THE MECHANISM OF THE HEART PUMP 1025 



raised so as to cause a positive pressure on the big veins and auricles, 

 the return flow of blood to the heart must come to an end. Thus 

 during extreme muscular efforts the glottis is fixed and a positive 

 pressure is produced in the thorax. The deficient circulation and the 

 deficient aeration of the blood thereby induced are shown by the 

 engorgement of the superficial veins and the blueness of the surface. 

 Weber showed that by a forcible expiration, with the glottis closed, 

 the pulse might disappear at the wrist and the circulation be brought 

 for a time to a standstill, so that even loss of consciousness might 

 supervene. 



Since the heart during its systole diminishes its own volume by 

 the expulsion of blood from the thorax, it becomes smaller, and the 

 space thus provided in the chest cavity is taken up by an expansion 

 of the veins, auricles, and lungs. To this systolic diminution of intra- 

 thoracic pressure is due the ' cardio-pneumatic ' movements. These 

 are recorded by attaching one nostril to a delicate tambour by means 

 of a tube, while the other nostril and the mouth are kept closed. If a 

 carotid pulse tracing be taken at the same time it will be found that 

 there is a fall of the lever attached to the nasal cavity synchronous 

 with the rise of the pressure in the arteries, due to the expulsion of 

 blood from the heart. 



The normal filling of the heart during diastole can be prevented 

 by anything which hinders its expansion, such as the presence of 

 fluid in the pericardial cavity. The same effect may be produced 

 experimentally. If oil be allowed to flow into the pericardium under 

 pressure, when the pressure of the oil rises to about 60 rnm. the 

 pressure of the vena cava rises to a height just above that obtaining 

 in the pericardial cavity. On increasing the pressure, a point is finally 

 reached at which no more blood can be driven from the veins to the 

 heart, so that the arterial blood-pressure falls to zero and death ensues. 

 In order to maintain the arterial pressure it is necessary that the 

 amount of blood driven into the arterial system by the contraction 

 of the left ventricle should be exactly equal to that leaving the arteries 

 to pass into the capillaries during the period which elapses between 

 each systole. 



Over-filling of the heart is prevented to a certain extent by the 

 resistance of its walls. The danger of over-filling is therefore most 

 marked in the right ventricle. An important part is played moreover 

 by the pericardium in this regard. Even when beating normally, the 

 heart during diastole tends to protrude through a slit made in the 

 pericardium, and Barnard has shown that the right auriculo-ventri- 

 cular valve ceases to be entirely efficient when the pericardium has 

 been freely opened, the closure of this valve being dependent on 

 the support afforded to the heart by the pericardium. 



65 



