PHYSIOLOGY OF THE CIRCULATION. 67 



much thinner, their structure more porous, and their 

 elasticity less marked. 



Again, the arterial blood passes from one great 

 trunk, through several intermediate branches, into 

 an infinite number of minute twigs; whereas, in the 

 veins, innumerable small currents meet in more 

 capacious vessels, by the convergence of which two 

 or three large streams are ultimately formed. 



It may be further remarked, that the arterial 

 blood encounters numerous obstacles to its free 

 passage into the veins ; whereas, in the latter 

 vessels, as a general rule, every facility is afforded 

 in order to secure the free return of the vital 

 fluid. A consideration of these and other pecu- 

 liarities, therefore, naturally suggests the opinion 

 that the two great systems of blood-vessels are in 

 health subservient to totally distinct, if not absolutely 

 opposite, functions ; and I shall now proceed to con- 

 sider the facts which bear upon this conjecture. 



PART I. 



OF THE USES SERVED IN THE ANIMAL ECONOMY BY THE VA- 

 RIOUS IMPEDIMENTS WHICH THE BLOOD ENCOUNTERS DURING 

 ITS PASSAGE FROM THE VENTRICLES INTO THE VEINS. 



THE existence of numerous impediments to the free 

 passage of the arterial blood is a fact so generally 

 known and acknowledged, as to render unnecessary 

 anything more than a brief mention of some of the 

 most important obstructing causes. 



F 2: 



