278 



THE CIRCULATION 



physiology of the circulation. We are obliged, however, to confine 

 ourselves here strictly to the human physiology, and therefore must omit 

 more than this mere reference to the structural facts, however important, 

 described by the correlated sciences. 



The Causes of the Circulation are at least five in number, namely: the 

 contractile energy of the heart in systole (contraction), the elastic recoil 

 of the arterial walls, the thoracic suction, the pressure of the body's 

 muscles on ,the valve-supplied veins, and the suction of the auricles during 

 diastole (relaxation). Of these, the first, chiefly ventricular contraction, 

 unaided, is adequate to continue the circulation. 



THE CONTRACTION OF THE HEART (considered as a mere pump for 

 the present), occurs on the average seventy-five times every minute. 

 The comparatively long time of three-tenths of a second is required for 

 he ventricles to contract. This is preceded by the auricular contraction 



FIG. 146 



Heart-cells of an adult in cross-section. 

 (Minervini.) 



which lasts one-tenth of a second. 

 As already noted, the auricular 

 beat merely fills the ventricles and 

 for our present purpose need not be 

 discussed. The ventricular systole 

 (contraction) is essentially a con- 

 striction of the middle layer of mus- 

 cle-fibers. This narrows the caliber 

 of the chamber considered as a tube 

 closed at the lower end by the other 

 layers. The middle fibers furnish the chief motive power. The others, 

 oblique, help to bring about the shortening of the ventricle, the forward 

 up-rise of its apex against the chest-wall, and the partial descent of the 

 heart's base as the whole heart twists, shortens, and becomes flatter 

 antero-posteriorly. The exact meaning of all parts of the muscular 

 structure is not yet well understood, for the whole process is one of great 

 complexity, having in it the natures of both cross-striated and of smooth 

 muscle. The former kind of muscle causes the beat and the latter the 

 tonus of the heart. This tonus is as yet little known, but apparently is 



Fresh human heart-muscle: K, nucleus; 

 Kl, cement-substance; P, pigment-masses. 

 224 /!. (Schiefferdecker and Kossel.) 



