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 vein ; 

 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 

 parts. 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- 



