CHAP, iv.] THE VASCULAR MECHANISM. 143 



The semilunar valves are, during the ventricular systole, pressed 

 outwards towards but not close to the arterial walls, reflex currents 

 probably keeping them in an intermediate position, their orifice 

 forming an equilateral triangle with curved sides ; they thus offer 

 little obstacle to the escape of blood from the cavities of the 

 ventricles. The ventricle propels the blood with great force and 

 rapidity into the aorta and the whole contents are speedily ejected. 

 Now, when in a closed channel a rapid current suddenly ceases, 

 a negative pressure makes its appearance in the rear of the fluid, 

 and sets up a reflux current. So when the last portions of blood 

 leave the ventricle a negative pressure makes its appearance behind 

 them in the ventricle, and leads to a reflux current from the aorta 

 towards the ventricle. This alone would tend to bring the 

 valves together; but in all probability it is not till a short 

 (variable) time afterwards, that upon the commencing diastolic 

 relaxation of the ventricle, the elastic rebound of the arterial walls 

 completely fills and renders tense the pockets, causing their free 

 margins to come into close and firm contact, and thus entirely 

 blocking the way. The corpora Arantii meet in the centre, and the 

 thin membranous festoons or lunulse are brought into exact apposi- 

 tion. As in the tricuspid valves, so here, while the pressure of the 

 blood is borne by the tougher bodies of the several valves, each two 

 thin adjacent lunulsfi, pressed together by the blood acting on both 

 sides of them, are kept in complete contact, without any strain 

 being put upon them; in this way the orifice is closed in a 

 most efficient manner. 



The ingenious view put forward by Briicke that during the ven- 

 tricular systole, the flaps are pressed back flat against the arterial walls, 

 and in the case of the aorta completely cover up the orifices of the 

 coronary arteries, so that the flow of blood from the aorta into the 

 coronary arteries can take place only during the ventricular diastole 

 or at the very beginning of the systole, and not at all during the systole 

 itself, has been disproved. 



The Sounds of the Heart. 



When the ear is applied to the chest, either directly or by 

 means of a stethoscope, two sounds are heard, the first a com- 

 paratively long dull booming sound, the second a short sharp 

 budden one. Between the first and second sounds, the interval 

 of time is very short, too short to be measurable, but between the 

 second and the succeeding first sound there is a distinct pause. 

 The sounds have been likened to the pronunciation of the syllables, 

 lubb, dup, so that the cardiac cycle, as far as the sounds are 

 concerned, might be represented by : lubb, dup, pause. 



