CIRCULATION. 135 



very near the chest. These same forces produce a slight suction within the 

 ventricles, relaxed in their diastole. But a very slight suction occurs at each 

 ventricular diastole even after the chest has been opened. The causes of this 

 arc -till obscure; but it is to be borne in mind that the relaxing wall of the 

 ventricle, flabby as it is, possesses some little elasticity, especially at the auriculo- 

 ventricular ring, and therefore may tend to resume a somewhat different form 

 from that due to its contraction. As the result of this slight elastic recoil, a 

 feeble suction may occur. 



N. The Functions of the Auricles. 



Connections of the Auricle. — Into the right and left auricles open the 

 systemic and pulmonary veins respectively, and each auricle may justly be re- 

 garded as the enlarged termination of that venous system with which it is con- 

 nected. Until modern times the terms of anatomy reflected this view, and 

 from the ancient Greeks to a time later than Harvey, the word " heart "com- 

 monly meant the ventricles only, as it still does in the language of the 

 slaughter-house. This termination of the venous system, the auricle, com- 

 municates directly with the ventricle, at the auriculo-ventrieular ring, by an 

 aperture so wide that, when the cuspid valve is freely open, auricle and ven- 

 tricle together seem to form but a single chamber. 



The Auricle a Feeble Force-pump ; the Pressure of its Systole. — The 

 wall of the auricle is thin and distensible; it is also muscular and contractile. 

 But the slightest inspection of the dead heart shows how little force can be 

 exerted by the contraction of so thin a sheet of muscle. In the Avail of the 

 appendix, however, the muscular structure is more vigorously developed -than 

 over the rest of the auricle. The auricle, then, should be a very feeble force- 

 pump ; and such in fact, it is ; for the highest pressure scarcely rises above 20 

 millimeters of mercury in the right auricle of the dog, 1 and an auricular sys- 

 tole often produces a pressure of only 5 or 10 millimeters. 2 This would be 

 but a small fraction of the maximum ventricular pressure of the same heart. 

 The auricle, however, is equal to its work of completing the filling of the 

 ventricle; and the feebleness of the auricle will not surprise us when we 

 consider that, at the beginning of its systole, the pressure exerted by the 

 contents of the relaxed ventricle is but little above that of the atmosphere, 

 and offers small resistance to the injection of an additional quantity of 

 blood. 



The systole of the auricles is so conspicuous a part of the cardiac cycle when 

 the beating heart is looked at, that its necessity is easily overrated. Even Har- 

 vey, in attacking the errors of his day, was led by imperfect methods to estimate 

 too highly the work of the auricular systole (see p. 184). The error, although 

 a gross one, is not rare, of considering the systole of the auricles to be as im- 

 portant for the charging of the ventricles as the systole of the ventricles is for 

 the charging of the arteries. On page 98 the proof has already been given 



1 Goltz und <i:iulo: op. fit., p. 106. 



1 YV. T. Porter : op. cit., p. 533. 8. de Jager : op. <-ii., p. 506. 



