234 VALVES OF THE HEAET. 



very great distress is experienced; and there may be even 

 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 venae cavse, e, e' } 

 and transmits it into the ventricle, b, by the orifice, c. On 

 either side of this orifice are seen the membranous folds, 

 which are kept in their places by the tendinous cords, d. 

 a Now when the blood is passing from 



a to 5, these folds yield to the current ; 

 L. / hut when the cavity b is filled and begins 



to contract, the blood presses against 

 ff their under sides, so as to make them 



close against each other, as far as they 

 b are permitted to do by the tendinous 



cords. In this manner the aperture is 



completely shut, and no blood can flow 

 Fig. 127. SECTION OP ONE back. A valve of this kind exists on 

 HEART> each 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 right side 

 has three pointed divisions, to which the tendinous cords 

 are attached, and it is hence called the tricuspid valve; 



