134 



THE VASCULAR MECHANISM. 



different tissues, surrounded by capillary networks. But the walls of the 

 capillaries are so thin and of such a nature that certain of the constituents 

 of the blood pass from the interior of the capillary, through the capillary 

 wall to the elements of the tissue outside the capillary, and similarly certain 

 of the constituents of the tissue, to wit, certain substances the result of the 

 metabolism continually going on in the tissue, pass from the tissue outside 

 the capillary through the capillary wall into the blood flowing through the 

 capillary. Thus as we have already said, 13, there is a continual inter- 

 change of material between the blood in the capillary and the elements of 

 the tissue outside the capillary, the lymph acting as middle-man. By this 

 interchange the tissue lives on the blood, and the blood is affected by its 

 passage through the tissue. In the small arteries which end in, and in the 

 small veins which begin in the capillaries, a similar interchange takes place ; 

 but the amount of interchange diminishes as, passing in each direction from 

 the capillaries, the walls of the arteries and veins become thicker ; and 

 indeed; in all but the minute veins and arteries, the interchange is so small 

 that it may practically be neglected. It is in the capillaries (and minute 

 arteries and veins) that the business of the blood is done ; it is in these that 

 the interchange takes place ; and the object of the vascular mechanism is to 

 cause the blood to flow through these in a manner best adapted for carryino- 

 on this interchange under varying circumstances. The use of the arteries is 

 in the main simply to carry the blood in a suitable manner from the heart 

 to the capillaries, the use of the veins is in the main simply to carry the 

 blood from the capillaries back to the heart, and the use of the heart is in 

 the main simply to drive the blood in a suitable manner through the 

 arteries into the capillaries and from the capillaries back along the veins 

 to itself again. The structure of these several parts is adapted to these 

 several uses. 



Main Features of the Apparatus. 



101. We may now pass briefly in review some of the main features of 

 the several parts of the vascular apparatus heart, arteries, veins, and 

 capilliaries. 



The heart is a muscular pump that is, a pump the force of whose strokes 

 is supplied by the contraction of muscular fibres working intermittently, the 

 strokes being repeated so many times (in man about 72 times) a minute. 

 It is so constructed and furnished with valves in such a way "that at each 

 stroke it drives a certain quantity of blood with a certain force and a certain 

 rapidity from the left ventricle into the aorta, and so into the arteries, receiv- 

 ing during the stroke and the interval between that stroke and the next the 

 same quantity of blood from the veins into the right auricle. We omit for 

 simplicity's sake the pulmonary circulation by which the same quantity of 

 blood is driven at each stroke from the right ventricle into the lungs and 

 received into the left auricle. The rhythm of the beat, that is the frequency 

 of repetition of the strokes, and the characters of each beat or stroke, are 

 determined by changes taking place in the tissues of the heart itself, though 

 they are also influenced by causes working from without. 



The arteries are tubes, with relatively stout walls, branching from the 

 aorta all over the body. The constitution of their walls, especially of their 

 middle coat, gives the arteries two salient properties. In the first place 

 they are very elastic, in the sense that they will stretch readily, both length- 

 wise and crosswise, when pulled, and return readily to their former size and 

 shape when the pull is taken off. If fluid be driven into one end of a piece 

 of artery, the other end of which is tied, the artery will swell out to a very 

 great extent, but return immediately to its former calibre when the fluid is 



