XXIX. THE BLOOD 





Function of the Blood. — We have seen in the preceding chapter 

 that the chief function of the digestive tract is to change foods 

 to such form that they can be absorbed through the walls of the 

 food tube. The food, after it has passed through the intestine 



walls, ultimately reaches the 

 blood and becomes, as we shall 

 see, a part of this tissue. By 

 means of a system of closed 

 tubes, this fluid tissue circu- 

 lates to all parts of the body, 

 depositing its burden of food 

 at the places where it is most 

 needed and where it will be 

 used, either in the rejDair or 

 building of tissues or in the 

 release of energy. 





■>w. 





Nucleated blood cells of a frog, as seen vmder 

 the compound microscope; a, colorless 

 corpuscles. 



Laboratory Exercise. — Examine 

 a prepared slide of the blood of a 

 frog. Note that three constituents 

 are found: (1) Ovoid cells, each 

 containing a nucleus. What color 

 do these bodies have? They are called the red corpuscles. (2) Other 

 colorless corpuscles of irregular form may be seen. What can you say of 

 the number of colorless corpuscles as compared with the red corpuscles? 

 Notice that the colorless corpuscles have the power to change their shape. 

 They are said to be amoeboid. Like the amoeba, they also have the power 

 to take up particles of food and other materials and ingest them. (3) The 

 colorless fluid in which the corpuscles float is known as the plasma. 



Composition of Plasma. — The plasma of blood (when chemically 

 examined in man) is found to be largely (about 90 per cent) water. 

 It also contains a considerable amount of proteid, some sugar, 

 fat, and mineral material. It is, then, the medium which holds the 

 fluid food (or at least part of it) that has been absorbed from the 

 food within the intestine. When the blood returns from the tis- 



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