TELOCITY OF THE CIRCULATION. 189 



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 con- 

 cluded 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 

 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 approximated to the truth. 

 Still, the most careful of the estimates thus made accord 



