124 THE CIRCULATION. 



again when the piston is forced down, i.e., when the 

 ventricle contracts, and the other, i.e., the arterial, valve 

 opens. The diagrams on the preceding page illustrate 

 this very well. 



During auricular contraction, the force of the blood pro- 

 pelled into the ventricle is transmitted in all directions, but 

 being insufficient to raise the semilunar valves, it is ex- 

 pended in distending the ventricle, and in raising and 

 gradually closing the auriculo-ventricular valves, which, 

 when the ventricle is full, form a complete septum between 

 it and the auricle. This elevation of the auriculo-ventricular 

 valves is, no doubt, materially aided by the action of the 

 elastic tissue which Dr. Markham has shown to exist so 

 largely in their structure, especially on the auricular sur- 

 face. When the ventricle contracts, the edges of the valves 

 are maintained in apposition by the simultaneous contrac- 

 tion of the columnce carnete, which are enabled thus to act 

 by the arrangement of their tendinous cords just men- 

 tioned. In this position the segments of the valves are 

 held secure, even though the form and size of the orifice 

 and the ventricle may change during the continued con- 

 traction; for the border -pieces are held by their mutual 

 apposition and the equal pressure of the blood on their 

 ventricular surfaces ; and the middle pieces are secure by 

 their great strength, and by the attachment of the ten- 

 dinous cords along their margins, these cords being always 

 held tight by the contraction of the musculi papittares. A 

 peculiar advantage, derived from the projection of these 

 columns into the cavity of the ventricle, seems to be, that 

 they prevent the valve from being everted into the auricle ; 

 for, when the ventricle contracts, and the parts of its walls 

 to which, through the medium of the columns, the ten- 

 dinous cords are affixed, approach the auriculo-ventricular 

 orifices, there would be a tendency to slackness of the 

 cords, and the valves might be everted, if it were not that 

 while the wall of the ventricle is drawn towards the orifice, 



