198 MAIN FEATURES OF APPARATUS. [BOOK i. 



the blood vessels ; the connective tissue in it bears blood vessels 

 and lymphatics. In the case of the auriculo-ventricular valves 

 these blood vessels of the endocardium traverse the whole valve, 

 but in the case of the semilunar valves stop short near the 

 attached border so that the greater part of the valve is bloodless. 



Main Features of the Apparatus. 



112. We may now 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 

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

 as we have seen, especially of the 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 lengthways 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 let out. This elasticity is 

 as we have seen chiefly due to the elastic elements in the coats, 

 elastic membranes and feltworks, but the muscular fibres being 

 themselves also elastic contribute to the result. By reason of 

 their possessing such stout elastic walls the arteries when empty 

 do not collapse but remain as open tubes. In the second place 

 the arteries by virtue of their muscular elements are contractile ; 

 when stimulated either directly as by applying an electric or 

 mechanical stimulus to the arterial walls or indirectly by means 

 of the so-called vaso-motor nerves, which we shall have to study 



