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HAND-BOOK OF PHYSIOLOGY. 



Each of these cords is attached by its outer extremities to the outer end 

 of the free margin of its valve, and in the middle to the corpus Arantii; 

 they thus enclose a lunated space from a line to a line and a half in width, 

 in which space the substance of the valve is much thinner and more pliant 

 than elsewhere. When the valves are pressed down, all these parts or 

 spaces of their surfaces come into contact, and the closure of the arterial 

 orifice is thus secured by the apposition not of the mere edges of the 

 valves, but of all those thin lunated parts of each which lie between the 

 free edges and the cords next below them. These parts are firmly pressed 

 together, and the greater the pressure that falls on them the closer and 

 more secure is their apposition. The corpora Arantii meet at the centre 

 of the arterial orifice when the valves are down, and they probably assist 

 in the closure; but they are not essential to it, for, not unfrequently, 

 they are wanting in the valves of the pulmonary artery, which are then 

 extended in larger, thin, flapping margins. In 

 valves of this form, also, the inlaid cords are less 

 distinct than in those with corpora Arantii; yet the 

 closure by contact of their surfaces is not less 

 secure. 



It has been clearly shown that this pressure of the 

 blood is not entirely sustained by the valves alone, but 

 in part by the muscular substance of the ventricle 

 (Savory). By making vertical sections (Fig. 100) 

 through various parts of the tendinous rings it is pos- 

 sible to show clearly that the aorta and pulmonary 

 artery, expanding toward their termination, are sit- 

 uated upon the outer edge of the thick upper border 

 of the ventricles, and that consequently the portion 

 of each semilunar valve adjacent to the vessel passes 

 over and rests upon the muscular substance being 

 thus supported, as it were, on a kind of muscular floor 

 formed by the upper border of the ventricle. The result of this arrange- 

 ment is that the reflux of the blood is most efficiently sustained by the 

 ventricular wall. l 



As soon as the auricles have completed their contraction they begin 

 again to dilate, and to be refilled with blood, which flows into them in a 

 steady stream through the great venous trunks. They are thus filling 

 during all the time in which the ventricles are contracting; and the con- 

 traction of the ventricle* being ended, these also again dilate, and receive 

 again the blood that flows into them from the auricles. By the time that 

 the ventricles are thus from one-third to two-thirds full, the auricles are 



1 Savory's preparations, illustrating this and other points in relation to the struc- 

 ture and functions of the valves of the heart, are in the Museum of St. Bartholomew's- 

 Hospital. 



FIG. 100. Vertical sec- 

 tion through the aorta 

 at its junction with the 

 left ventricle, a, Section 

 of aorta. 66, Section of 

 two valves, c, Section of 

 wall of ventricle, d, In- 

 ternal surface of ven- 

 tricle. 



