CIRCULATION. 427 



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-ventricular ring, by an 

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

 tricle together seem to form hut 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 fdead heart shows how little force can be 

 exerted by the confoaction of so thin a sheet of muscle. In the wall 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. 426). 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 390 the proof has already been given 

 that the work of the heart may entirely suffice to maintain the circulation with- 

 out aid from any subsidiary source of energy. It must now be added that the 

 ventricles can, for a time, maintain the circulation without the aid of the auric- 

 ular systole a clear proof that this systole is not a sine qua non for the 

 working of the cardiac pump. 



If in an animal, not only anaesthetized but so drugged that all its skeletal 

 muscles are paralyzed, artificial respiration be established and the chest be 

 opened, the circulation continues. If the artificial respiration be suspended 

 for a time, the lungs collapse, asphyxia begins, and the blood accumulates 

 conspicuously in the veins and in the heart. Presently the muscular walls 

 of the auricles may become paralyzed by overdistention, and their systoles 

 may cease, while the ventricles continue at work and may maintain a circu- 

 lation, although of course an abnormal one. After the renewal of artificial 

 respiration, it may not be till several beats of the ventricles have succeeded, 



1 Goltz und Gaule : op, cit., p. 106. 



1 W. T. Porter : op. cit., p. 533. S. de Jager : op. cit., p. 506. 



