PHYSIOLOGIC APPARATUS. 727 



auriculo-ventricular, and the ventriculo-conic junctions the continuity is to 

 some extent interrupted by bands of circularly disposed fibrous tissue, serving 

 for the support of the valves, which momentarily interfere with the ready 

 passage of the contraction wave from one division of the heart to another. 

 The frog heart receives its nutritive material from the blood flowing through 

 its cavities. During the diastole the blood, under the influence of the slight 

 pressure developed, passes from the interior of the heart into a system of 

 irregular passage-ways or channels which penetrate the heart-wall in all 

 directions, and thus comes into direct contact with the heart-cells. With 

 the beginning of the systole the blood is forced out of these channels into the 

 interior of the ventricle, bringing with it the products of tissue metabolism. 

 The Heart Beat. If the heart while beating is lifted up by a ligature 

 attached to the apex it will be observed that the contraction begins in the 

 walls of the sinus venosus, then passes to the auricles, thence to the ventricle 

 and finally to the conus; from this it may be inferred that the physiologic 

 stimulus acts primarily in the walls of the sinus from which its effect, viz., 

 the excitation process, is conducted from one cavity to another in quick 

 succession. 



