294 THE HUMAN BODY 



the connection of the various vessels with the chambers of the 

 heart can be better seen. Opening into the right auricle are the 

 superior and inferior vense cavse (cs and ct) and proceeding from 

 the right ventricle the pulmonary artery, P. Opening into the 

 left auricle are the right and left pulmonary veins (pd and ps) and 

 springing from the left ventricle the aorta, A . 



The Interior of the Heart. The communication of each auricle 

 with its ventricle is also represented in the diagram Fig. 102, and 

 the valves which are present at those points and at the origin of 

 the pulmonary artery and that of the aorta. Internally the auricles 

 are for the most part smooth, but from each a hollow pouch, the 

 auricular appendage, projects over the base of the corresponding 

 ventricle as seen at Adx and ^.s in Figs. 103 and 104. These 

 pouches have somewhat the shape of a dog's ear and have given 

 their name to the whole auricle. Their interior is roughened by 

 muscular elevations, covered by endocardium, known as the fleshy 

 columns (columnce carnaz). On the inside of the ventricles (Fig. 

 105) similar fleshy columns are very prominent. 



The Auriculoventricular Valves. These are known as right 

 and left, or as the tricuspid and mitral valves respectively. The 

 mitral valve (Fig. 105) consists of two flaps of the endocardium 

 fixed by their bases to the margins of the auriculoventricular 

 aperture and with their edges hanging down into the ventricle 

 when the heart is empty. These unattached edges are not, how- 

 ever, free, but have fixed to them a number of stout connective- 

 tissue cords, the cordce tendinece, which are fixed below to muscular 

 elevations, the papillary muscles, Mpm and Mpl, on the interior 

 of the ventricle. The cords are long enough to let the valve flaps 

 rise into a horizontal position and so close the opening between 

 auricle and ventricle which lies between them, and passes up be- 

 hind the opened aorta, Sp, represented in the figure. The tricus- 

 pid valve is like the mitral, but with three flaps instead of two. 



Semilunar Valves. These are six in number: three at the mouth 

 of the aorta, Fig. 105, and three, quite like them, at the mouth 

 of the pulmonary artery. Each is a strong crescentic pouch fixed 

 by its more curved border, and with its free edge turned away 

 from the heart. When the valves are in action these free edges 

 meet across the vessel and prevent blood from flowing back into 

 the ventricle. In the middle of the free border of each valve is a 



