116 



THE CIRCULATION. 



The function of the arteries is to convey blood from the 

 heart to all parts of the body, and each tissue which enters 

 into the construction of an artery has a special purpose to 

 serve in this distribution. 



(1.) The external coat forms a strong and tough investment, 



FIG. 40. 



FIG. 41. 



FIG. 40. Muscular fibre-cells from human arteries, magnified 350 diameters 

 (Kolliker). a, natural state ; ft, treated with acetic acid. 



FIG. 41. Portion of fenestrated membrane from the crural artery, magnified 200 

 diameters, a, b, c, perforations (from Henle). 



which, though capable of extension, appears principally de- 

 signed to strengthen the arteries and to guard against their 

 excessive distension from the force of the heart's action. In 

 it, too, the little vasa vasorum find a suitable tissue in which to 

 subdivide for the supply of the arterial coats. 



(2.) The purpose of the elastic tissue, which enters so largely 

 into the formation of all the coats of the arteries, is, 1st. To 

 guard the arteries from the suddenly exerted pressure to which 

 they are subjected at eac-h contraction of the ventricles. In 

 every such contraction, the contents of the ventricles are forced 

 into the arteries more quickly than they can be discharged 

 into and through the capillaries. The blood therefore being, 

 for an instant, resisted in its onward course, a part of the force 

 with which it was impelled is directed against the sides of the 

 arteries; under this force, which might burst a brittle tube, 

 their elastic walls dilate, stretching enough to receive the 

 blood, and as they stretch, becoming more tense and more 

 resisting. Thus, by yielding, they, as it were, break the shock 



