. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Internal 1 reimportation ;63 and controlled, one can conceive how complex the stopcock system of the human machine is" (Keith). If the varying work of the heart were not regulated by some kind of automatic device for adjusting the blood pres- sure and controlling the flow, disaster would inevitably follow whenever in the countless exigencies of life, a sudden extra load is thrown upon this faithful pump. The tireless beat of the heart itself is initia
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Internal 1 reimportation ;63 and controlled, one can conceive how complex the stopcock system of the human machine is" (Keith). If the varying work of the heart were not regulated by some kind of automatic device for adjusting the blood pres- sure and controlling the flow, disaster would inevitably follow whenever in the countless exigencies of life, a sudden extra load is thrown upon this faithful pump. The tireless beat of the heart itself is initiated and regulated at the sinu- auricular node (Fig. 299). This "pace setter of the vertebrate heart" is a narrow zone of tissue that marks the transitional region between the sinus venosus and the atrium in the fish heart, and which becomes incorporated Ventral Aorta Auricle Sinu-Auricular Junction-, Sinus Venosus -. —it*— Con us Arteriosus die of His .-"-^--Ventricle Fig. 299. Diagrammatic section of a heart to show location of timing mechanism. (After Keith.) as a part of the auricle in higher vertebrates. Another indispensable part of the mechanism of the throbbing heart is the auriculo-ventricular node, a dense network of cardiac muscle fibers connecting the auricular and ven- tricular walls, and acting somewhat like the "timer" in an automobile. Across this bridge the initiator)' stimulus, originating in the sinu-auricular node, is transmitted to the ventricle completing the heart-beat. The auric- ulo-ventricular node was discovered in the human heart by His in 1893, and is consequently known as the bundle of His. Although the heart beats in successive throbs, a constant flow of blood is maintained because the elastic arteries, stretched by the pressure gen- erated by the ventricular contraction, gradually contract until they air suddenly distended again by the next "beat" of the ventricle. There is thus a < (instant flow of blood
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