ix THE HEART 81 



and left auricles above, and the right and left ventricles 

 below. 



On the back of the heart, in the middle line, just above 

 tin- transverse mass of fat, lies a thin collapsed tube, the 

 inferior vena cava. The vessel may have been cut off short, 

 in which case its opening only will be seen. If the little finger 

 be gently passed into this opening, its tip will be felt to reach 

 inside the right auricle. Make the opening a little larger with 

 scissors ; then put the finger into the right auricle again and 

 pass it straight upwards till it comes out of another opening at 

 the top of the auricle. This is the opening of the superior 

 vena cava. More or less length of this vessel may be present. 

 There are no other vessels opening from without into the right 

 auricle. Put the finger into the right auricle and pass it down- 

 wards, the tip of the finger will go through the opening into the 

 right ventricle and can be felt to be inside that chamber. 



Just to the left of the inferior vena cava is an opening 

 leading into the left auricle. It is the opening of one of the 

 pulmonary veins. Put your little finger or a penholder 

 into it, and passing it upwards feel for another opening ; if 

 you find one, it will be that of the other pulmonary vein. There 

 are in the sheep two pulmonary veins, one bringing blood from 

 the right lung and one from the left lung, but sometimes they 

 unite into one vessel just before they open into the left auricle. 

 In man there arc four pulmonary veins, two coming from each 

 lung, and they all open separately into the left auricle. 



The tip of the finger can be passed down from the left auricle 

 into the left ventricle. The wall of the left ventricle will then 

 be felt to be very thick. You cannot pass your finger from one 

 auricle to the other or from one ventricle to the other, for 

 the two auricles are completely separated from each other, and 

 the two ventricles are completely separated from each other by 

 a partition running through the heart, dividing the right side 

 from the left. On the right side there is an opening leading 

 from the right auricle to the right ventricle, and on the left side 

 an opening leading from the left auricle to the left ventricle. 



Cut open the right auricle from the inferior vena cava to 

 the superior vena cava, and notice that the wall is thin and 

 smooth on the inside except in the appendix, and there 

 it is very irregular and formed into small pits by the wall 



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