THE CIRCULATORY SYSTEM 



159 



thin elastic layer on its outer side; (5) a muscular and 

 elastic coat; (<?) a connective tissue coat, forming the 

 outside of the vessel. The 

 very largest arteries have 

 more of the elastic in pro- 

 portion to the muscular 

 tissue. Owing to the pres- 

 ence of the elastic tissues, 

 arteries may be stretched 

 lengthwise, or distended 

 by pressure from within, 

 and will contract when 

 the stretching force dis 

 appears. Arteries have 

 no valves; those at the 



Fig. 91. Cross section of an artery. 



, , a smooth inner coat, 



beginning Ot the aorta 6 middle or muscular coat. 



c outer or connective tissue coat. 



d small artery to nourish the large one. 



and the pulmonary ar- 

 tery belong to the heart. 

 Branches of the smaller arteries often run into one 

 another, so that there is more than one path for the 

 blood from point to point; if, by any means, one becomes 

 closed, the blood can still pass round by another way. 



220. Veins are the vessels which carry the blood toward 

 the heart. Great arteries open from the ventricles. The 

 great veins open into the auricles. Their walls are 

 thinner than those of the arteries, and collapse when the 

 veins are empty. They contain less of the elastic and 

 muscular tissues than arteries. Veins are supplied with 

 many semilunar valves, which prevent the blood from 

 flowing in the wrong direction. 



221. Vascular and Nervous Supply of Blood Vessels. The 

 coats of arteries and veins are supplied with their own 

 arteries, capillaries, lymphatics, and veins by means of 



