AN INTRODUCTION TO ANIMAL BIOLOGY 51 



any animal with many layers of cells the outermost cells 

 would get insufficient food if they had to depend upon ab- 

 sorption by layers, as in the case of the experiment with 

 paper. Obviously absorption directly from the stomach 

 and intestine would not be possible in an animal like the 

 frog, in which, as we have already learned by dissection, 

 many organs are at some distance from the digestive cavities 

 of the stomach and intestine. Clearly, then, there must 

 be some method of transporting digested food from the diges- 

 tive organs (stomach and intestine) to all parts of the 

 body. This is one work of the blood, which, as we have 

 already found, flows in a set of tubes, leading to all parts 

 of the body. The details of the way in which blood carries 

 dissolved food from the stomach and intestine to all parts 

 of the body will be studied in a later lesson in connection 

 with the same process in the human body. The important 

 point in this lesson is that the frog needs blood and a mechan- 

 ism (pumping heart) for moving it in order to carry absorbed 

 food from the stomach and intestine to all parts of the body; 

 that is, to every living cell where it can be used as a supply 

 of energy and as material for repair and growth. We shall 

 later find still other important reasons why the frog needs 

 blood. 



47. Changes in Living Cells : Oxidation. We turn now 

 to the changes which occur in the living cells everywhere 

 in the body of the frog. As has already been stated (42), 

 the continuous activity of the living animal causes a wearing 

 out of the body-substance ; that is to say, of the particles of 

 cell-substance in the tissues composing the body. The wearing 

 out changes are largely due to the chemical union of oxygen 

 with the cell-substances, producing a kind of slow combustion 

 or oxidation. We commonly think of combustion as the 

 rapid burning of substances in the open air ; but essentially 

 the same kind of change may take place slowly. For ex- 

 ample, a piece of iron wire can be quickly burned (oxidized) 



