CIRCULATION.] PHYSIOLOGY. - 55 



that very frequently, directly you had passed one of 

 these valves, you came to a spot where one vein 

 joined another. 



Well, but for these differences, your journey along 

 the veins would be very like your journey along the 

 arteries, and at last you would find yourself in a great 

 vein, whose name you would learn to be the vena 

 cava, or hollow vein (and because, though there is 

 but one aorta, there are two great "hollow veins," 

 the superior vena cava or upper hollow vein), 

 and from thence your next tep would be into the 

 heart again. So you see, starting fi*om the capillary 

 (you started from a capillary in the arm, but you 

 might have started from any capillary anywhere), 

 whether you go along the arteries or whether you 

 go along the veins, you at last come to the heart 



Before we go on any further we must learn some- 

 thing about the heart. 



27. Go and ask the butcher for a sheep's pluck. 

 There will most probably be one hanging up in his 

 shop. Look at it before he takes it down. The hook 

 on which it is hanging has been thrust through the 

 windpipe. You will see that the sheep's windpipe is, 

 like the rabbit's, all banded with rings of cartilage, only 

 very much larger and coarser. Below the windpipe 

 come the spongy lungs, and between them lies the 

 heart, which perhaps is covered up with a skin and so 

 not easily seen. Hanging to the heart and lungs is 

 the great mass of the liver. When you have got the 

 pluck home, cut away the liver, cut away the skin 

 (pericardium, it is called) which is covering the heart, 

 if it has not been cut away already, and lay the 

 ixings out on a table with the heart between them. 



teaik 



