162 A TEXTBOOK OF PHYSIOLOGY 



aspirated from the veins into the right side of the heart and lungs; 

 conversely, at each expiration the thoracic pressure increases, and the 

 blood is expressed from the lungs into the left side of the heart. While 

 the respiratory pump at all times renders important aid to the cir- 

 culation of the blood, its action becomes of supreme importance during 

 such an exercise as running. The runner pants for breath, and this 

 not only increases the intake of oxygen, but maintains the diastolic 

 filling of the heart. It is of importance to grasp the fact that the 

 circulation of the blood depends not only on the heart, but on the 

 vigour of the respiration and the activity of the skeletal muscles. 

 Muscular exercise is for this reason a sine qua non for the maintenance 

 of vigorous mental and bodily health. 



An experiment which throws light on the filling of the heart is 

 the following: A pressure-bottle filled with oil is connected by a 

 T-piece with (1) an oil manometer, (2) a tube tied into the pericardial 

 sac. So soon as the pressure of the oil is raised to 60 to 70 mm. 

 oil, the arterial pressure falls by 20 to 30 mm. Hg, while the vena 

 cava pressure rises to about 5 mm. Hg. By a pressure of at most 

 240 to 300 mm. oil, the arterial pressure is brought to zero. By 

 no possible means in the anaesthetized animal can the vena cava 

 pressure rise beyond this pressure, and thus the heart is unable to 

 fill. The quantity of blood thrown into the aorta by each contrac- 

 tion of the left ventricle must correspond to that entering the right 

 ventricle during diastole, otherwise the blood will become congested 

 in the veins, and the circulation quickly come to an end. 



In the dog, either the vena cava superior, or the vena cava inferior 

 below the liver may be completely occluded, and yet no change of 

 pressure in arterial pressure is indicated by the arterial manometer. 

 If on the other hand the vena cava inferior between the liver and the 

 heart be compressed, there occurs an immediate and marked descent 

 of the arterial pressure. Thus a rabbit, in which the portal vein has 

 been ligatured, perishes within a few minutes, owing to the rapid 

 accumulation of the blood in the portal tributaries. The capacity 

 of these is so great that they are sufficient to hold all the blood ; the 

 plasma, too, rapidly leaks out of the capillaries when the circulation is 

 thus arrested. The filling of the heart is thus enormously diminished 

 while the aorta continues to empty itself by its elastic reaction into 

 these veins. 



On occluding the pulmonary artery of an animal the left heart 

 empties, and the arterial pressure falls towards zero. The pressure 

 in the vense cavse rises, but only by a very few millimetres of Hg. 

 If one vena cava be half occluded, the venous pressure rises distal to 

 the obstruction, but only by a few millimetres of Hg, and for a short 

 while. A few hours after the pressure in the veins is found to be 

 normal. It is important to note that the oedema, or dropsy, which 

 follows such obstruction, or occurs in heart disease, is not caused by 

 transudation due to a rise of venous pressure, but by nutritive changes 

 in the tissues which follow the obstructed flow. 



If the thorax of a dog or cat be compressed, the arterial pressure 



