136 THE BLOOD AND ITS CIRCULATION. 



ventricle (Fig. 39) empties itself completely, the contact 

 becoming perfect between its sides and the auricular prolon- 

 gation. 



The result of this mechanism, which is so simple, and yet 

 so generally misunderstood, is that no reflux of blood into 

 the auricle can take place : the auricle, even by means of 

 the mechanism which we have described, exercises a sort of 

 suction upon the venous blood, its cavity being continued so 

 far into the ventricle. We see, also, that when the ventricu- 

 lar systole is complete, the lengthened tube, the hollow cone 

 which unites the ventricle and the auricle, is full of blood, 

 and that a slight and rapid contraction of the auricle is suffi- 

 cient to drive this blood into the ventricle and fill it. 



Nearly all the standard works admit, without discussion, 

 the theory of the occlusion of the auriculo-ventricular ori- 

 fices by the simple mechanism of a plug or valve, just as in 

 the case of the arterial orifices (see farther on), but without 

 remarking the entire difference of structure which distin- 

 guishes the auriculo-ventricular valves from the semilunar 

 valves of the aorta and of the pulmonary artery. This 

 theory has become, up to a certain point, the property of 

 Chauveau and Faivre, on account of the interesting experi- 

 ments which they have so often made upon horses, killed 

 instantaneously by section of the bulb, and in which artifi- 

 cial respiration was kept up. " If, under these circumstances, 

 the finger is introduced into one of the auricles, and the 

 auriculo-ventricular orifice explored, the tricuspid valves 

 will, at the moment that the ventricles begin to contract, be 

 felt to straighten, appose their edges, and stretch in such 

 a manner as to become convex, and form concave domes 

 above the ventricular cavity." This method of proof 

 does not always furnish such decided results; the finger 

 thus introduced has given to many other observers quite 

 different sensations. Onimus 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, pressing themselves against the sides of the 

 ventricles, have the effect of driving the blood, engulfed 

 between them and the corresponding sides, into the arterial 

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

 tricular membranes. This is the only theory which accounts 

 for the existence and arrangement of the papillary muscles. 



