ON INFLAMMATION. 
321 
the arteries are curiously contrived tubes for the mechanical 
conveyance of the blood. The heart is the propelling* power ; 
the artery is the tube through which the fluid runs, little or not 
at all adding to the velocity of the current, but admirably con¬ 
structed to remove every possible impediment to the stream, 
with intuitive skill to adapt itself to the variations of the current, 
and even with power to take up the action when the central 
machine is weakened or deranged. 
The arteries, however, diminish in size as they recede from the 
heart, and they terminate in vessels as small as a hair, or not a 
thousandth part of the bulk of a hair. There are, through every 
assignable portion of the course of the arteries, myriads of these 
microscopic vessels given out to, and losing themselves in, the 
neighbouring parts, and in which the important functions of se¬ 
cretion and nutrition are performed. These are prolongations of 
the arteries; and the coats of the main vessel may still be traced 
so far, at least, as the power of vision, assisted by art, can 
reach; but w ith this difference, that the elastic coat is diminished 
in thickness and in pow er, and the muscular coat proportionably 
increased in bulk and strength. The reason of this is plain. 
The grand source of resistance to the passage of fluids through 
tubes, living or dead, is the attraction between the fluid and the 
wall cr side of the tube ; and this is powerful in the proportion 
which the internal surface of the tube bears to the quantity of 
fluid contained. In tubes that admit of mechanical admeasure¬ 
ment, the proportion of surface, and consequently the force of 
attraction most rapidly increase with the diminution of calibre, 
and in minute tubes every part of the fluid is w ithin the sphere 
of attraction. Therefore it is, that, in capillary tubes, a fluid 
does not obey the strong power of gravitation, hut rises in the 
vessel, and that in proportion to the smallness of the bore. The 
ratio has been accurately calculated, and the mechanical effect 
almost exceeds belief. 
So, in the capillaries of the animal frame, w hen the w hole of the 
fluid enclosed within these tubes feels the attractive force of the 
walls of the vessels, the power of the heart, like that of gravi¬ 
tation, is counteracted and unavailing. The heart has, and can 
have, nothing to do directly with the circulation of the blood 
through the capillaries; but another power is called into exer¬ 
cise, the muscular coat of the vessels, which was increasing 
with their diminution of size, and at length constitutes the main 
bulk of their parietes or coverings. The muscular coat is the 
grand agent here, and the elastic remains, although shrunk in 
bulk, to act as an antagonist to the muscular: so that, although 
tire larger trunks may be considered as little more than mechani- 
