THE BEABT. 505 



tendinous cords; 4, An internal face, which becomes superior when the 

 valve IS raised to close the opening, at which period it constitutes the floor 

 01 the auricular cavity. 



Pulmonary opening.— This orifice represents the embouchure of the 

 piilmonary artery. Situated in front and to the left of the preceding, and 

 a little higher, it occupies the summit of a kind of infundibulum formed by 

 the left cimpartraent of the ventricle being prolonged upwards. It is 

 perfectly circular, smaller than the artery to which it gives origin, as well 

 as the auriculo -ventricular opening, from which it is separated by a species 

 01 muscular spur, to which is •> r 



Fig. 261. 



attached the principal festoon 

 of the tricuspid valve. 



The pulmonary opening is 

 furnished with three valves : the 

 sigmoid (or semicircular), sus- 

 pended over the entrance to the 

 pulmonary artery, and, as has 

 been ingeniously remarked (by 

 Winslow), like three pigeon's 

 nests joined in a triangle. 

 These valves are remarkable for 

 their thinness; a circumstance 

 which does not interfere with 

 their solidity. They present : 

 an external, convex border, at- 

 tached to the margin of the 

 orifice and to the walls of the 

 pulmonary artery; a free bor- 

 der, straight when pulled tense, 

 concave when left to itself, and 

 sometimes provided in its middle 

 with a small, though very hard, 

 tubercle, the nodule of Arantius 

 {noduli Arantii) ; a superior, 

 concave face ; and an inferior, convex one. The sigmoid valves are raised 

 and applied to the walls of the vessel whose entrance they garnish, when 

 the ventricle contracts and sends the venous blood into the lung. When 

 this contraction ceases, they fall back one against the other by that part 

 of their inferior face next to their free border, so as to oppose the reflux of 

 the blood into the ventricular cavity.' 



Eight Auricle. — The cavity of the right auricle represents a very 

 concave lid or cover surmounting the auriculo-ventricular opening, and 

 is prolonged, anteriorly, by a curved cul-de-sac. It offers for study this 

 anterior cul-de-sac, a posterior, external, and internal wall, as well as a superior 



' It has been repeated, ad nauseam, that the occlusion of the arterial openings results 

 from the juxtaposition of the free harder of the sigmoid valves; even tlie small tubercle 

 in the middle of this border has been considered to play its part in closing the triangular 

 central space left when these valves meet. In passing the iinger into the pulmonary 

 artery of a living animal, to explore the function of these membranous folds, it is readily 

 perceived that they come in contact by a large poition of their convex face, and not alone 

 by their free border. This arrangement is such, thiit we have with much difficulty tried 

 to produce an insufficiency of contact by keeping one of the valves up against the 

 ■walls of the vessel with tho finger ; but the others came down against the finger and 

 applied themsolves around it so as to exactly close the orifice. 



SECTION or THE HEART AT THE LEVEL OF THE 

 VALVES. 



P, Pulmonary artery ; A, Aorta ; M, Mitral valve ; 

 T, Tricuspid valve. 



