ACTION OF THE VALVES OF THE MAMMALIAN HEART. 187 



In the human heart the drawing towards the septum of the external wall of the 

 ventricle is favoured by the muscular trabecule in the lower part of the ventricle. The 

 auriculo-ventricular ring is drawn downwards and inwards by the chordae tendineas 

 passing to it, while the infundibular cusp of the tricuspid from the lines of traction of its 

 chordae tendineae must be flattened against the bulging septum. Its posterior margin is 

 approximated to the anterior edge of the posterior cusp, which with its posterior edge in 

 contact with the posterior margin of the septal cusp is pulled downwards into the 

 posterior angle of the ventricle against the curve of the septum. 



Left Ventricle. 



The Cavity of the Ventricle becomes narrower from side to side, and wider from before 

 backwards. At the same time the posterior cusp of the Mitral Valves is raised from the 

 ventricular wall and pulled forward by the chordae tendineae towards the anterior cusp, 

 which is at the same time pulled backwards so that the two are applied face to face (fig. 11). 



And now, one function of the papillary muscles and anterior cusp of the valve becomes 

 very apparent. By their action on the membranous part of the auriculo-ventricular ring 

 forming the posterior wall of the aorta, from which the cusp takes origin, they help to 

 keep open the aortic orifice, which would tend to be pressed upon and closed by the con- 

 traction of the muscular fibres extending;; round the aortic and mitral orifices. The 

 direction obliquely backwards taken by the membrane between the aorta and the 

 auriculo-ventricular orifice is well seen in figs. 8 and 11. 



Onimus partly appreciated this action of the large segment of the mitral valve 

 {Journal de V Anatomie, t. 2, p. 376). 



2nd — Period of Expulsion. 



It is, of course, impossible to fix the ventricle in this phase. But from a study 

 of the first and third period we can form a clear picture of what occurs during this 

 period. On the right side, the blood collected in front and to the right of the tricuspid 

 valve and accumulated in the conus is shot into the pulmonary artery, the outer wall 

 approaching the septum and the auriculo-ventricular orifice being narrowed. 



On the left side, the posterior-ventricular wall contracts on the posterior cusp of the 

 mitral, forcing the blood round the valve, to be expelled along with the mass of blood 

 accumulated in front of the anterior cusp. 



3rd — Period of Eesidual Contraction. 

 Right Ventricle. 



The auriculo-ventricular opening is reduced to crescentic slit. The cavity of the 

 ventricle is flattened from side to side and obliterated ; except just under the pulmonary 

 artery, where a small cavity, resembling a flattened and inverted cone, is left filled with 



VOL. XXXVII. PART I. (NO. 12). 2 F 



