68 THE HEART 



their contraction to lengthen out the ventricle ; the longitudinal 

 layer from the arterial base will tend by their contraction to 

 shorten it. Between them, therefore, they render the apex a 

 fixed point. The apex thus being fixed, the force of their com- 

 bined contraction is to empty the ventricle of its contents. The 

 significance of the longitudinal layer of trabeculae to the auricle is 

 quite clear. This layer is the opponent of the musculi pectinati 

 of the auricle. The apex being a fixed point, its contraction in 

 ventricular systole draws down the A-V groove towards the apex. 

 By the action, therefore, of these opponent sets of muscles in the 

 auricle and in the ventricle, the well-known to-and-fro movement 

 of the heart at the A-V groove is executed (A, B, Fig. 4). It 

 should be noted that this movement of the A-V groove towards 

 the apex in ventricular systole expands the auricle and at the 

 same time produces therein not only more room but also a 

 negative pressure facilitating the flow of blood thereto. 



During joint diastole two things happen (1) The base of heart 

 replaces itself owing to the relaxation of the ventricular fibres ; 

 (2) the ventricle opens out to fill with blood. A glance at the 

 tracing of the jugular pulse given on page 88 will reveal these 

 movements of the auricle and ventricle during the cardiac cycle. 

 The positive wave a takes place during auricular systole ; the fall 

 v is due to the contraction of the ventricle inducing the negative 

 pressure in the auricle ; lastly, during diastole there is, first, a rise 

 due to the reposition of the A-V groove, and secondly, a fall owing 

 to the relaxation of the ventricle. 



Thus far the right side of the heart has occupied our attention. 

 The manner in which the left auricle acts is more obscure, and has 

 had far less attention paid to it. It is situated between the roots 

 of the lungs, being firmly attached to each. By this means it is 

 bound down in the unopened thorax to the chest wall. Behind 

 it are the unyielding structures of the posterior mediastinum. 

 Above, too, are unyielding structures, the pulmonary arteries and 

 bronchi ; to the latter and to the trachea the upper border of the 

 left auricle is firmly bound.. There is left, therefore, only (a) the 

 anterior surface in contact with the ascending aorta, and (6) the 

 floor in contact with the left ventricle. Anatomical evidence, 

 therefore, points to the fact that in systole of the left auricle the 

 anterior wall moves backwards and the ventricle upwards. The 

 former movement would involve a backward movement of the 



