FUNCTION OF THE VALVES. 101 



ing the auriculo-ventricular valves, which, when the ventricle 

 is full, form a complete septum between it and the auricle. 

 This elevation of the auriculo-veutricular 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, es- 

 pecially on the auricular surface. When the ventricle con- 

 tracts, the edges of the valves are maintained in apposition by 

 the simultaneous contraction of the musculi papillares, which 

 are enabled thus to act by the arrangement of their tendinous 

 cords just mentioned. 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 

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

 apposition and the equal pressure of the blood on their ven- 

 tricular surfaces ; and the middle-pieces are secure by their 

 great strength, and by the attachment of the tendinous cords 

 along their margins, these cords being always held tight by 

 the contraction of the musculi papillares. A peculiar advan- 

 tage, derived from the projection of these columns into the 

 cavity of the ventricle, seems to be, that they prevent the 

 valve from being converted into the auricle ; for, when the 

 ventricle contracts, and the parts of its walls to which, through 

 the medium of the columns, the tendinous 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, the end of the simultaneously con- 

 tracting fleshy column is drawn away from it, and the cords 

 are held tight. 



What has been said applies equally to the auriculo-ven- 

 tricular valves on both sides of the heart, and of both alike 

 the closure is generally complete every time the ventricles 

 contract. But in some circumstances, the closure of the tri- 

 cuspid valve is not complete, and a certain quantity of blood 

 is forced back into the auricle ; and, since this may be advan- 

 tageous, by preventing the overfilling of the vessels of the 

 lungs, it has been called the safety-valve action of this valve 

 (Hunter, Wilkinson King). The circumstances in which it 

 usually happens are those in which the vessels of the lung are 

 already full enough when the right ventricle contracts, as, 

 e. g., in certain pulmonary diseases, in very active exertion, 

 and in great efforts. In these cases, perhaps, because the right 

 ventricle cannot contract quickly or completely enough, the 

 tricuspid valve does not completely close, and the regurgitation 

 of blood may be indicated by a pulsation in the jugular veins 

 synchronous with that in the carotid arteries. 



