THE THORAX 983 



filled with blood, the segments are in contact, and the ventricular systole 

 now takes place. At the same time the musculi papillares contract. Blood 

 is forced against the segments of the valve, but it cannot, in health, enter the 

 auricle, because the segments are maintained in close contact, and are prevented 

 from being driven back into the auricle, by the chordae tendineae, which are 

 under the control of the musculi papillares. If there were no musculi 

 papillares, in which case the chordae tendineae would spring directly from 

 the wall of the ventricle, then the segments of the valve would not be held 

 tight, but would, under the pressure of the blood, be driven back into the 

 auricle, and regurgitation of blood would of necessity occur. The explanation 

 of this lies in the fact that when the ventricle contracts a kind of screwing up 

 or wringing movement takes place in its wall, as, so to speak, in wringing 

 a wet towel. The effect of this peculiar action is to approximate successive 

 parts of the ventricular wall to the auriculo-ventricular orifice, and this would 

 have the effect of relaxing the chordae tendineae, and so allowing the segments 

 of the valve to be driven back into the auricle. The chordae tendineae, 

 however, spring from musculi papillares, and these are elevations of the wall 

 of the ventricle, directed towards the auriculo-ventricular orifice. During 

 the wringing movement, therefore, of the ventricular wall in systole the 

 musculi papillares, as stated, contract, and so maintain the chordae tendineae 

 taut, or tightly drawn. 



When the mitral valve opposes the entrance of blood into the left ventricle 

 the cardiac affection is known as mitral obstruction (stenosis). When the 

 mitral valve is incompetent, and allows regurgitation of blood to take place 

 from the left ventricle into the left auricle, the cardiac affection is known as 

 mitral regurgitation (incompetence). 



The aortic orifice is circular, and is separated from the auriculo- 

 ventricular orifice by the anterior or aortic cusp of the mitral valve. 

 It is guarded by the aortic valve, which, by means of its segments, 

 prevents regurgitation of blood from the aorta into the left ventricle 

 during the elastic recoil of the arterial wall. It is composed of three 

 semilunar or sigmoid segments, consisting of fibrous tissue, covered 

 on their ventricular surfaces by endocardium, and on their 



Orifice of Corpus Arantii Lunula Orifice of 



Coronary Artery J Coronarj- Artery 



Fig. 404. — The Aortic Semilunar Valves. 



arterial surfaces by the endothelial lining of the arter}-. Each 

 segment is attached by its convex border to the wall of the artery 

 at the place where it springs from the ventricle. The other border 

 of the segment is free, and is directed away from the ventricle. 

 Each segment, therefore, is so disposed as to allow the blood to 

 pass freely into the aorta from the left ventricle. The free border 

 of each segment is strengthened by a band of fibrous tissue, and at 



