96 PHYSIOLOGY OF THE CIRCULATION. 



same power of absorbing an external stagnant fluid. 

 But the difficulties preventing the successful per- 

 formance of this experiment were much more con- 

 siderable than any yet encountered ; for the mere 

 weight of the incumbent flaccid walls opposed so 

 much resistance to the passage of fluid through the 

 tube, that a copious exudation occurred through any 

 lateral openings existing in it; and I could only 

 obtain a satisfactory result by carefully observing, 

 and in the performance of the experiment strictly 

 adhering to, the conditions under which the return 

 of venous blood is effected in animals. 



The most important of these conditions may be 

 thus stated : - 



1. The different converging streams always flow 

 from narrow into wider tubes. 



2. Those flaccid porous tubes are placed in a 

 dense medium, the pressure of which on their 

 external surface is much greater than that of the 

 atmosphere ; for, in addition to this latter force, we 

 have, in the living body, various other powers, the 

 operation of which greatly increases the pressure 

 acting on the exterior of the blood-vessels ; as, for 

 instance, the elasticity and contractility of the skin, 

 the weight and lateral pressure of adjacent parts, 

 muscular action, &c. 



3. The walls of the terminating portion of the 

 lar<re venous trunks are strengthened and rendered 



O O 



comparatively rigid by their adhesion to the dia- 

 phragm, pericardium, and pleura-. 



4. The flow of venous blood into its proper recep- 

 tacle the right auricle is facilitated by the 



