192 PHYSIOLOGY CHAP. 



- and are correct up to a certain point, but are all more or less 

 . incomplete and inadequate. A complete theory can only be 

 , obtained by weaving the several partial hypotheses together. 



1 Prior to Lower (1679) there was no well-founded theory of the 

 mechanism of the auriculo- ventricular valves. 



He formulated a theory of the passive systolic closure of these 

 valves by regurgitation of the blood, which had a great success. 

 Yieussens, Window, Haller, Senac, Magendie and others adopted 

 much the same opinion. 



Meckel (1825) and Parchappe (1848) brought forward a very 

 different theory, and admitted- an active systolic closure of the 

 auriculo-ventricular orifice by approximation of the folds of the 

 valves, which are kept tense by the papillary muscles connected 

 with them. This theory, too, found its followers, including 

 Berard, Surmay, and See, the last of whom (reasoning from the 

 position of the valves in the hardened human heart) modified it in 

 several particulars, while retaining the main idea of an active 

 . systolic closure. This theory, like the preceding, assumed a 

 certain reflux of blood from ventricle to auricle. The difference is, 

 that while on the former it was the reflux that closed the valves, 

 on the latter it is the movement of active closure of the valve that 

 drives back some at least of the blood contained in the conns 

 valvularis into the auricles. 



Kiirschner (1840) was the first to ascribe an active function in 

 the expulsion of the blood to the cuspid valves. He admitted, 

 even if obscurely, that there must be a presystolic closure of the 

 valves, brought about more particularly by the contraction of the 

 auriculo-valvular fibres, which he described. At the beginning of 

 systole the valves are tense, and the chordae tendineae prevent 

 their reversal into the auricular cavity ; with the progress of 

 systole, however, they are drawn down by the contraction of the 

 papillary muscles, and thus assist in emptying the ventricles in 

 the direction of the arteries. 



Baumgarten (1843) and Weber (1848) adopted these theories ; 

 and, in order to give a more satisfactory explanation of the pre- 

 systolic closure of the auriculo-ventricular orifices, they included 

 in their considerations the low specific gravity of the valves, in 

 consequence of which they open with a light pressure, as well as 

 the elastic tension into which the ventricles are thrown as 

 the effect of the presystolic wave. The same explanation was 

 adopted by Ludwig, Friedreich, Vierordt, and others in Germany. 



In France, Kliss, with the intention of further developing. and 

 completing Kiirschner's theory, proposed a new hypothesis. He 

 assumed that a hollow cone was formed during systole, in conse- 

 quence of the contraction of the papillary muscles, which, as it 

 descends into the ventricular cavity, tends to bring the segments 

 of the valves into close proximity with the walls of the ventricle, 



