THE GENERAL ANIMAL FUNCTIONS. 65 



exchanges between the blood and the other tissues occur. This 

 is the region of capillaries. The blood system has capillaries 

 in the walls of the digestive tract, in the respiratory organs, 

 and in and about all the tissues receiving a direct blood supply. 

 The capillary region is that for which the rest exists ; it is the 



FIG. 32. 



FIG. 32. A scheme to represent the circulation of the blood, in its essential features. 

 The arrows indicate the course of the blood, a, arteries; aur., auricle or receiving por- 

 tion of the heart; d, digestive tract; c. d., capillaries of the digestive tract; c.r., capil- 

 laries 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 neces- 

 sary to secure circulation? Which secure the real objects for which the 

 circulation .exists ? Why are valves essential ? What common work occurs 

 in the three classes of capillaries figured above? What special type of 

 work is characteristic of each of the three? 



physiologically important part of the system. Fig. 32 illus- 

 trates the arrangement of parts found in a common type of 

 circulatory apparatus. 



93. Demonstration. Circulation of blood in tail of tadpole; in the 

 web of the foot of a frog; or in the fin of small fish. Distinguish veins 

 and arteries. Notice behavior of corpuscles in passing through small capil- 

 laries. Compare rate of flow in vessels of different size. 



94. The Excretory System and Function. Beside the 

 carbon dioxid eliminated from the blood in the lungs or gills, 

 other waste products of oxidation are to be removed from the 

 tissues where they are produced. Important among these are 



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