178. THE CIRCULATION. 



has been observed by Mr. J. Blake, who found that 

 nitrate of baryta injected into the jugular vein of a horse 

 could be detected in blood drawn from the carotid artery 

 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 

 complete 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 sub- 

 stance 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 substances, 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 ex- 

 tremities 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 mode of estimating the general velocity of the 

 circulating blood, is by calculating it from the quantity of 

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



