228 ANIMAL PHYSIOLOGY. 
those of the blood-vessels can not be considerable. Whence, 
then, arises that friction which keeps the arterial vessels always 
distended by its backward influence? The microscopic study 
of the circulation helps to answer this question. The plas- 
ticity of the corpuscles and of the vessel walls themselves 
must be taken into account, in consequence of which a drag- 
ging influence is exerted whenever the corpuscles touch the 
wall, which must constantly happen with vast numbers of 
them in the smallest vessels and especially in the capillaries. 
The arrangement of capillaries into a mesh-work, must also, in 
consequence of so many angles, be a source of much friction. 
The action of the corpuscles on one another may be com- 
pared to a crowd of people hurrying along a narrow passage— 
the obstruction comes from interaction of .a variety of forces, 
owing to the crowd itself rather than the nature of the thor- 
oughfare. We must set down a great deal to the influence of 
the corpuscles on one another, as they are carried along, accord- 
ing to mechanical principles; but, as we shall see later, other 
and more subtile factors play a part in the capillary circulation. 
Owing to the peripheral resistance and the pumping force of 
the heart, the arteries become distended, so that, during cardiac 
diastole, their recoil, owing to the closure of the semilunar 
valves, forces on the blood in a steady stream. It follows, then, 
that the main force of the heart is spent in distending the 
arteries, and that the immediate propelling force of the circu- 
lation is the elasticity of the arteries in which the heart stores 
up the energy of its systole for the moment. 
BLOOD-PRESSURE. 
Keeping in mind our schematic representation of the circu- 
lation, we should expect that the blood must exercise a certain 
pressure everywhere throughout the vascular system; that this 
blood-pressure would be highest in the heart itself; considera- 
ble in the whole arterial system, though gradually diminishing 
toward the capillaries, in which it would be feeble; lower still 
in the smaller veins; and at its minimum where the great veins 
enter the heart. Actual experiments confirm the truth of these 
views; and, as the subject is one of considerable importance, 
we shall direct attention to the methods of estimating and re- 
cording an animal’s blood-pressure. 
First of all, the well-known fact that, when an artery is cut, 
the issuing stream spurts a certain distance, as when a water- 
