THE CIRCULATORY SYSTEM. 589 



The veins, then, through the intermedium of the capillaries, having taken up the blood, commence 

 the return journey, receiving tributary veins from all directions, and getting larger and fewer in 

 number as they near the heart. The veins thus formed unite at last to form one large vessel, 

 the vena cava, which pours the blood into the right auricle. The blood has not yet quite 

 completed the circulation, however ; for, be it remembered, it is still dark and impure blood, and 

 unfit for the further nutrition of the body, until the effete matter with which it is loaded has 

 been burned off in the lungs. For this reason and purpose it is received by the right ventricle 

 from the right auricle, and by the former pumped into the lungs through the medium of the 

 pulmonary artery ; and here the same minute subdivision of arteries takes place that we noticed 

 throughout the general circulation, the artery ending in capillaries, which are spread out around 

 the air-cells of the lungs, and the blood in them absorbs oxygen, and gives off carbonic acid 

 and water, which is exhaled. The capillaries of the pulmonary artery join those of the pulmonary 

 veins, which, uniting and re-uniting, finally pour the purified blood into the left auricle, from 

 which it descends to the ventricle of the same side, and so is once more pumped away to carry 

 nutriment to every portion of the body. And thus the wheel of life goes round. 



We usually speak of arterial blood being red, and venous blue or purple; but the reader 

 will observe that in the lesser circulation, or that through the lungs, matters are simply reversed, 

 for here it is the arteries that contain the dark blood, and the veins the red. In considering the cir- 

 culation of the blood, there is one thing that cannot fail to strike any one who has some knowledge 

 of mechanics. There are openings between the auricles and ventricles through which the blood 

 has to pass. Well, on the contraction of this ventricle (say the left) necessary to send the blood 

 with force in the aorta, why does it not just as easily flow back again into the auricle as go 

 onward. To prevent this a valve is placed in the orifice between the two chambers, called the 

 mitral valve, and this valve is so arranged that it opens to permit the blood to flow into the 

 ventricle, but closes as the ventricle contracts, just as the air gets freely in through the lower 

 opening of a pair of bellows, but cannot get out again. 



Now this has an important bearing with regard to heart disease, for it sometimes happens 

 that this valve or any other valve in the heart, for that matter becomes the subject of disease, 

 gets either insufficient or thickened by deposit, so that it cannot close properly, and then at 

 every contraction of the heart a portion of blood regurgitates, which forms one very serious kind 

 of heart disease. 



Another question that will doubtless present itself to the intelligent reader is : What power 

 or force is it that sends the blood upwards and onwards through the veins, the contractile force 

 of the left ventricle, which has pumped the blood through the arteries, being naturally to a great 

 extent expended at the capillaries. There is a certain resiliency in the arteries themselves, 

 which gives the blood in the venous capillaries a start, at all events, to begin with, but on the 

 dilatation of the right ventricle it. will naturally form a vacuum, and so, after the manner of a pump, 

 it will suck the blood towards it. Again, the walls of the veins themselves have a certain 

 contractility which aids the flow of the blood, and the action of surrounding muscles will also 

 help the work. " But," it may be said, " this latter is a force that may act in both directions." 

 True enough ; but we find in the venous system a beautiful provision of Nature which determines 

 the flow of the blood in the right direction only. This is accomplished by means of valves or 

 little pockets, having their mouths pointed in the direction in which the blood is flowing. 

 These valves are simply little pouches formed of the lining membrane of the veins themselves. 

 Now it must be evident to every one that so long as the blood flows in one direction, these pockets 

 will lie flat against the inside of the vein, and present no obstacle, but if any attempt is made by 



