ACTION OF THE VALVES OF THE MAMMALIAN HEART. 183 



lary muscles contract, they will therefore tend to pull the rings downwards and inwards 

 and thus to diminish the auriculo-ventricular opening and the ventricular cavity, and to 

 assist in the expulsion of the blood. 



The left ventricle forms a central cylindrical core to the heart ; and to one side of 

 this the right ventricle is attached along the anterior and posterior sulci (figs. 1, 2, 3). 



The right ventricle is formed of an outer and an inner wall — the latter, the so-called 

 septum, being really part of the wall of the left ventricle, and bulging into the cavity 

 of the right ventricle as a surface that is convex, not only from before backwards, but 

 also from above downwards (figs. 1, 2, 3, 7, 10). 



It is triangular in shape. At the anterior and upper angle is the orifice of the pul- 

 monary artery. The inferior angle is at the apex. The anterior and posterior sides of 

 the triangle correspond to the anterior and posterior sulci. The superior side is com- 

 posed in front of the pulmonary orifice ; behind this, of the upper and right wall of 

 the conus arteriosus ; and still further back, at the posterior and upper angle is the 

 auriculo-ventricular orifice (fig. 3). 



This opening, in the normal position of the heart of Man, faces to the left forwards 

 and downwards. It is surrounded by muscular fibres. In diastole it is elliptical, but in 

 the fully contracted condition it is reduced to little more than a slit. 



The distribution of the papillary muscles varies in different animals ; but in all, an 

 anterior group of small muscular processes is present, situated just behind and below the 

 pulmonary orifice, having a somewhat horizontal direction, and connected by chordae 

 tendineae with the anterior borders of the infundibular and septal cusps of the tricuspid 

 valve (figs. 3, 6). The arrangement of the anterior and posterior sets of papillary 

 muscles varies considerably. In the Eabbit (fig. 4), where there is only a small amount 

 of trabecular tissue towards the apex of the ventricle, these muscles arise from the 

 septum. In Man (fig. 6), where the lower or apical third of the ventricle is composed of 

 a network of muscular trabeculae, the papillary muscles take origin most usually from 

 fibres of that network running between the septum and the outer wall — sometimes in 

 close relationship to the septum, sometimes more intimately connected with the external 

 wall. In the latter case the governor band is well developed. 



They may be described as four in number — if we include the anterior muscle situated 

 under the pulmonary artery (fig. 6). 



1st. Superior, situated just under the pulmonary orifice, directed backwards and to 

 the right, and sending chordae to the anterior margins of the septal and infundibular 

 cusps of the tricuspid. These chordae have a transverse direction. 



2nd. Anterior takes origin from the trabecular tissue at the apex towards its anterior 

 part, and is directed upwards. It gives off chordae to the posterior margin of the 

 infundibular cusp, and to the anterior margin of the posterior cusp. 



3rd. Posterior takes origin from the trabecular tissue near the apex posteriorly, 

 is directed upwards, and gives off chordae to the posterior margins of the posterior and 

 septal cusps of the tricuspid. 



