GENERAL ANIMAL FUNCTIONS. 83 



need to be valves to prevent or retard back-flow; finally 

 the blood vessels must have very thin walls in all those 

 regions where they are to give up or take in elements (capil- 

 laries}. The capillaries are functionally the important 

 part of the system. The rest exists for them. They are 



FIG. 12. A scheme to represent circulation of the blood. The arrows indicate 

 the course of the blood, a, arteries; aur, auricle or receiving portion of the heart; d, 

 digestive tract; c.d., capillaries of the digestive tract; C.T., capillaries of the respiratory 

 organs; c.s., capillaries of the system; va, valves; ve, veins; vt, ventricle. 



Questions on the Figure. What portions of the apparatus are 

 necessary in order to secure circulation? In which regions is the 

 real work of the blood done? Why are valves essential? What 

 common work is done in the three sets of capillaries figured above? 

 What special type of work is done in each of the three? 



found in all regions of the system that must be nourished 

 and relieved of waste ; especially in the respiratory, digest- 

 ive, and excretory systems. (Study Fig. 12.) 



88. Blood. For the purposes named, only the fluid 

 portions of the blood are absolutely essential ; but in most 

 instances there are also living cells in the blood, known 

 as blood cells or corpuscles. The blood cells may have 

 as their function the better handling of the O, as in the 



