THE BLOOD-VESSELS AND CIRCULATION 135 



This action of the elastic walls of the arteries may be well illustrated by the 

 following experiments : Fasten one end of a long glass or metal tube to the 

 stop-cock of a water pipe by means of an india-rubber tube as short as possible, 

 and fix a small nozzle to its other end. Then turn the cock on and off alter- 

 nately and rapidly, to imitate the intermittent discharges of the ventricles. 

 The water will issue from the other end of the pipe in a series of jets, each jet 

 ceasing the moment the water is turned off. Next, substitute a long india- 

 rubber tube for the one just used, and repeat the experiment. It will now be 

 found that a continuous stream issues from the tube. The pressure of water 

 stretches the elastic tube, and, when the stream is turned off, the india-rubber 

 recoils on the water, and the intermittent flow is changed into a continuous 

 stream. 



As the blood passes from the larger into the smaller arteries, and then into 

 the capillaries, it comes in contact with a gradually increasing amount of 

 surface. This multiplication of surface greatly increases the resistance offered 

 to the blood by friction, which also tends to equalise the current as well as 

 to reduce its velocity. Not only is the surface increased, but also the total 

 sectional area of the small arteries or capillaries of any part is much greater 

 than that of the large artery or arteries supplying that part, so much so that the 

 velocity of the blood in the capillaries is only ^ that in the aorta. As the 

 blood passes onward into the veins, the velocity again increases, owing to the 

 decrease of area, but the circulation in these vessels is less rapid than in the 

 corresponding arteries, the capacity of the former being two or three times 

 that of the latter. These facts explain how it is that, while the flow of blood 

 from a cut artery is intermittent and rapid, the stream from a vein of the same 

 size is continuous and slower. 



The time required for a portion of the blood to make a complete circulation, 

 i.e. to pass through arteries, capillaries, and veins, is less than one minute. 

 Seeing that the blood passes so slowly through the capillaries this may appear 

 incredible, but it must be remembered that each portion of the blood passes 

 through only a very short distance of capillary tube, probably less than 

 i of an inch in any case. 



We may now view the circulation as a whole, tracing the 

 course of the blood from a certain point till it arrives at that 

 same point again. The blood which has been collected from 

 all parts by the venae cavse is poured into the right auricle. It 

 then passes into the right ventricle, and is forced through the 

 pulmonary arteries into the lungs. After circulating through the 

 capillaries of the lungs, where it gives up carbonic acid gas 

 and absorbs oxygen, it is collected by the pulmonary veins, and 

 conveyed by them into the left auricle, and thence into the left 

 ventricle. This ventricle contracts, forcing the blood through 

 the aorta and its branches into capillary networks in all the 

 tissues, excepting those few, such as the outer skin, the nails, hair, 

 enamel of the teeth, &c., which are bloodless. It is then collected 

 up by veins which convey it directly or indirectly into the venae 

 cavae, thus completing a revolution. 



It will thus be seen that there are two distinct circulations by 

 which the blood can pass from one side of the heart to the other. 

 One is called the pulmonary circulation, by which it passes from 



