LOCAL VARIATIONS. 303 



When these results were first published, it was thought to be 

 doubtful whether the circulation were really as rapid as they would 

 make it appear. It was thought that the saline matter which was 

 injected, " travelled faster than the blood ;" that it became " diffused'' 

 through the circulating fluid; that it transuded through dividing 

 membranes ; or passed round to the point at which it was detected, 

 by some short and irregular route. 



But none of these explanations have ever been found to be cor- 

 rect. They are all really more improbable than the fact which 

 they are intended to explain. The physical diffusion of liquids 

 does not take place with such rapidity as that manifested by the 

 circulation ; and there is no other route so likely to give passage to 

 the injected fluid, as the bloodvessels and the movement of the blood 

 itself. Beside, the first experiments of Poisseuille and others have 

 not been since invalidated, in any essential particular. It was found, 

 it is true, that certain other substances, injected at the same time 

 with the saline matter, might hasten or retard the circulation to a 

 certain degree. But these variations were not very marked, and 

 never exceeded the limits of from eighteen to forty-five seconds. 

 There is no doubt that the blood itself makes the same circuit in 

 very nearly the same interval of time. 



The truth is, however, that we cannot fix upon any absolutely 

 uniform rate which shall express the time required by the entire 

 blood to pass the round of the whole vascular system, and return 

 to a given point. The circulation of the blood, far from being a 

 simple phenomenon, like a current of water through a circular tube, 

 is, on the contrary, extremely complicated in all its anatomical and 

 physiological conditions ; and it differs in rapidity, as well as in its 

 physical and chemical phenomena, in different parts of the circu- 

 latory apparatus. We have already seen how much the form of 

 the capillary plexus varies in different organs. In some the vascu- 

 lar network is close, in others comparatively open. In some its 

 meshes are circular in shape, in others polygonal, in others rectan- 

 gular. In some the vessels are arranged in twisted loops, in others 

 they communicate by irregular but abundant inosculations. The 

 mere distance from the heart at which an organ is situated must 

 modify to some extent the time required for its blood to return 

 again to the centre of the circulation. The blood which passes 

 through the coronary arteries and the capillaries of the heart, 

 for example, must be returned to the right auricle in a compara- 

 tively short time; while that which is carried by the carotids into 



