106 PHYSIOLOGY OF THE CIRCULATION. 



of the short arm of the syphon so as to close it ; and 

 the long arm then speedily became emptied. 



In addition to those already mentioned, there is 

 one other condition which, in the animal body, is 

 found to be essential to the proper circulation of the 

 blood, and more particularly to the due performance 

 of the function of absorption ; for it has been clearly 

 proved by the experiments of Hales, and by the ex- 

 perience of all practical anatomists, that in general 

 none other than a thick or viscid fluid can pass 

 through the minute blood-vessels without the occur- 

 rence of a very copious effusion. And the truth of 

 this observation is also strikingly exemplified in 

 certain phenomena of disease. As to the manner in 

 which this physical property of the blood assists 

 absorption, it does not appear to me that it exercises 

 any other than an indirect action, viz. that of render- 

 ing the blood less apt to escape through the pores of 

 its containing vessels: for as a certain amount of 

 lateral pressure is required to overcome the cohesive 

 force of the particles of blood, and as the impeding 

 causes which occasion that lateral pressure are, in 

 the veins, very few in number, and trivial in their 

 character, it is probable that, in a state of perfect 

 health, little or no effusion takes place through the 

 walls of this portion of the circulating system. To 

 render the chain of evidence more complete, I re- 

 peated the same experiments with membranous tubes, 

 observing every particular as before, with the single 

 exception of substituting a viscid fluid, viz. common 

 size, for water ; and absorption went on quite as well 

 as when the more mobile liquid was used ; there 



