of Edinburgh, Session 1863-64. 
195 
veins, which tends to urge the segments together, while it assists 
the onward flow of the blood. In the arteries, as, e. g., at the origin 
of the pulmonary artery and aorta, the segments of the valve are 
of a semilunar shape, and always three in number. They consist 
of a doubling of the lining membrane, and contain within their 
fold certain tendinous bands which run in well-marked directions ; 
the structure closely resembling that met with in the auriculo- 
ventricular or highest form of valve.* The cusps, moreover, are 
semi-opaque, stronger, and more ample than in the veins. 
In the arteries the dilatations behind the segments, commonly 
called the Sinuses of Valsalva, are very large, and, as the author has 
ascertained, curve towards each other in such a manner as ensures 
that the blood will act upon the segments only in certain directions : 
in fact, that they will be urged by it towards each other, and fixed by 
a distinctly spiral wedging movement ; the margins of the segments 
flattening themselves against each other whilst in action, and form- 
ing an inverted tripartite dome, the strength of which is limited 
only by the strength of the materials used in its construction. 
The action of the arterial semilunar valves may also be considered 
mechanical, the inconvenience which might be supposed to result 
from an excess of vital contractility in the arteries and ventricles, 
between which the valves are situated, being counteracted by the 
existence at this point of comparatively unyielding fibro-cartila- 
ginous rings. Strongly contrasting with the arterial semilunar 
valves in mammals whose area of activity is, so to speak, circum- 
scribed, are the semilunar valves situated within the bulbus arteri- 
osus of fishes, which is an actively contracting structure. In these 
cases the segments, as a rule, are more numerous than in the veins 
and arteries, and arranged in tiers, so that the blood which is not 
caught by one tier falls into and is supported by the next. The 
number of segments, moreover, obviates the evil effects which might 
accrue from the displacement to which they are subjected by the 
contraction of the bulb. 
In the semilunar valves of fishes we find the first trace of what 
may be regarded as chords tendine99, in the shape of tendinous 
The arrangement of tJie tendinous bands in the semilunar and auriculo- 
ventricular valves, and the structure generally, is described at length. 
