234 VALVES OF THE HEART. 
very great distress is experienced; and there may be ever 
danger of rupture of the heart or ‘large vessels, or of sudden 
cessation of the heart’s action, causing instant death. Such 
persons ought, therefore, carefully to refrain from any violent 
muscular movement, and also to avoid giving way to strong 
mental emotions.—In syncope or fainting, the heart's action is 
so weakened as to be scarcely perceptible, though it does not 
entirely cease ; and this state may be brought on by several 
causes which make a strong impression on the nervous system, 
such as violent mental emotion (whether joy, or grief, or terror), 
sudden loss of blood, and the like. 
272. The blood which has been received by each ventricle 
from its auricle, is prevented from being driven back into the 
latter, on the contraction of the former, by a valve that guards 
the aperture through which it entered. This valve consists of 
a membranous fold, surrounding the borders of the aperture, 
and so connected with the neighbouring parts, as to yield when 
the blood passes from the auricle into the ventricle, but to 
be tightened so as completely to close the aperture when 
the blood presses in the contrary direction. The manner 
in which these valves act will be seen from fig. 127, which 
is a section of the right auricle with its ventricle. The 
auricle, a, receives its blood from the two venxe cave, ¢, €; 
and transmits it into the ventricle, 6, by the orifice, ¢ On 
either side of this orifice are seen the membranous folds, 
which are kept in their places by the tendinous cords, d. 
Now when the blood is passing from 
a to b, these folds yield to the current ; 
but when the cavity 0 is filled and begins 
to contract, the blood presses against 
their under sides, so as to make them 
close against each other, as far as they 
are permitted to do by the tendinous 
cords. In this manner the aperture is 
completely shut, and no blood can flow 
Fig. 127.—Sxctrow ory one back. A ,valve of this kind exists on — 
Sipe ov tHE HEART. cach side of the heart; but there is 
a slight difference between the forms of the two, whence 
they have received different names. That on the vight side 
has three pointed divisions, to which the tendinous cords 
are attached, and it is hence called the tricuspid valve ; 
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