102 PHYSIOLOGY OF THE CIRCULATION. 



petually flowing from the capillaries into the con- 

 tiguous venous radicles, the calibre of which is much 

 greater than that of the vessels supplying them. At 

 this point, therefore, the conditions are evidently 

 favourable to absorption. And on tracing the course 

 of the veins to their final termination in two or three 

 large trunks, the individual streams are, in the same 

 manner, constantly found to pass from narrow to 

 wider tubes. In the smaller vessels this arrange- 

 ment is obviously calculated to add to the absorbing 

 power of the contained streams. In the large veins, 

 the walls of which are too thick to be readily per- 

 meated by liquids, its chief use is apparently that of 

 economising the impelling force of the vis a tergo, by 

 diminishing friction. 



2. The next point to be noticed is the medium in 

 which the membranous tube was placed: and the 

 necessity of a denser medium than atmospheric air 

 was proved by a repeated observation of the fact, 

 that when the membranous tube was unsupported, 

 the whole of the fluid constituting the stream escaped 

 through the lateral openings of the same apparatus 

 which, when immersed in water, not only permitted 

 the stream to traverse it freely, but also enabled the 

 current to carry with it a considerable quantity of 

 the external stagnant fluid. It is difficult to form 

 any estimate of the average amount of pressure 

 acting on the exterior of the blood-vessels of the 

 living body. It must necessarily vary much in 

 different animals, and in different parts of the same 

 body. I have, however, been careful not to employ 

 a greater amount of external pressure in these ex- 

 periments than might reasonably be supposed to 



