CIRCULATION OF THE BLOOD. 



129 



extremities. The arterial branches wherever given off divide and sub- 

 divide, until the calibre of each subdivision becomes very minute, and 

 these minute vessels pass into capillaries. Arteries are, as a rule, placed 

 in situations v protected from pressure and other dangers, and are, with 

 few exceptions, straight in their course, and frequently communicate with 

 other arteries (anastomose or inosculate). The branches are usually 

 given off at an acute angle, and the area of the branches of an artery gen- 

 erally exceeds that of the parent trunk; and as the distance from the 

 origin is increased, the area of the combined branches is increased also. 



After death, arteries are usually found dilated (not collapsed as the 

 veins are) and empty, and it was to this fact that their name was given 

 them, as the ancients believed that they conveyed air to the various parts 

 of the body. As regards the arterial system of the lungs (pulmonary 

 system) it begins at the right ventricle in the pulmonary artery, and is 

 distributed much as the arteries belonging to the general systemic cir- 

 culation. 



Structure. The walls of the arteries are composed of three principal 

 coats, termed the external or tunica adventitia, the middle or tunica 

 madia, and the internal coat or tunica intima. 



The external coat or tunica adventitia (Figs. 107 and 111, t. a.), the 

 strongest and toughest part of the wall of the artery, is formed of areolar 



FIG. 107. 



Fio. 108. 



FIG. 107. Minute artery viewed in longitudinal section, e. Nucleated endothelial membrane, 

 With faint nuclei in lumen, looked at from above, i. Thin elastic tunica intima. m. Muscular coat 

 or tunica media. , Tunica adventitia. (Klein and Noble Smith.) X 250. 



FIG. 108. Portion of fenestrated membrane from the femoral artery. X 200. a, 6, c. Perfo- 

 rations. t.Henle.) 



tissue, with which is mingled throughout a network of elastic fibres. 

 At the inner part of this outer coat the elastic network forms in most 

 arteries so distinct a layer as to be sometimes called the external elastic 

 coat (Fig. 123, e.e.). 



The middle coat (Fig. 107, m) is composed of both muscular and 

 VOL. I. 9 



