ACTION OF THE VALVES OF THE MAMMALIAN HEART. 185 



Position of Valves in Ventricular Systole. 



In studying the position of the valves during this period, its division into three 

 phases, more or less distinctly marked, must be remembered. 



1st. Latent Period, — before the opening of the aortic and pulmonary valves, during 

 which the pressure in the ventricle is being got up. 



2nd. Expulsion Period —-during which the great mass of blood is being expelled 

 through the arterial orifices. 



3rd. Period of Residual Contraction, — during which the ventricles remain con- 

 tracted, and may expel any blood not driven out during the last period. According to 

 HtJRTHLE this period is simply the terminal stage of the second period. 



1st — Latent Period. 



Right Ventricle. 



The heart having been fixed as described on p. 181, the following condition of the 

 various parts was observed : — 



a. Tricuspid Valve. — On making sections, the septal cusp is found more or less 

 closely applied to the septum, while the two external cusps are pulled towards the septum 

 (fig. 10), and the infundibular cusp is pressed closely against it from the action of the 

 superior and anterior sets of papillary muscles. At the same time the outer part of the 

 auriculo-ventricular ring is pulled downwards and inwards, by the combined action of 

 the papillary muscles and chordas inserted into the ring and the muscular fibres 

 surrounding the orifice. 



b. Ventricular Cavity. — The external wall of the ventricle is pulled nearer to the 

 septum ; and the anterior wall, under and in the region of the conus, bulges forward. 

 This is due to the greater thickness and power of the muscular fibres at the apex and 

 right side of the ventricle, and to the comparative thinness of the wall in the region of 

 the conus. This I believe to be a matter of some importance in explaining the increase 

 in the antero-posterior diameter of the heart, and the diminution in the transverse 

 diameter described by various investigators. 



The change in the antero-posterior and transverse diameters of the heart may be shown 

 not only by tracings taken from the living organ, but can also be demonstrated by fixing 

 the heart in the various phases of the cardiac cycle. 



This has already been done by Ludwig and Hesse (Arch, f Anat. u. Phys., Bd. 18), 

 but their observations refer merely to the state of diastole and to the third stage of systole 

 when the ventricles have already expelled their contents. 



By adopting the method already described I have been able to fix the heart in the 

 first stage of ventricular systole, and by careful measurement to show that a distinct 

 increase in the antero-posterior and a diminution in the transverse diameter of the organ 

 occurs. This is clearly shown in the accompanying tables and set of figures. 



