THE HEART 107 



contraction of the circular fibres results in considerable 

 diminution of the diameter of the base of the heart (the 

 clinical importance of which we shall see immediately), 

 whilst the total length of the ventricles is not increased, 

 owing to the fact that the transverse thickening of the 

 circular fibres of the wall is neutralized by the contrac- 

 tion of the opposing longitudinal fibres.* 



The result is that the walls of the heart approach 

 nearer to the septum, and in consequence the blood is, 

 as it were, ' wrung out ' into the aorta and pulmonary 

 artery. It must not be supposed, however, that the ex- 

 pulsion of the contained blood takes place the moment 

 the ventricular wall enters into contraction. During 

 an appreciable interval ' the presphygmic interval ' 

 which varies in the human heart from seven to ten- 

 hundredths of a second, the muscle of the wall is engaged 

 in compressing the contained blood in order to get up a 

 pressure sufficient to overcome the resistance in the 

 aorta and pulmonary artery, and it is only when a 

 sufficient degree of pressure has been attained that the 

 semilunar valves fly open and the blood is expelled. 

 Thus one might divide the period of systole into (1) a 

 compression time, during which pressure is brought to 

 bear on the contained blood, and (2) an expression 

 time, which is occupied by the expulsion of the blood 

 into the aorta and pulmonary artery, t It is during the 



* For a very interesting description of the functional anatomy of 

 the musculature of the heart the reader should consult a paper by 

 Dr. Arthur Keith in the Journal of Anatomy and Physiology, 1907, 

 vol. xlii. 



f See Martius : ' Der Herzstoss des Gesunden und Kranken 

 Menschen.' Volkmann's Samml. Klin. Vort. 1894, Inn. Med. t 

 Ko. 34, p. 171. 



