194 



THE HUMAN BODY. 



rounding soil; this is known as irrigation, and the capillaries 

 may be said to form an irrigation system for the body. In 

 a certain sense also they may be compared to the water 

 spigots of a house, which lie at the end of the supply tubes, 

 the arteries; but, to make the comparison more accurate, 

 we would have to imagine instead of ordinary spigots bags 

 of very fine muslin through which ,the water oozed when 

 the tap was turned on. The capillaries, though far the 

 smallest tubes in the vascular system, are 

 the really important parts; the heart, 

 arteries, and veins are all merely ar- 

 rangements for keeping the capillaries 

 full, and renewing the blood within 

 them. It is while flowing through these 

 and soaking through their walls that the 

 blood does its physiological work. 



Diagram of the Circulatory Organs. . 

 The general relationship of heart, arte- 

 ries, capillaries, and veins may be gath- 

 ered from the accompanying diagram 

 (Fig. 57). The heart is essentially a bag 

 with muscular walls, and internally di- 

 FIG. 57. The heart vided into four chambers (d, g, e, /). 



and blood-vessels dia- , 7 . N -ITT 



grammatically repre- TllOSC at 0116 end (d and 6) 1'GCeiVe blood 



from vessels opening into them and 

 known as the veins. From there the blood passes on to 

 the remaining chambers (g and/), which have very mus- 

 cular walls and, forcibly contracting, drive the blood out into 



By comparison with the water pipes of a house. What is the 

 use of heart, arteries, and veins? When does the blood do its physi- 

 ological work? 



Sketch on the blackboard a diagram of the circulatory organs. 

 What is briefly the structure of the heart? Where does the blood 

 enter the heart? From what vessels? Where does it go next? 



