346 CIRCULATION. 



from them, and carried on by the veins, and thus through the 

 entire circuit, before it has had time to diffuse itself suffi- 

 ciently to interfere with the observation. It is not apparent 

 how this objection can be overcome, for a substance must be 

 used which will mix with the blood, otherwise it could not 

 pass through the capillaries. The objection made by Mat- 

 teucci, especially as it does not appear how the difficulty can 

 be obviated, seems an unnecessary refinement ; for the ques- 

 tion itself is not one of vital importance, on which depends 

 an important physiological principle, but simply one to 

 which a tolerably close approximation of the exact truth is a 

 sufficient answer. It is interesting to know that the varied 

 and complicated actions which we have been studying effect 

 a single complete circuit of the blood in about half a min- 

 ute ; but it makes no great difference whether it be four or 

 five seconds more or less. In this statement, we must not be 

 understood as denying the value of the closest possible accu- 

 racy in physiological investigations ; but it is evident that 

 this accuracy is important in proportion to the importance 

 of the question, in itself, and in its physiological relations. 



There seems no reason why, with the above restrictions, 

 the results obtained by Hering should not be accepted, and 

 their application, as far as possible, made to the human 

 subject. 



Hering found that the rapidity of the circulation in dif- 

 ferent animals was in inverse ratio to their size, and in direct 

 ratio to the rapidity of the action of the heart. 



The following are the mean results in certain of the do- 

 mestic animals, taking the course from jugular to jugular, 

 when the blood passes through the lungs and through the 

 capillaries of the face and head : 



In the Horse, the circulation is accomplished in 27'3 seconds, 

 u j) ogj u M 15 . 2 



" Goat, " " 12-8 " 



" Rabbit, " " 6'9 " 1 



1 MILNE-EDWARDS, loc. cit. Vierordt found the mean rapidity in the horse 



