DISSECTION OF THE BACK AND THORAX. 129 



furrow round the anterior border of the heart, the incision should be 

 continued up in front of the left ventricular furrow, as far as the root 

 of the pulmonary artery. This will enable nearly the entire wall of 

 the right ventricle to be raised as a triangular flap, and will give a 

 good view of the cavity when looked into from below. 



The Cavity of the Eight Ventricle. This cavity is widest above, 

 and tapers to its lowest point ; and its shape is such that its transverse 

 section gives a crescentio outline, the wall of the ventricle being concave 

 towards the cavity, while the septum is convex in the same direction. 

 The inner surface of its wall is rendered irregular by muscular bands 

 and prominences — the columnm carnece, of which there are several 

 varieties: 1. Some of them have the form of bars or ridges sculptured 

 on the wall of the heart, to which they give a, sponge-like appearance. 

 2. Others, the trabeculce carnecv, are veritable bands or strings between 

 which and the wall of the ventricle the handle of a scalpel may be 

 passed. Of this variety two or tliree very tendinous strings, sometimes 

 more or less reticulate, stretch between the wall and the middle of the 

 septum ; and, since they are believed to prevent over-distension of the 

 ventricle, they have been named inoderator bands. Other strings occur 

 in the angle of junction of wall and septum, and still others stretch 

 between different parts of the wall. 3. A third variety are blunt, 

 nipple-shaped prominences called muscidi 'papillares, of whicli tliere are 

 commonly three in this cavity, one being placed on the wall and two on 

 the septum. Kadiating from each of these is a set of fibrous strings — 

 the chordce tendinece, which are attaclied by their other ends to the seg- 

 ments of the valve guarding the auriculo-ventricular opening. The 

 right auriculo-ventricular opening is situated at the base of the cavity, 

 and is a very large orifice. It is provided with a valve composed of 

 three main cusps or segments, and hence named tricuspid. Each of 

 these cusps is triangular in shape, being fixed by its base to the wall of 

 the heart, and having its edges free and directed towards those of the 

 adjacent cusps. When the blood stream is rushing through the opening 

 the segments of the valve hang down into the ventricle, and have one 

 surface directed towards the blood stream and the other to the wall of 

 the ventricle. The first of these surfaces is smooth ; the other is rough, 

 and to it and the apex and edges of the cusp the chordfe tendinea; are 

 attached. When, during contraction of the ventricle, the blood tends 

 to regurgitate through the opening, the cusps are forced inwards and 

 upwards, and, meeting each other, close the orifice. To the efficiency 

 of this action the chordse tendiuese passing from the musouli papillares 

 are essential; for, being attached to the edges and lower surfaces of 

 the cusps, they prevent the latter from being carried right up into the 

 auricle. The three principal cusps generally alternate around the 

 opening with three of much smaller size. There are three musouli 



