134 THE BLOOD AND ITS CIRCULATION. 



this time entirely different ; it is empty, completely relaxed, 

 and, consequently, offers no resistance whatever: the part 

 which it now plays in regard to the auricle is the same as 

 that previously sustained by the auricle in regard to the 

 veins ; and the elasticity of the muscle, when in a state of 

 repose, allows the ventricle to be distended (see physiology 

 of the muscle, p. 81) with as little resistance as would be 

 offered by a soap-bubble. Thus the blood of the contracted 

 auricle, meeting with a slight resistance from the veins, and 

 none at all from the ventricle, is precipitated into the latter, 

 and fills it. If the muscular tissue of the ventricle is dis- 

 eased, and its elasticity diminished, a certain reflux will 

 sometimes take place into the veins, which is one of the 

 causes of the pathological venous pulse : this venous pulse 

 always exists to a slight degree, but is usually scarcely per- 

 ceptible. 



The auricle is not, however, completely emptied, and its 

 opposite sides do not come in contact with each other. Its 

 rapid contraction being terminated, resumes the position of a 

 passive organ, and allows the blood which tills the venous 

 system to flow freely into Its cavity. 



Ventricle. The ventricle is hardly full before the blood, 

 by its contact with the walls of this cavity, occasions their 

 contraction. The ventricular systole thus immediately suc- 

 ceeds the auricular systole ; but the former lasts a long time, 

 because the ventricle is obliged to empty its contents into 

 a cavity which is already full of blood, and which offers 

 some resistance to the entrance of more. By this contrac- 

 tion and prolonged effort the contents of the ventricle pass 

 into the corresponding artery without any reflux towards the 

 auricle. 



How is this reflux towards the auricle prevented? By 

 means of a special apparatus called the auriculo-ventricular 

 valves, which really form a sort of sleeve or bag hanging 

 from the edges of the auricle into the ventricle, and alter- 

 nately approaching and withdrawing from the walls of the 

 latter. The name " valve " shows that the role of this organ 1 

 was not at first understood. It is now shown that the tri- 

 cuspid or mitral valve does not serve as a plug, but is only a 

 movable continuation of the auricle, acted upon by certain 



1 See V. L. Kohl, " Etude Critique sur la Physiologic de PAp- 

 pareil Auriculo-ventriculaire. These de Strasbourg, 1869, No. 

 231. 



