from one part to another of the 

 vascular system. It would, therefore, appear that a portion of 

 blood can traverse the entire course of the circulation, in the 

 horse, in half a minute ; of course it would require longer to 

 traverse the vessels of the most distant part of the extremities 

 than to go through those of the neck ; but taking an average 

 length of vessels to be traversed, and assuming, as we may, 

 that the movement of blood in the human subject, is not 

 slower than in the horse, it may be concluded that one minute, 

 which is the estimate usually adopted of the average time in 

 which the blood completes its entire circuit in man, is rather 

 above than below the actual rate. 



Another jnocle of estimating the general velocity of the cir- 

 culating blood, is by calculating it from the quantity of blood 

 supposed to be contained in the body, and from the quantity 

 which can pass through the heart in each of its actions. But 

 the conclusions arrived at by this method are less satisfactory. 

 For the estimates, both of the total quantity of blood, and of 

 the capacity of the cavities of the heart, have as yet only ap- 

 proximated to the truth. Still, the most careful of the esti- 

 mates thus made accord with those already mentioned ; for 

 Valentin has, from these data, calculated that the blood may 

 all pass through the heart in from 43 j to 62f seconds. 



The estimate for the speed at which the blood may be seen 

 moving in transparent parts, is not opposed to this. For, as 

 already stated (p. 137), though the movement through the 

 capillaries may be very slow, yet the length of capillary vessel 

 through which any portion of blood has to pass is very small. 



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