370 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the force generated by the contractions of the skeletal muscles throughout the 

 body (see p. 387). ' 



Mode of Working of the Pumping Mechanism. During each contrac- 

 tion or "systole" of the ventricles the blood is ejected into the arteries only, 

 because at that time the auriculo-ventricular openings are each closed by a valve. 

 During the immediately succeeding " diastole " of the ventricles, which con- 

 sists in the relaxation of their muscular walls and the dilatation of their 

 cavities, blood enters the ventricles by way of the auricles only, because at that 

 time the arterial openings are closed each by a valve which was open during 

 the ventricular systole ; and because the auriculo-ventricular valves which 

 were closed during the systole of the ventricles are open during their diastole. 

 During the first and longer part of the diastole of the ventricles the auricles, 

 too, are in diastole ; the whole heart is in repose; and blood is not only enter- 

 ing the auricles, but passing directly through them into the ventricles. 

 Near the end of the ventricular diastole a brief simultaneous systole of both 

 auricles takes place, during which they, too, narrow their cavities by the 

 muscular contraction of their walls, and eject into the ventricles blood which 

 had entered the auricles from the " systemic " and pulmonary veins respec- 

 tively. The systole of the auricles ends immediately before that of the ventri- 

 cles begins. The brief systole of the auricles is succeeded by their long dias- 

 tole, which corresponds in time with the whole of the ventricular systole and 

 with the greater part of the succeeding ventricular diastole. During the dias- 

 tole of the auricles blood is entering them out of the veins. Thus it is seen 

 that the direction in which the blood is forced is essentially determined by the 

 mechanism of the valves at the apertures of the ventricles ; and that it is due 

 to these valves that the blood moves only in the definite direction before 

 alluded to. In the words, again, of Harvey's note-book, at this point written 

 in English, the blood is perpetually transferred through the lungs into the 

 aorta " as by two clacks of a water bellows to rayse water." l 



Pulmonary Blood-path. In the birds and mammals the entire breadth of 

 the blood-path, at one part of the physiological circle, consists in the capillaries 

 spread out beneath the respiratory surface of the lungs. The right side of the 

 heart exists only to force the blood into and past this portion of its circuit, 

 where, as in the systemic capillaries, the friction due to the fineness of the tubes 

 causes much resistance to the flow. This great comparative development of the 

 pulmonary portion of the blood-path in the warm-blooded vertebrates is related 

 to the activity, in them, of the respiration of the tissues, which calls for a cor- 

 responding activity of function at the respiratory surface of the lungs, and for a 

 rapid renewal in every systemic capillary of the internal respiratory medium, the 

 blood. This rapid renewal implies a rapid circulation; and that the speed is 

 great with which the circuit of the heart and vessels is completed has been 

 proven by experiment, the method being too complicated for description here. 2 



1 Prelectwnes, etc., p. 80. 



2 Karl Vierordt : Die Erscheinungen und Oesetze der Stromgeschwindigkeiten des Blutes. 2te 

 Ausgabe, 1882. 



