58 SCIENCE PRIMERS, [§ V. 



of the body. S, F.C, is the superior vena cava, to 

 form which all the veins from the head and neck and 

 arms join, the vein in which you were journeying a 

 hitie Willie ago. / V.C. is the inferior vena cava, 

 made out of all the veins from the trunk and the legs. 

 Being veins, they have thin flabby walls; and their 

 sides fall flat together, so th?.t they seem nothing more 

 than little folds of skin, and it becomes very hard to 

 find the passage inside them. But when you have 

 found the opening into them, you will see that you 

 can stretch them out into quite wide tubes, and that 

 their walls, though very much thinner than those of 

 the aorta, so thin indeed that they are almost trans- 

 parent, are still after a fashion strong. If you put a 

 penholder or thin rod through either you will find 

 that they both seem to lead right into the middle of the 

 heart. With a little care you can pass a rod up /. V. C. 

 and bring the end of it out at the top of S. V. C. Of 

 course you will understand that both of these veins 

 have been cut off" short 



28. Before we go on any further with the sheep^s 

 heart, let me tell you something about it, by help of 

 the diagram in Fig. 6, which is meant to represent 

 the whole circulation. You must remember that this 

 figure is a diagram, and not a picture ; it does not 

 represent the way the blood-vessels are really arranged 

 in your own body. If you had no arms and no legs, 

 and if you only had a few capillaries at the top of 

 your head and at the bottom of your body, it might 

 be more like than it is. 



In the centre of the figure is the heart. This you 

 will see is completely divided by an upright partition 

 into two halves, a right half and a left half Each half 



Ul 



