Anatomy, physiology and hygiene . — BLOOD. — LYMPH. posed of three coats : first, a smooth, delicate, and slightlyelastic inner wall, similar to, and continuous with, theendocardium and the lining of the veins and the capil-laries ; next, a middle coat, composed ofelastic and muscular tissue; and lastly,a very strong outer coat, composed offibrous and elastic tissue with somemuscular As the arteries be-come smaller, the external coat disap-pears ; hence the very small arteries(arterioles) have but two coats. In thecapillaries (which connect the smallerarteries with the rootlets of the
Anatomy, physiology and hygiene . — BLOOD. — LYMPH. posed of three coats : first, a smooth, delicate, and slightlyelastic inner wall, similar to, and continuous with, theendocardium and the lining of the veins and the capil-laries ; next, a middle coat, composed ofelastic and muscular tissue; and lastly,a very strong outer coat, composed offibrous and elastic tissue with somemuscular As the arteries be-come smaller, the external coat disap-pears ; hence the very small arteries(arterioles) have but two coats. In thecapillaries (which connect the smallerarteries with the rootlets of the veins)the middle coat also disappears, and thethin, delicate, circular wall that remainsis well adapted for the transudation ofgases and smoothness of the lining wall prevents friction. Theelasticity of the arteries permits them to yield withoutdanger of bursting, as the blood is thrown into them witheach stroke of the heart, and also enables them to accom-modate themselves to the various movements of the Fig. 65. En-lvm- A Part of an A rteryveloping it arephatics and lymphaticglands. 1 The outer coat is so strong that when a surgeon ties (ligatures) anartery, it is not broken. The two broken internal coats arrest and clotthe blood and stop bleeding. Very seldom does a healthy artery rup-ture from the force of the blood current. The walls of the arteriesare nourished by blood conveyed to them by little arteries called vasavasorum. Corresponding vessels also supply the heart. The elasticityof the larger arteries will be best appreciated in the aorta of an ox orsheep. Like a piece of india-rubber, it yields when stretched, and imme-diately thereafter recovers itself. 2 If a large artery is ligatured, the small arteries which connect theportion of the artery below the ligature with that above, or with anotherartery, become enlarged and establish a collateral circulation. THE CIRCULATION. —BLOOD. —LYMPH. 153 Their contractility affords them the power of
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