382 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the flapping and closing of these valves, both curtain 

 and pocket, that make the characteristic sounds of the 

 beating heart, and it is the inflammation and thickening 

 of them that constitute the commonest and gravest dis- 

 ease of the heart. 



Blood Vessels.-^^Kco'Rt— In the dissection of 

 a body the arteries carfj^... £ ce distinguished from 

 the veins by the fact that t'^gfo 'ain their form as pipes, 



while the thinner veins colla, 



1 fi- 



Structure. — They consist of three coats : an outer thin, 



very tough fibrous coat; a middle elastic, muscular, or 

 proper coat; and an inner lining epithelial coat. The 

 outer coat gives toughness, the middle gives elasticity 

 and firmness, the inner is in some way necessary to the 

 life and integrity of the blood. In tying an artery the 

 surgeon draws the thread until he feels the cutting of 

 the middle and interior coats, and then secures the knot. 

 In aneurism the inner and middle coats are broken 

 and the tough outer coat is extended into a pulsating 

 tumor. The firmness of the middle coat is necessary 

 to maintain an open passage for the blood under power- 

 ful action of the heart, and by its elasticity it yields to 

 the impulse of the heart, and then, contracting, it carries 

 forward the blood in its course. It is this propagated 

 wave of swelling and contraction that constitutes the 

 pulse. From the great aortic trunk, of course, the 

 arteries branch and rebranch until they become of capil- 

 lary fineness in all the tissues, and finally grade into the 

 capillaries proper. The blood does not ooze through 

 the arterial walls ; these pipes do not leak. The nutri- 

 tion of the tissues is the function of the capillaries alone. 

 2. Veins. — The veins are much larger than the arteries, 

 and yet far less conspicuous, because they collapse when 

 empty. In structure they are similar to the arteries. 

 They, too, have three coats, but the middle coat is much 



