THE BLOOD AND JTS VESSELS. 
603 
every portion of the arteries, or the arteries terminating in them, 
are the capillary vessels: so minute that there can be in them no 
vis a tergo , for the attraction which the parietes of these tubes, 
so nearly in contact, exert on the fluid passing through them, 
is fully sufficient to neutralize any force impressed from behind. 
Then, that coat, little developed and powerless in the larger ves¬ 
sels, but comparatively increasing in bulk and in power as the 
vessel diminishes in calibre—the muscular coat—takes up the ac¬ 
tion, and, supplied with energy from the ganglionic system of 
nerves, contracts upon the contents of the capillaries, and con¬ 
tinues to propel the blood in its course. 
The capillary vessels having traversed every portion of the 
frame, every secretion having been duly performed, each tissue 
plied with the power of contractility, and each worn out part 
built up, the blood begins again to be collected in other vessels of 
a different construction ; exerting little or no power on the fluid 
which traverses them, and unaided by any propulsive power from 
the heart; yet the blood once more pursuing its course through 
the agency of that important machine, the principle of derivation 
now supplying that of percussion. 
When the piston is raised in the barrel of the pump, the water 
flows in to prevent the formation of a vacuum, and to restore the 
equilibrium of atmospheric pressure,* so when the heart, having 
contracted on its contents, resumes, by an inherent principle of 
elasticity, its natural form and state, that of dilation, the blood 
flows in from the cavas in the same way, and influenced by the 
same principle. As in the artificial pump the fluid would rush 
through the pipe, although it were a hundred yards in length, 
when the piston is raised, so, when the ventricle is suddenly 
dilated, the blood flows on from the remotest portion of the 
venous canal. 
The motion of the blood is much assisted in the extremities 
by the action of the muscles—exercise is the readiest way of 
quickening the circulation; and the assistance rendered by the 
muscles, when they compress the veins, is regulated by the 
valves, which will permit the blood to flow on, but not to take 
a retrograde course. 
Having thus considered the mechanism by which the circula¬ 
tion of the blood is effected, I should naturally proceed to the 
changes which take place in the blood during its circulation ; and 
as the majority of my little class is composed of those whom I 
had before the pleasure to see around me, and as the systems into 
which our subject has been thrown are distinct from each other, 
and each can be satisfactorily considered without any great re- 
