CIRCULATION.] PHYSIOLOGY, 73 



pulmonary veins, and if you slit them right up you 

 will find they open (by four openings) into a cavity on 

 the left side of the heart, almost exactly like that 

 cavity on the right side which we called the right 

 auricle (Fig. ii). This cavity is, in fact, the left 

 auricle ; out of it there is an opening into the left 

 ventricle, very like the opening from the right auricle 

 into the right ventricle. It too is guarded by flap 

 valves, exactly like the tricuspid valve, only there are 

 but two flaps instead of three (Fig. 9, m.v. i, m,v. 2). 

 Hence this valve is called the bicuspid, or more 

 frequently the mitral valve. Its flaps have little 

 threads by which they are fastened to the walls of the 

 ventricle, and in fact, except for there being two flaps 

 instead of three, the mitral valve is exactly like the 

 tricuspid valve, and acts exactly the same way. 



If you cut with a pair of scissors from the auricle 

 into the ventricle, you will find the left ventricle (Fig. 

 1 1 ) very much like the right ventricle, only its walls are 

 very much thicker, so much thicker that the left ven- 

 tricle takes up the greater part of the heart. You will 

 see this if you now look at the outside of a fresh heart. 



The auricles are so small and so covered up by fat 

 that from the outside you can hardl)' see them at all. 

 What you chiefly see are two little fleshy corners, one 

 of each auricle (Fig. 5, R.A. L.A.\ often called **the 

 auricular appendages." By far the greater part is 

 taken up by the ventricles — and if you look you will 

 see a band of fat slanting across the heart (Fig. 5, 3). 

 This marks the line of the fleshy division, or septum 

 as it is called, between the two ventricles. You will 

 notice that the point or apex of the heart belongs 

 altogether to the left ventricle. 



