188 DR D. NOEL PATON ON THE 



blood. The septal cusp of the valve is applied flat against the septum ; the infundibular 

 and posterior lie flat against it, — only a small wedge-shaped mass of blood continuous 

 with the auricular contents lying between the valves at their upper part (fig. 12). 



Occasionally extremely instructive casts of the inter-valvular space may be seen in 

 post-mortem examination of the human heart in which a blood clot has formed. This clot 

 shows a thin flattened anterior part where the anterior cusp has been pulled against the 

 septum, and a thicker, more conical posterior portion. Such a cast is figured by 

 Pettigkew (Proc. of the Royal Soc, vol. xxiii. part iii., 1864). 



Left Ventricle. 



The auriculo- ventricular orifice is reduced to a transverse slit. The cavity of the 

 ventricle is entirely obliterated, except for a cylindrical part filled with blood im- 

 mediately under the aortic orifice. The posterior cusp of the mitral is applied against 

 the posterior wall of the ventricle ; and the anterior lies in front of it, and applied to it 

 throughout the lower part of its extent. A wedge of blood from the auricles extends 

 down between the upper part of the valves. No strain is put on the membranes, which 

 mutually support one another (fig. 13). 



Such a series of observations seem to demonstrate beyond a doubt that the mechanism 

 of the auriculo-ventricular valves is very different from that so universally described. 



Instead of the cusps of the valves being floated into a horizontal position to form a 

 septum between auricles and ventricles, they are simply applied face to face, and thus 

 prevent all regurgitation without being subjected to any strain. At the same time, 

 their depressed position gives the ventricles a core upon which they can contract to 

 completely empty themselves into the arteries. 



It may be objected to this view that it does not account for the valves being closed 

 before the ventricular systole begins, so as to prevent regurgitation at the commencement 

 of the systole. But such a closure, before ventricular systole, is not necessary, for it has 

 been shown that the auricles do not relax until after the commencement of ventricular 

 systole, and of course, until these chambers pass into diastole, no reflux flow is possible. 

 Hence a closure at the commencement of the systole is all that is required. 



It may perhaps also be urged that, although in these preparations the valves are 

 found closed as above described, they may have become occluded in the manner usually 

 described, and subsequently pulled downwards, as described by Kuhschner (Wagner's 

 Handwbrterbuch, Bd. ii. S. 60, 1844), Lcjdwig ' (Lehrbuch du Physiologic, Bd. ii. S. 61, 

 1856), and Pettigeew (Trans, of the Royal Soc. of Edin., vol. 23, part iii., 1864). A 

 moment's consideration will show that this is impossible. For, once closed in the hori- 

 zontal position, it would be impossible to have them pulled downwards until blood had 

 left the ventricles, since the fluid blood is incompressible. 



Again, the recent researches of Roy and Adami (Practitioner, 1890) on the action of 

 the papillary muscles, independently of and later than the general heart muscle, might 



