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A TEXTBOOK OF PHYSIOLOGY 



the ventricles are antagonists. The circular fibres, by their con- 

 traction, tend to lengthen the apex-base diameter. The longitudinal 

 fibres resist this, and the two together wring the blood out of the 

 heart. The apex is maintained as a fixed point by this antagonistic 

 action, and thus the longitudinal fibres are enabled to expand the 

 auricles by pulling down the floor of these chambers. This action is 

 important, as it contributes to the filling of the auricles simultaneously 

 with the emptying of the ventricles. Tracings of the jugular pulse 

 give evidence of s-uch action. 



In the case of the auricles, the longitudinal musculi pectinati not 

 only help the circular fibres to expel the blood, but draw up the base 

 of the ventricle to meet its load of blood. Thus, the A.-V. groove is 



FIG. 59. To SHOW THE ANTAGONISTIC ACTION OF THE MUSCULATURES OF THE RIGHT 

 AURICLE AND VENTRICLE. (Keith.) 



A, position of A.-V. groove at end of auricular systole; B, at end of ventricular systole. 



pulled up during auricular systole, and down during ventricular 

 systole (A, B, Fig. 59). The posterior and upper borders of the left 

 auricle lie against the unyielding structures of the posterior medi- 

 astinum, the pulmonary artery, and bronchi, the floor and anterior 

 part being in contact with the base of the ventricle and ascending 

 aorta respectively. The latter parts alone are free to move during 

 systole. Thus, the left ventricular base is drawn up and the aorta 

 back on auricular systole. 



Modes of Examining the Living Heart. The physiologist or physi- 

 cian has many means at his disposal of examining the heart's 

 action. Its efficiency may be tested by noting how much its 



