PHYSIOLOGY OF THE CIRCULATION. 109 



under the microscope *, observed the coloured sub- 

 stance to appear first in the most minute venous 

 radicles, and then in the larger veins. 



I have before remarked, that the absorbing power 

 of streams is cceteris paribus proportioned to their 

 velocity ; and, as connected with the point now 

 under consideration, I may refer to the interesting 

 fact, that natural philosophers, from reasoning upon 

 the physical arrangement of the system of blood- 

 vessels, long since concluded that the point where 

 the blood flows with the greatest velocity, is at the 

 commencement of the veins, f 



The next question, viz. as to the means by which 

 the passage of fluids into the interior of the blood- 

 vessels is accomplished, involves an examination of 

 the nature of the absorbing power exercised by all 

 rapid streams ; and in the following brief explanation 

 of this action, I have endeavoured to limit myself as 

 closely as possible to the relation of such facts as 

 appeared best calculated to render intelligible the 

 process of vascular absorption in animals. 



It is well known that a fluid in rapid motion does 

 not press equally in all directions, its lateral being 

 invariably less than its onward pressure ; for if those 

 forces were at all equal, every liquid jet, instead of 

 contracting somewhat after escaping from an orifice, 



* " Rechcrches Experimentales sur 1'absorption et Pexhalation," 

 par Michel Fodera, p. 26. Paris, 1824. 



f " L'endroit ou il (le sang) doit se mouvoir avec le plus de 

 rapidite c'est a Forigiiie des veines." Musschenbroek, Essai de 

 Physique, t. i. p. 392. Leyden, 1 793. 



