PHYSIOLOGY OF THE CIRCULATION. 83 



But the observation which bears most directly on 

 the present point is this that on examining these 

 hypertrophied kidneys at the end of a few hours, I 

 generally found the primary divisions of the renal 

 artery imbedded in a quantity of liquor sanguinis, 

 which the unnatural compression of the contained 

 blood had expelled through their porous coats. 

 Since, therefore, an additional degree of compression 

 of the blood is capable of forcing its most viscid 

 parts through the coats of vessels which, from their 

 thickness, must oppose a very great resistance to 

 the passage of fluid through their pores, we cannot, 

 I think, avoid the conclusion, that a constant, though 

 slow, and, to the naked eye, imperceptible, process of 

 albuminous effusion through the thin porous coats 

 of the minute arteries, is a necessary consequence of 

 the existence in each of them of a distending column 

 of blood. This process of effusion must necessarily 

 take place whenever the pressure acting on the 

 internal surface of the membranous tube is greater 

 than that of the medium in which the tube or vessel 

 is placed. And if, at any one point, the resistance 

 of the medium be less than the outward or lateral 

 pressure of the contained fluid, then effusion will 

 occur at that point only. For so long as an amount 

 of pressure greater than the dilating or lateral force 

 of the contained fluid is applied on the external 

 surface of a porous tube, the escape of that fluid 

 through these minute lateral openings is evidently 

 rendered as impossible as though the latter did not 

 exist at all. In the living body the different tissues 

 situated between the smaller blood-vessels, will re- 



G 2 



