66 



SCIENCE PRIMERS, 



[§V. 



The valves in the veins, then, let the 

 hlood pass easily from the capillaries to the 

 heart, but won*t let it go the other way. 

 If you bare your arm you may see some of the 

 veins in the skin, in which the blood is running up 

 from the hand towards the shoulder. If with your 

 finger you press one of these veins back towards the 

 hand it will swell up, and if you look carefully you 

 may see little knots here and there caused by the 

 bulging out of the watch-pocket valves. If you press 

 it the other way, towards the elbow, you will empty 

 it easily, and if with another finger you prevent the 

 blood getting into it from behind, that is from the 

 hand, the vein will remain empty a very long time. 



The presence of valves in the veins, then, is one 

 reason why the blood moves in one direction, but 

 other reasons, and these the chief ones, are to be 

 found in the heart. 



Let us now go back to the sheep's heart 



30. You know from the diagram that the two great 

 veins, the superior and inferior vena cava, open into 

 the right auricle. If you slit up these two veins in 

 the sheep's heart, you will find that they end by 

 separate openings in a small cavity, the inside of 

 which is for the most part smooth, and the walls of 

 which, made, as you will at once see, of muscle, are 

 not very thick. This small cavity is the right auricle, 

 shown in Fig. 8, R.A.^ where the great veins have not 

 been slit up, but the front of the auricle has been cut 

 away. In this auricle, beside the openings into the two 

 great veins and another one which belongs to a vein 

 coming from the heart itself (Fig. 8, b) there is quite a 

 large one^ leading straight downwards, intp which you 



