150 MAIN FEATURES OF THE APPARATUS. [BOOK i. 



is a continual interchange 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 carrying 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. 



94. We may pass briefly in review some of the main 

 features of the several parts of the vascular apparatus, heart, 

 arteries, veins and capillaries. 



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, receiving 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 pul- 

 monary circulation by which the same quantity of blood is driven 

 at the stroke from the right ventricle into the lungs and received 

 into the left auricle. The rhythm of the beat, that is the fre- 

 quency 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 



