THE MECHANISM OF THE CIRCULATION 447 



blood flows through them at the rate of about one twenty-fifth of an inch 

 in a second. 



The veins have thinner walls than the arteries, are more numerous than 

 they, and have much greater capacity, containing, according to the estimate 

 of Haller, about twice as much blood. They communicate with each other 

 by large branches very frequently in their passage to the heart, and in 

 structure very much resemble the arteries. The principal instance where 

 veins do not convey the blood directly to the heart is the great portal vein, 

 which conducts the blood from the intestines to the liver, and there breaks 

 up into a second system of capillaries, which unite together again to form 

 the hepatic vein; a similar arrangement occurs in the case of the kidney. 



In the veins of the neck and limbs, however, differences are found in 

 the form of numerous valves, usually arranged in pairs, and at tolerably 

 regular distances from each other. These valves are composed of a reflexion 

 of the inner coat, strengthened with some connective-tissue fibres, and near 

 their base have also a small amount of involuntary muscle. The function 

 discharged by the veins is to convey the blood back to the heart. The 

 veins collectively, though there are a few exceptions, convey the blood to 

 the heart, and run side by side with the arteries after which they are 

 named. The radial artery is thus accompanied by the radial vein or veins, 

 the brachial artery by the brachial veins. There are often two or more 

 veins to one artery. The veins are more exposed than the arteries, as is 

 seen in those of the neck, face, body, and limbs. Their capacity is greater 

 than that of the arteries, and the blood flows through them with a uniform 

 and continuous current, but more slowly, the velocity of the current being 

 about 1 foot per second. They communicate freely with one another, and 

 hence obstruction in any one vein is of less importance than in the case of 

 an artery, since the flow of blood hindered or arrested in one channel of the 

 former easily finds escape by another. 



The Chief Arteries and Veins. — We may now, with advantage, 

 take a short survey of the chief vessels that have their origin in the 

 heart, and which are engaged in the distribution of the blood through the 

 system. The large arteries rarely join each other as do the veins, and 

 the blood which traverses them always pursues the same direction. The 

 capillaries, on the other hand, freely anastomose or unite together, and 

 the blood they contain sometimes runs in one, sometimes in the opposite 

 direction, through them; and as many arteries open into the same capillary 

 net-work, this arrangement efliectively prevents the serious consequences 

 that would result in the case of obstruction or division of an artery, unless, 

 indeed, the artery injured is a very large one, and is the parent trunk of 

 the several arteries which open into the same capillary plexus. If, for 



