184 DR D. NOEL PATON ON THE 



4Jh. A number of small prominences arising from the septum give chordae to the 

 inferior margin of the septal cusp, and bind it down to the septum. 



Contraction of the superior and anterior of these muscles, through their direction and 

 from the curvature of the septum, will stretch the infundibular cusp against that wall of 

 the ventricle. At the same time the various papillary muscles will pull the different 

 cusps together, while the posterior papillary muscle will pull the posterior cusps against 

 the curved septum. 



5th. Left Ventricle. — The left auriculo- ventricular orifice is situated posteriorly, 

 facing downwards, forwards, and to the left (in the normal position of the heart in Man). 

 In diastole, it is nearly circular (fig. 5). In systole, it is reduced to a transverse slit. 

 In front and to the right of this is the aortic orifice, separated from it only by a mem- 

 branous wall. 



The anatomical position and characters of the two cusps of the mitral valve, and the 

 separation of the papillary muscles into two sets, an anterior or left (figs. 1 and 2), the 

 chordse of which are connected with the left edges of the anterior and posterior cusps, 

 and a posterior or right (figs. 1 and 2), with cords attached to the right edges of the 

 cusps, are so well known as merely to require mention. The chordse connected with the 

 posterior cusp are largely inserted into the auriculo-ventricular ring. 



Action of the Valves. 

 Position of Valves in Ventricular Diastole. 



When an attempt is made to study this question in the usual way by dissection of 

 the heart, the results are rendered fallacious by the fact that rigor-mortis is frequently 

 present, or that the organ is so limp that the relations of the various parts are not 

 preserved. 



It may be best investigated in hearts prepared as described on p. 182. 



In order to demonstrate the position of the parts of the tricuspid valve, sections are 

 best prepared in a vertical transverse plane (fig. 5, A). For the mitral valve, on the 

 other hand, the sections should run in a plane (fig. 5, B), passing through the orifice of 

 the aorta and the left auriculo-ventricular orifice. 



Tricuspid Valve. — The septal cusp is applied to the wall of the septum (fig. 7). 

 The two outer cusps extend downwards and somewhat inwards, so that a flattened 

 funnel-shaped orifice, narrow in front and broad behind, between auricle and ventricle is 

 formed (figs. 3 and 7). 



Mitral Valve. — The posterior cusp lies against the posterior wall of the ventricle. 

 The anterior cusp extends downwards, and somewhat to the left (figs. 8 and 9). 



