328 THE HUMAN BODY 



artery, the aorta, Aa. It forms an arch over the base of the heart 

 and then runs down behind it at the back of the chest. From the 

 convexity of the arch of the aorta several great branches are 

 given off, Ssi, Cs, Ab; but before that, close to the heart, the aorta 

 gives off two coronary arteries, branches of which are seen at crd 

 and crs lying in the groove over the partition between the ventri- 

 cles, and which carry to the substance of the organ that blood 

 which comes back through the coronary sinus. Into the left au- 

 ricle open two right and two left pulmonary veins, ps and pd, 

 which are formed by the union of smaller veins proceeding from 

 the lungs. 



In the diagram Fig. 101 from which the branches of the great 

 vessels near the heart have been omitted for the sake of clearness, 

 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 venae cavae (cs and ci) 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. 101, 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 As in Figs. 102 and 103. 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 carnce). On the inside of the ventricles (Fig. 

 104) 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. 104) 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 



