THE CIRCULATION. Gl 



laries, and drawing blood, so close together they are. If 

 we could take out from the body of a man all the flesh 

 and bone, leaving the blood-vessels, there would still be 

 sufficient to form a perfect figure. 



5, The arteries divide up into the capillaries, and the 

 capillaries unite to form the veins. The little veins thus 

 formed, unite to form larger veins, and so on, until at last 

 they are all gathered into two large veins which enter the 

 heart. These are called the vena cava superior, and the 

 vena cava inferior. 



6, The walls of the capillaries are very thin. Although 

 there are no openings in them, a portion of the blood 

 soaks through into the surrounding tissues; and, on the 

 other hand, fluids containing waste matter soak into them, 

 to be carried away. 



THE HEART. 



SECTION II. 1, The heart is made of muscle, and is 

 hollow. It is the pump which keeps the blood moving. 



2, It is situated in the chest, resting on the diaphragm, 

 chiefly on the left side of the middle line. It is shaped 

 like a pear, with the small end pointing down, and to the 

 left. 



3, It is inclosed in a sac called the pericardium. This 

 sac is also pear-shaped; but the large end is on the dia- 

 phragm, and the small end points up. 



4, The pericardium is lined with a very smooth mem- 

 brane. The heart is covered with a continuation of the 

 same membrane. This membrane is kept glossy and 

 moist by a fluid that it gives out. When the heart moves 

 in its sac, these two smooth membranes rub together with- 

 out friction. 



