CIRCULATION OF THE BLOOD. 



513 



anism of a plug or valve, just as in the ease of the arterial orifices (see 

 farther on), but without remarking the entire difference of structure 

 which distinguishes the auriculo-ventricular valves from the semi-lunar 

 valves of the aorta and of the pulmonary artery. This theory has be- 

 come, up to a certain point, the property of Chauveau and Paivre, on 

 account of the interesting experiments which they have so often made 

 upon horses killed instantaneously by section of the bulb, and in which 

 artificial respiration was kept up. ' If, under these circumstances, the 

 finger is introduced into one of the auricles, and the auriculo-ventricular 

 orifices explored, the tricuspid valves will, at the moment that the ventricles 

 begin to contract, be felt to straighten, push their borders, and stretch in 

 such a manner as to become convex, and form a concave dome above 

 the ventricular cavity.' This method of proof does not always furnish 



Fig. 209. — Diagram of the Auriculo- 

 Ventricular Apparatus during 

 Ventricular Systole, after Ktfss. 

 (Bcaunis.) 



1. During the first part of systole. 2. At the com- 

 pletion of systole. AV, valvular cone ; O, auricle ; V 

 ventricle ; A, aorta or pulmonary artery. 



Fig. 210. —Diagram of the Auriculo- 

 Ventricular Apparatus during 

 Ventricular Diastole, after Kuss. 

 (Beaunis.) 

 V, vena cava; O, auricle; Vf ventricle; A, artery; 



1, valvular cone ; 2, arterial infundibulum. 



such decided results, and many observers, among others Spring and 

 Onimus, have met with one entirely different. The latter found the 

 auriculo-ventricular orifices effaced by the contraction of the muscular 

 fibres, which, at this level, really form a sphincter (this is the case in the 

 heart of birds, but not of the mammalia). The papillary muscles, being 

 now contracted, lower the valves, and these, supporting themselves 

 against the sides of the ventricles, have the effect of driving the blood 

 ingulfed between them and the corresponding sides into the arterial 

 orifices. Such is, in short, the working of the auriculo-ventricular mem- 

 branes. This is the only theory which accounts for the existence and 

 arrangement of the papillary muscles." 



The action of the semi-lunar valves is more simple. The semi-lunar 

 valves are composed of three free folds of serous membrane, which during 



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