166 STUDIES IN ADVANCED PHYSIOLOGY. 



to note in this connection that the heart in many of the 

 lower animals is forcibly expanded as well as contracted. 

 In the case of the cray-fish and lobster, for instance, large 

 muscular cords run from the edges of the heart to the body 

 wall, and by their contraction the heart is forcibly dis- 

 tended. 



During this filling of the heart, as has been stated be- 

 fore, the blood does not accumulate in the auricle, but runs 

 unhindered into the ventricle until both ventricle and auricle 

 are passively filled. At this point the systole of the auricle 

 begins. The auricle does not completely empty itself, the 

 contraction probably going no farther than the reduction of 

 the dimensions of the auricle to about that of the vena 



cava. 



The Auricular Contraction. 



The auricular contraction begins at the mouth of the veins 

 and runs like a wave toward the ventricles. This explains 

 why the blood is not forced back into the veins rather than 

 into the already filled ventricle. As has been stated, this time 

 occupies about one-tenth of a heart-beat period. While the 

 auricular contraction lasts, the on-rushing blood of the veins 

 is of course checked for an instant. This slight checking of 

 the venous stream caused by the inability of the stream to 

 drop into the auricle during the systole of that structure, 

 causes a slight pulsation along the veins known as the venous 

 pulse. This venous pulse is not at all so well marked as the ar- 

 terial pulse , and is due to an entirely different thing. It starts , 

 however, from the mouth of the large veins at the auricle 

 and runs along the veins much like an arterial pulse along 

 arteries. By the contraction of the auricles the already 

 filled ventricle is forcibly distended and so enabled to throw 

 more blood out on its systole. Further, by the pressing of 

 the blood into the ventricles the auriculo-ventricular valves 

 have, in a way already described, been closed. At this 

 point, of course, the systole of the ventricles begins. Dur- 

 ing this period the auricles act as reservoirs for the entering 

 venous blood and in *Hs role they probably perform their 



