ANATOMY OF THE HEART AND BLOOD - VESSELS. 217 



chambers of the heart can be better seen. Opening into the 

 right auricle are the superior and inferior venae cavae (cs and 

 ci) and proceeding from the right ventricle tine pulmonary 

 artery, P. Opening into the left auricle are the right and 

 left pulmonary veins (pel and ps) and springing from the left 

 ventricle the aorta, A. 



The Interior of the Heart. The communication of each 

 auricle with jits ventricle is also represented in the diagram 

 Fig. 89, and the valves which are present at those points 

 and at the origin of the pulmonary artery and that of the 

 aorta. Interjnally the auricles are for the most part smooth^ 

 but from eacji a hollow pouch, the auricular appendage, pro- 

 jects over th^ base of the corresponding ventricle as seen at 

 Adx and As ,ln Figs. 90 and 91. These pouches have some- 

 what 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 col- 

 umns (columned car nee). On the inside of the ventricles (Fig. 

 92) similar fleshy columns are very prominent. 



The Auriculo-Ventricular Valves. These are known as 

 right and left, or as the tricuspid and mitral valves respec- 

 tively. The mitral valve (Fig. 92) consists of two flaps of the 

 endocardium fixed by their bases to the margins of the auric- 

 ulo-ventricular aperture and with their edges hanging down 

 into the ventricle when the heart is empty. These unattached 

 edges are not however free, but have fixed to them a number 

 of stout connective-tissue cords, the cor dee 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 riae into a horizontal 

 position and so close the opening between auricle and ven- 

 tricle which lies between them, and passes up behind the 

 opened aorta, Sp, represented in the figure. The tricuspid 

 , 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. 92, 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 pre- 

 vent blood from flowing back into the ventricle. In the 

 middle of the free border of each valve is a little cartilagi- 



