Practieum V. The Left Auricle. S3 



and the pericardial line cut a window in it at least as large as the triangle 

 indicated on PI. XII. 



a. Admitting the light, look toward the ventricle and note that the 

 mouth of the artery is surrounded by three membranous SEMILUNAR (or 

 sigmoid) VALVES. 



b. Pass the scissors point from the ventricle into the artery and cut 

 toward the middle of the window already made. On divaricating the 

 sides of the artery it will be found that (i) one of the valves has been 

 medisected ; (2) each constitutes a sort of pocket ; and (3) opposite each 

 there is in the wall a depression, a SINUS OF VALSALVA. 



c. The semilunar valves permit the blood to pass freely from the ventricle into the 

 artery, but a reflux is prevented by some of the, blood getting behind the valves and clos- 

 ing them ; without the sinuses the valves might be pressed so closely against the walls as 

 to prevent their closing. 



14. Opening the Left Auricle. Note the two portions, the atrium 

 dorsad, crossed by the azygous vein ; the appendix ventrad. Cut a win- 

 dow in the appendix so as to expose its entire cavity. 



a. Note the general resemblance to the right appendix, but the greater 

 prominence of the trabeculae. 



b. This prominence of the trabeculae and the concomitant depth of the interspaces 

 gives to the wall a spongy character which is even more marked in all parts of the heart 

 in the human fetus, and is permanent with some lower animals, e. g., the turtle. 



c. Between the appendix and the atrium, toward the base of the heart, 

 is a prominent RIDGE, coinciding approximately with the azygous vein. 



15. The Pulmonary Veins. Hold the heart so as to see the depth 

 of the atrium and note that it presents a RIDGE at right angles with the 

 one just mentioned. 



a. At -either side of this ridge is a cavity into which open the PUL- 

 MONARY VEINS of that side ; PL XIV. 



b. From the appendix cut through the ridge and azygous in two 

 lines converging in the cavity for the left pulmonary veins. 



c. Follow up the incision to the end of a vein which is tied ; this is 

 the one through which the left side of the heart was injected. The others 

 are probably tied up in the roots of the lungs. 



16. The Fossa Ovalis. Hold the heart so that the auricular septum 

 is between the eye and light. Near the ridge between the pulmonary 

 veins is a depressed translucent area about one cm. in diameter. It is thin 

 and yields to pressure. In the right atrium it is at the orifice of the post- 

 cava. The depression is the FOSSA OVALIS ; PI. XIV. 



a. In the fetus there was here an orifice, the foramen ovale, between the two auricles 

 so that the blood from the right passed into the left instead of into the right ventricle ; at 

 or soon after birth it should close. Sometimes, especially in young individuals, there 

 persists a slit-like remnant of the orifice to which the attention of the instructor should 

 be called. 



b. Draw the left auricle, locating the fossa ovalis. 



17. Opening the Left Ventricle. With the scalpel transect the heart 

 just caudad of the septal attachment of the moderator band. 



a. The cavity thus exposed is the LEFT VENTRICLE. 



b. Make an outline of the cut end of the apical piece ; supply the 

 wall of the left ventricle from memory. 



c. The much greater thickness of the left walls is required in order to force the blood 

 through the arteries of the entire body. 



d. Slit the septum from the cut surface to the apex of the left ven- 

 tricle ; note that at the tip the wall is no thicker than that of the right, 

 illustrating the resisting power of living muscle. 



e. The left ventricle forms the apex of the heart, and the right is as 

 it were laid upon it. 



