THE MECHANISM OF THE HEART PUMP 1017 



pressure in the ventricle were due to the sudden cessation of flow through the first 

 part of the aorta, we ought to obtain with the minimum manometer a negative 

 pressure at the root of the aorta equal to that found in the ventricle. But this 

 is not the case, so that the cause of the negative pressure must be sought in 

 the ventricle itself. It is probably due to the fact that during ventricular contrac- 

 tion the base of the heart, including the orifices of the pulmonary artery and 

 aorta, is constricted. Directly the ventricle relaxes, the pressure of blood in these 

 two trunks causes a dilatation of their bases and therefore of the base of the 

 heart. This dilatation of the base of the heart increases its capacity, and so 

 creates a negative pressure in the ventricular cavities. This mode of production 

 of the negative pressure may be illustrated experimentally by connecting a 

 manometer with the interior of either of the ventricles of an excised heart that 

 has ceased beating, and then forcing fluid into the aortic and pulmonary arteries. 

 With each distension of the arteries so produced the mercury in the manometer 

 sinks, showing the production of a negative pressure in the ventricles. 



CHANGES IN FORM OF THE HEART 



As the heart walls are perfectly flaccid during diastole, the shape 

 of this organ as a whole will depend upon the position in which the 

 heart is lying and the direction of its support. Thus if the chest and 

 the pericardium be opened and the animal be in the supine position, 

 the heart during diastole will be flattened from before backwards as 

 a result of the simple weight of its contents. In this position therefore 

 systole will be accompanied by a shortening in the lateral and ver- 

 tical directions and a lengthening in the sagittal direction. During 

 systole, when the heart becomes tense and all its fibres are firmly 

 contracted, the heart, whatever its previous condition, takes the form 

 of a truncated cone. Under normal circumstances the heart in the 

 unopened chest lies in the pericardium, which is attached above to the 

 great vessels and below to the central tendon of the diaphragm. It 

 is supported laterally by the lungs, which, however, owing to their 

 elasticity, have very little influence on the diastolic shape of the 

 heart. 



When the heart is freed from the pericardium, the obliquity of its 

 fibres causes the apex to move forwards and to the right during systole ; 

 this movement is normally prevented by the attachment of the pericar- 

 dium to the central tendon of the diaphragm, so that the most movable 

 part of the heart comes to be the base. If three needles be passed 

 through the chest wall so that their points lie, one in the base, one about 

 the centre of the ventricles, and one in the apex of the ventricles, each 

 ventricular systole is found to be accompanied by a movement of the 

 needle in the base of the heart downwards, a slighter movement in the. 

 same direction of the needle in the middle of the ventricles, and prac- 

 tically no movement at all of the needle which is thrust into the apex. 

 During systole the base of the heart moves downwards towards 

 the apex. This movement is determined partly by the shortening 

 of the fibres of which the ventricular wall is composed, partly by the 



