THE NUTRITION OF THE HEART 161 



Similarly, when the thorax is opened, the heart cannot fill itself 

 unless aided by a positive pressure in the veins. 



If the pressure in the venae cavae were not positive, then negative 

 pressure occurring in the heart cavities would lead to a collapse of the 

 thin-walled venae cavae, and not to suction of blood from the veins. 

 In hydraulic engineering, the efforts of engineers are directed towards 

 making the water enter the system without shock. If the negative 

 pressure had any sudden and decided action, there would be shock 

 and consequent loss of energy. 



The driving force of the heart is sufficient by itself to maintain, 

 P/J any rate for a time, and in the horizontal position of the animal, a 

 circulation when the thorax is opened, artificial respiration established, 

 and the muscles paralyzed by injection of curari. Under these con- 

 ditions, the circulation may fail altogether in the vertical posture 

 when gravity opposes the return (see later, the effect of posture, p. 194). 



Normally, the filling of the heart is largely under the control of 

 the respiratory pump. In the closed thorax, the pressure is less than 

 that of the atmosphere by that amount which is required to overcome 

 the pulmonary elasticity and expand the lungs to the "size of the thoracic 

 cavity. In ordinary inspiration, this pressure is equivalent to 9 mm. 

 Hg; in the position of the deepest inspiration, it may sink to 30 mm. 

 Hg. On the one hand, the extrathoracic veins are under a pressure 

 made positive by the compressive action of the skeletal muscles, 

 contraction of muscular walls of viscera, and the respiratory 

 pump; on the other hand, the intrathoracic veins and the heart 

 are under a slight negative pressure, which in inspiration may 

 become 9 to 30 mm. Hg. The venous blood is thus pressed and 

 aspirated into the heart from the venous cistern. Each descent of 

 the diaphragm compresses the abdominal organs, and if in sequence 

 to, or synchronously with, the inspiratory movements of the thorax 

 the abdominal muscles be thrown into contraction, then the respira- 

 tory muscles act powerfully on the venous cistern, not only as a suction 

 but also as a force pump. To prevent overdistension of the right 

 heart, the breath is always held when the abdominal muscles are 

 forcibly contracted e.g., on straining at stool, or when the abdomen 

 is compressed. The pericardium, too, supports the heart and limits 

 its distension. The venous cistern in its turn is filled by the force 

 of the heart-beat (the vis a tergo), but in particular by the muscular 

 movements of the limbs and viscera aided by the valvular action of 

 the veins. During any violent exercise, such as running, the skeletal 

 muscles alternately contract and expand, and a full tide of blood flows 

 through the locomotor organs. The stroke of the heart is then both 

 more energetic and more frequent, and the blood circulates with in- 

 creased velocity. Under these conditions, the filling of the heart is 

 maintained by the pumping action of the skeletal and respiratory 

 muscles. The abdominal wall is contracted, and the reserve of blood 

 is driven from the splanchnic vessels to fill the dilated vessels of the 

 lo ^omotor organs. At each respiration the pressure within the thoracic 

 cavity becomes less than that of the atmosphere, and the blood is 



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