12 THE MECHANISM OF THE CIRCULATION. 



found to take place only in the final stage, when the arterial pressure is fall- 

 ing to zero. 



In pathological conditions of hypertrophy and dilatation of the heart, due 

 to a prolonged continuance of obstruction in the pulmonary circulation or in 

 the orifices of the heart, regurgitation takes place, and the great veins may then 

 pulsate equally with the arteries. 



The semilunar valves. It was maintained by Burdach l that the 

 semilunar valves close at the end of systole, in consequence of their own 

 elasticity. This is at once disproved by the fact that these valves do 

 not assume the closed position when the aorta and ventricle are filled 

 with fluid at the normal aortic pressure. By Weber 2 the closure was 

 attributed simply to the agency of the aortic pressure at the moment 

 when the ventricle passes into diastole. Since by its systole the ventricle 

 never completely empties itself, there would be, if Weber's hypothesis 

 were true, a column of blood extending through the arterial orifice at 

 the termination of systole, and considerable regurgitation would thus 

 take place before the closure was effected. 



Now the curves of intra ventricular pressure do not give us the 

 slightest evidence of regurgitation. Some other method of closure must 

 therefore exist. Light is thrown upon this problem by the following 

 experiment. 3 A long glass syringe is filled with water containing a fine 

 powder such as lycopodium spores. The syringe is held in the vertical 

 position, and the fluid column is set in motion by a slight upward push 

 given to the piston. The velocity of movement in the axis of the 

 column is double as great as the velocity of the whole column. Close 

 to the wall of the syringe, owing to viscosity, the movement of the 

 fluid is so slow that the lycopodium spores are seen to be here overtaken 

 by the piston. When this happens, an eddy drags the spores back into 

 the axis, they thence mount rapidly to the top of the column, and there 

 by an eddy once more reach the wall of the syringe. At the moment 

 when the piston ceases to move, the column of fluid is divided into an 

 inner core which runs upward with the axial velocity, and an outer 

 mantle, in which the movement is in the opposite direction. It is this 

 centripetal eddy which produces the closure of the valves in the aorta 

 and pulmonary artery. The sinuses of Valsalva are dilatations in which 

 eddies are bound to be produced, and there can be no doubt that these 

 eddies cause the valves to approximate more and more, and finally close 

 them at the instant when the ventricular pressure ceases to be higher 

 than that in the aorta. 



It has been suggested that the closure of the valves is likewise aided by 

 the contraction of the longitudinal bands of muscle fibres which spring from 

 the margins of the aortic or pulmonary orifices. These muscular bands are 

 assumed to contract the orifices into cleft-like openings, bringing the 

 segments of the valves almost into apposition at the end of systole. 4 The 

 eddies in the sinuses of Valsalva complete the work of these muscular bands. 



Moens 5 explained the closure of the semilunar valves in a different manner. 



1 Ber. d. anat. Anstalt zu Konigsbcry, Leipzig, 1820, Bd. iii. S. 25. 



2 Letter to Hamernjk, Vrtljschr. f. d. prakt. ffcillc., Frag, 1848, Bd. xx. S. 106. 



3 Ceradini, "Die Mechanismus d'er halbmondformigen Klappen," Leipzig, 1872, S. 46 ; 

 Centralbl.f. d. med. Wisscnsch., Berlin, 1872, S. 135. 



4 Krehl, Abhandl. d. math.-phys. Cl. d. k. scichs. Gesellsch. d. Wissensch., 1891, 

 Bd. xvii. S. 348. 



5 Arch.f. d. gcs. Physiol., Bonn, 1879. Bd. xx. S. 524. 



