THE HEART BEAT. 565 



manometer, which registered the lowest pressure reached during 

 the period of observation. By this method they and others have 

 shown that in an animal (dog) with an opened thorax the pressure 

 in the interior of the ventricles may be negative to an extent equal 

 to 20, 30, or even 50 mms. of mercury. Moreover, by the use of 

 some form of elastic manometer, such as the Hiirthle instrument 

 (p. 498). it has been shown that this negative pressure occurs at the 

 end of the period of relaxation, at the time, therefore, at which it 

 might be supposed to exert a marked influence upon the inflow of 

 venous blood. It should b6 added, however, that a negative 

 pressure can not be shown for every heart beat. It may be absent 

 altogether or slight in amount, varying, no doubt, with the force of 

 contraction and the condition of the heart. With regard to the 

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

 venous blood to the ventricles there has been some difference of 

 opinion. 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 the tube 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 importance to the negative pressure as a factor in moving 

 the blood. 



Occlusion of the Coronary Vessels. The coronary vessels'sup- 

 ply 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 



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

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

 Baltimore, 1895. See also, for confirmatory results, von den Velden, "Zeit- 

 schrift f. exp. Pathol. u. Therapie," 1906, iii., 432. 



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



