66 PHYSIOLOGY OF THE CIRCULATION. 



the mechanism of these processes be fully understood, 

 till some force more active, more manageable, and 

 more comprehensible than any yet advanced, is 

 applied to the elucidation of the subject. 



In the course of the following remarks I shall 

 endeavour to demonstrate that such a force does 

 exist in the circulation of the blood ; and that this 

 agent produces all the effects in question by directly 

 or indirectly occasioning a disproportion between 

 the amount of pressure acting on the internal and 

 external surfaces of the minute blood-vessels. The 

 researches on which this opinion is based were under- 

 taken with no other object in view than the discovery 

 of the laws by which the passage of substances into 

 and out of the blood-vessels is regulated. I have at 

 present therefore nothing to say concerning the 

 means by which the different tissues and cells imbibe 

 and assimilate their nutriment, nor of the mode in 

 which the embryo obtains a supply of plastic matter 

 previously to the existence of its circulation. 



In these cases, the capillarity of the investing 

 membranes may, perhaps, in the absence of any 

 more active force, still be considered as the chief 

 instrument employed for this purpose. 



On examining the apparatus contrived for the 

 passage of the blood to and from the heart, we 

 observe considerable differences to prevail in the 

 structure of the two great systems of blood-vessels, 

 and in the course of the currents through them. 



Thus, the large arteries are thick, strong, and 

 highly elastic, while the coats of the veins are 



