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_ STRUCTURE AND DISTRIBUTION OF ARTERIES AND VEINS. 219 
the heart, and must be strong enough to resist the force of its 
contraction ; otherwise, as there is a considerable impediment 
to its onward flow, produced by the minuteness of the tubes 
through which it has to pass, and the friction to which it is 
subjected against their sides, their walls would give way, and 
they would burst. They have, accordingly, a tough elastic 
fibrous coat, which contains also more or less of non-striated 
muscular fibre. On the other hand, the Veins receive the 
blood after the heart's power over it has been almost ex- 
pended in forcing it through the capillary system, and when 
it is consequently moving much more slowly. They are very 
large in proportion to the arteries ; so that, if we were to cut 
across a limb at any place, and to estimate the respective areas 
of all the veins and arteries, we should find that of the veins 
two or three times as great as that of the arteries. Hence the 
pressure on their walls is much less ; and their strength does 
not require to be so great. Accordingly we find their walls 
much thinner, and the tough elastic fibrous coat almost entirely 
wanting. 
249. The difference in the force with which the blood 
presses on the walls of the arteries and veins, is seen when 
these vessels are wounded. If a small incision be made into 
an artery, the blood spouts from it to a great distance ; but if 
a similar incision be made in a vein, the blood merely flows 
out, unless we stop its passage to the heart, by making pres- 
sure on the vein above the orifice, as in ordinary blood-letting 
(§ 277). Hence much greater pressure is requisite to check 
bleeding from an artery, than to stop bleeding from a yein ; 
and it frequently happens that no amount of pressure can 
prevent the continued drain of blood from the former, so that 
it becomes necessary to stop the flow of blood through the 
artery altogether, by tying a ligature tightly round it. 
250. The Arteries are for the most part so distributed, that 
their trunks lie at a considerable distance from the surface of 
the body, so as to be secluded from injury ; and they are often 
specially protected by particular arrangements of the bony 
paris. Of the Veins, on the other hand, a large proportion lie 
near the surface, and they are consequently more liable to be 
injured ; but, for the reason just stated, wounds in them are 
_ of comparatively little consequence. 
251. The ultimate ramifications of the Arteries are conti- 
