480 SEMILUNAR VALVES. 



columns. The anterior valve or curtain is the largest, and is so placed as 

 to prevent the filling of the pulmonary artery during the distension of the 

 ventricle. The rig/it valve or curtain is of smaller size, and is situated 

 on the right side of the auriculo-ventricular opening. The third valve, or 

 "fixed curtain" is connected by its cords to the septum ventriculorum. 

 The cords (chordse tendineae) of the anterior curtain are attached, princi- 

 pally, to a long column (columna carnea), which is connected with the 

 "right or thin and yielding wall of the ventricle." From the lower part 

 of this column a transverse muscular band, the "long moderator band" is 

 stretched to the septum ventriculorum, or "solid wall" of the ventricle. 

 The right curtain is connected, by means of its cords, partly with the long 

 column, and partly with its own proper column, the second column, which 

 is also attached to the "yielding wall" of the ventricle. A third and 

 smaller column is generally connected with the right curtain. The "fixed 

 curtain" is so named from its attachment to the "solid wall" of the ven- 

 tricle, by means of cords only, without fleshy columns. 



From this arrangement of the valves it follows, that if the right ven- 

 tricle be over distended, the thin or " yielding wall" w r ill give way, and 

 carry with it the columns of the anterior and right valves. The cords 

 connected with these columns will draw down the edges of the correspond- 

 ing valves, and produce an opening between the curtains, through which 

 the superabundant blood may escape into the auricle, and the ventricle be 

 relieved from over-pressure. This mechanism is therefore adapted to 

 fulfil the "function of a safety valve." 



The Columna earners (fleshy columns) is a name expressive of the 

 appearance of the internal walls of the ventricles, which seem formed of 

 muscular columns interlacing in almost every direction. They are divided, 

 according to the manner of their connexion, into three sets. 1. The 

 greater number are attached by the whole of one side, and merely form 

 convexities into the cavity of the ventricle. 2. Others are connected by 

 both extremities, being free in the middle. 3.- A few (columns papillares) 

 are attached by one extremity to the walls of the heart, and by the other 

 give insertion to the chordae tendineaB. 



The Semilunar valves, three in number, are situated around the com- 

 mencement of the pulmonary artery, being formed by a folding of its 

 lining membrane, strengthened by a thin layer of fibrous tissue. They 

 are attached by their convex borders, and free by the concave, which are 

 directed upwards in the course of the vessel, so that, during the current 

 of the blood along the artery, they are pressed against the sides of the 

 cylinder ; but if any attempt at regurgitation ensue, they are immediately 

 expanded, and effectually close the entrance of the tube. The margins 

 of the valves are thicker than the rest of their extent, and each valve pre- 

 sents in the centre of this margin a small nbro-cartilaginous tubercle, 

 called corpus Arantii* which locks in with the other two during the 

 closure of the valves, and secures the triangular space that would other- 

 wise be left by the approximation of three semilunar folds. 



Between the semilunar valves and the cylinder of the artery are three 

 pouches, called the pulmonary sinuses (sinuses of Valsalva). Similar 



* Julius Csesar Arantius, Professor of Medicine in Bologna. He was a disciple of 

 Vesalius, one of the founders of modern anatomy. His treatise "De Humano Fcetu" 

 was published a* Rome, in 1564. 



