CIRCULATION. 403 



mitral, Dot only in the complete distinctness of their segments, but also in 

 their mechanism. The tendinous cords are wholly lacking, and each segment 

 depends upon its direct connection with the arterial wall to prevent reversal 

 into the ventricle during the diastole of the latter. If the artery be carefully 

 laid open by cutting exactly between two of the segments, each of the three is 

 seen to have the form of a pocket with its opening turned away from the heart 

 (see Fig. 103). Behind each segment, the artery is dilated into one of the hol- 

 lows or " sinuses " of Valsalva. 1 As the valve lies immediately above the 

 base of the ventricle the segments rest upon the top of the thick muscular 

 wall of the latter, which affords them a powerful support (see Fig. 104). 

 Each segment is attached by the whole length of its longer edge to the artery, 

 while the free margin is formed by the shorter edge. It is this arrangement 

 which renders reversal of a segment impossible (see Fig. 103). 



FIG. 104. Diagram to illustrate the mechanism of FIG. 105. Diagram to illustrate the mechanism of 

 the semilunar valve. the semilunar valve and corpora Arantii. 



While the blood is streaming from the ventricle into the artery, the three 

 segments are pressed away by the stream from the centre of the vessel, but 

 never nearly so far as to touch its wall. At all times, therefore, a pouch ex- 

 ists behind each segment, which pouch freely communicates with the general 

 cavity of the artery. As the ventricular systole nears its end, the ventricular 

 cavity doubtless becomes narrowed just below the root of the artery, and with 

 it the arterial aperture itself, while currents enter the sinuses of Valsalva. 

 Thus for a double reason the three segments of the valve are approximated, 

 and probably the last blood pressed out of the ventricle issues through a nar- 

 row chink between them. The instant that the pressure in the ventricle falls 

 below the arterial pressure, the three segments must be brought together by 

 the superior pressure within the artery, and tightly closed by its forcible recoil, 

 without regurgitation having occurred in the process (see Figs. 104, 105). 2 



Lunulse and their Uses. Each segment of a semilunar valve, when 

 closed, is in firm contact with its fellows not only at its free margin but also 

 over a considerable surface, marked in the anatomy of the segment by the 

 two "lunulaB" or little crescents, each of which occupies the surface of the 

 segment from one of its ends to the middle of its free margin, the shorter edge 



1 Named from the Italian physician and anatomist Valsalva of Bologna, born in 1666. 



2 L. Krehl: "Beitrage zur Kenntniss der Fiillung und Entleerung des Herzens," Abhand- 

 lungen der math.-physischen Ciasse der k. sdchsischen Gesellschaft der Wissenschaften, 1891, Bd. xvii. 

 No. 5, p. 360. 



