328 



THE CIRCULATION. 



Fig. 108. 



TRANSVERSE SECTION OF THE 

 BULLOCK'S HEART IN THE STATE 

 OP CADAVERIC RIGIDITY. a. Cav- 

 ity of the Left Ventricle, b. Cavity of 

 the Right Ventricle. 



Fig. 109. 



A transverse section of the heart, in its contracted condition, shows the 

 relative volume of the muscular walls of the right and left ventricles, 



and the difference in form of the two 

 cavities. 



The left ventricle forms a thick 

 muscular tube, with its cavity nearly 

 in the centre of the cardiac mass ; 

 while the right ventricle has the ap- 

 pearance of a comparatively incon- 

 siderable layer of fibres, attached to 

 the lateral surface of the organ, and 

 enclosing a cavity of a more linear 

 and flattened form. 



The surperficial cardiac fibres, which 

 make the visible part of the wall of 

 the right ventricle, run obliquely from 

 right to left and from above down- 

 ward, toward the heart's apex; but the more deeply seated layers, 

 belonging to the left ventricle, take more and more a horizontal or 

 circular course, being wrappped round the ven- 

 tricle, almost like those of the small intestine. 

 Whenever these muscular fibres contract, they 

 must, of course, swell in the direction of their 

 thickness ; and the effect produced by this simul- 

 taneous swelling of all the circular fibres is to 

 increase the longitudinal diameter of the ven- 

 tricle, at the same time that its sides are drawn 

 together and its calibre diminished. In the sys- 

 tole of the ventricle, accordingly, its muscular 

 fibres contract upon its contents, like the fingers 

 of a closed hand, and the blood is expelled from 

 its cavity very much as the fluids of the intestinal 

 canal are forced onward by the contracting cir- 

 cular fibres of the muscular coat. 



fihythm of the Heart's Action. The succession of phenomena in the 

 heart's action is peculiar and somewhat complicated. Each pulsation 

 is made up of a double series of contractions and relaxations. The two 

 auricles contract together, and afterward the two ventricles ; and in 

 each case the contraction is immediately followed by a relaxation. The 

 auricular contraction is short and feeble, and occupies the first part of 

 the time of a pulsation. The ventricular contraction is longer and more 

 powerful, and occupies the latter part of the same period. Following 

 the ventricular contraction there comes a short interval of repose, after 

 which the auricular contraction again recurs. The auricular and ven- 

 tricular contractions, however, do not alternate distinctly with each 

 other, like the strokes of the two pistons in a double forcing-pump. On 

 the contrary, they are connected and continuous. The contraction, 



LEFT VENTRICLE OF 

 BULLOCK'S HEART, 

 showing its deep fibres. 



