562 CIRCULATION OF BLOOD AND LYMPH. 



indicate that this factor has little or no actual influence upon the 

 venous flow. These authors used an isolated dog's heart kept 

 beating by an artificial supply of blood. At a given moment the 

 stream of blood into the vena cava was shut off and the auricle of 

 the heart was brought into communication with a U tube filled 

 with blood. It was found that the auricle took blood from this 

 tube only so long as the pressure in it was positive. Although the 

 heart continued to beat vigorously, whatever negative pressure was 

 present in the ventricle was unable to suck any blood into the 

 auricle from the U tube. Porter* also has shown that at the time 

 of a strong negative pressure in the ventricle the auricle may give 

 little or no evidence of a similar fall in pressure. It would seem 

 most probable, therefore, that the negative pressure observed under 

 certain conditions in the ventricles is a fleeting phenomenon, and 

 disappears with the entrance of the first portion of the blood from 

 the auricles. While it may be of value in accelerating the opening 

 of the auriculo- ventricular valves, its influence does not extend to an 

 actual suction of the blood from the veins toward the heart. 

 Other authors, however, on theoretical grounds attribute more 

 actual importance to the negative pressure as a factor in moving 

 the blood. In one respect it would seem that the contractions of 

 the ventricle must exert a direct influence in accelerating the in- 

 flow of venous blood into the heart. In the paragraph upon the 

 venous pulse (p. 528) it will be recalled that the steep fall of 

 pressure in the auricles immediately after the c wave is attributed 

 mainly to the fact that the contracting ventricles pull the flow of the 

 auricles downward toward the apex as the blood is discharging 

 from the ventricular cavities into the aorta and pulmonary artery. 

 This action for a brief period must exert a suction effect in drawing 

 blood from the veins into the auricles. 



Occlusion of the Coronary Vessels. The coronary vessels 

 supply the tissues of the heart with nutrition, including oxygen, 

 so that if the circulation is interrupted the normal contractions soon 

 cease. The branches of the large coronaries form what are known 

 as terminal arteries, that is, each supplies a separate region of the 

 musculature, and although anastomoses may exist they appear 

 to be too incomplete to allow a collateral circulation to be estab- 

 lished when one of the main arteries is occluded. The portion of 

 the heart supplied by it dies, or to use the pathological term, under- 

 goes necrosis. On account of the pathological interests involved 

 the known serious results that may follow occlusion of any of the 

 coronary vessels or even any interference with the normal structure 

 of the vessels a number of investigations have been made upon ani- 

 mals to determine the effect of occluding one or more of the coronary 

 * "Journal of Physiology," 13, 513, 1892. 



