494 CIRCULATION OF BLOOD AND LYMPH. 



the ventricle and the resulting negative pressure to the sudden in- 

 jection of the coronary system from the aorta at the beginning of 

 diastole. The heart in contracting exerts a force greater than that 

 of the blood in the coronary vessels, and probably, therefore, these 

 vessels are emptied and their cavities obliterated in part. At the 

 beginning of diastole they are reinjected with blood under a pressure 

 of perhaps 100 mms. of mercury, and this fact seems to offer a 

 probable explanation for a partial dilatation of the ventricular cavity 

 and a production of negative pressure in the brief interval before the 

 opening of the auriculo- ventricular valves. No view, however, has 

 met with general acceptance, and the cause or causes that produce the 

 negative intraventricular pressure are still a subject for investiga- 

 tion. Regarding the second question proposed above, namely, 

 the extent of the influence of this negative pressure on the flow 

 of venous blood to the ventricles, much diversity of opinion also 

 exists. Direct experiments made by Martin and Donaldson* 

 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 f 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 im- 

 portance to the negative pressure as a factor in moving the blood. 

 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 



* Martin and Donaldson, " Studies from the Biological Laboratory, Johns 

 Hopkins University," 4, 37, 1887; also Martin's " Physiological Papers," 

 Baltimore, 1895. 



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



