OF THE CAPILLARY VESSELS. 253 



many times, must have caused, and indeed have generally 

 caused to reject the supposed parenchyma interposed between 

 the terminations of the arteries and veins, in rendering evident 

 the ramifications which are not visible to the naked eye, the 

 microscopical divisions, and thus establishing a direct commu- 

 nication between them. 



Minute injections and microscopical observations, soon led 

 anatomists to admit, that instead of the parenchyma of the an- 

 cients, every thing is composed of vessels in the body; an 

 opinion which yet divides all the cultivators of the science. 



377. The sanguineous capillary vessels are the last ramifi- 

 cations of the arteries, and the first radicles of the veins, or 

 rather they are intermediate between the arteries and veins, 

 and, as has been remarked in comparing them to the portal 

 system, foreign or indifferent to both. It is in these vessels 

 that, insensibly and without any fixed point, the arteries are 

 converted into veins; of which we may judge by the succes- 

 sive increase or diminution of the size of the vessels in the one 

 or the other direction, by the direction in which the successive 

 divisions or union are made, and at the extremity of the fins 

 and the tail of fishes, by the opposite direction of the course 

 of the blood. However, the capillary vessels have been gene- 

 rally described as the last divisions of the arteries, rather than 

 the beginning of the veins. Whether this be well founded 

 and depend upon the small veins being larger than the small 

 arteries, acquiring a considerable volume after a few re- 

 unions; or whether it is because the veins, almost all provided 

 with valves, and more difficult to inject than the arteries, have 

 been less the object of investigation. These two reasons may 

 have contributed to give more currency to the opinion in ques- 

 tion. 



378. All the capillary vessels, however, have not the same 

 volume. In this respect three degrees of them may be esta- 

 blished, by taking as the largest those which begin to be in- 

 visible to the naked eye, and as the smallest those which ad- 

 mit only one single red globule of blood at a time, and the di- 

 ameter of which, of course, is not much larger than the glo- 

 bule itself. ( 72.) 



