322 CIRCULATION OF NUTRITIVE FLUID. 



system ; which is small compared with that which the fluid possesses in 

 the arteries. 



571. The natural movements of the Heart are accompanied by cer- 

 tain sounds, which are heard when the ear is applied over the cardiac 

 region ; and an acquaintance with these sounds and with their causes 

 is of much importance, since the alterations which they undergo in dis- 

 ease, afford us some of our most accurate information in regard to the 

 nature of the morbid affection. Concurrently with the impulse of the 

 heart against the chest, a dull and prolonged sound is heard ; this, 

 which is termed the first sound, marks the ventricular systole, and is 

 synchronous with the pulsation in the arteries. The second sound, 

 which is short and sharp, follows immediately upon the conclusion of 

 the first ; and it must therefore be produced during the first stage of 

 the Ventricular diastole, before the systole of the Auricles has com- 

 menced. It is followed by a brief interval of repose, which occurs 

 during the remainder of the Ventricular diastole and the Auricular 

 systole ; and this is succeeded by a recurrence of the first sound. If 

 the whole period between two successive pulsations be divided into four 

 parts, it is estimated that the first sound usually occupies two of these ; 

 and the second sound, and the interval, one part each. 



572. Now in order to understand the causes of these sounds, it is 

 necessary to study the course of the blood through the heart a little 

 more in detail. When the Ventricles, distended with blood, are con- 

 tracting upon their contents, they eject them forcibly through the narrow 

 orifices of the aorta and pulmonary artery ; and the semilunar valves, 

 which guard these orifices, are thrown back against the walls of the 

 arteries. The regurgitation of the blood into the auricles is prevented 

 by the action of the mitral and tricuspid valves ; but the flaps of these 

 do riot suddenly fall against each other, when the blood first begins to 

 press them together ; being restrained by the chordce tendinece. The 

 connexion of these with the carnece columnce, which form part of the 

 ventricular walls, and contract simultaneously with them, appears to 

 have this use, that the flaps of the valves, which are completely thrown 

 back during the preceding rush of blood from the auricles to the ven- 

 tricles, may be drawn into a favourable position for the blood to get 

 behind them and bring them together, so as completely to close the 

 orifice. As soon as the ventricular diastole begins to take place (even 

 before the contraction of the auricles has commenced), there will be a 

 tendency of the blood, that has just been propelled into the aorta and 

 pulmonary artery, to flow back to the heart ; but this regurgitation is 

 completely prevented by the semilunar valves of these orifices, which 

 are immediately filled-out by the backward tendency of the blood, and 

 which meet in such a manner as completely to close the orifices. This 

 closure is much more sudden than that of the mitral and tricuspid valves, 

 being altogether unrestrained. 



573. The first sound is certainly in part due to the impulse of the 

 heart against the thoracic parietes ; as is proved by the fact, that when 

 the impulse is prevented, the sound is much diminished in intensity ; 

 also by the circumstance, that, when the ventricles contract with vigour, 

 the greatest intensity of the sound is over the point of percussion. But 



