2o Animal Life and Intelligence. 



CHAPTEE II. 



THE PEOCESS OF LIFE. 



In the foregoing chapter, on " The Nature of Animal Life," 

 we have seen that animals breathe, feed, grow, are sensitive, 

 exhibit various activities, and reproduce their kind. These 

 may be regarded as primary life-processes, in virtue of 

 which the animal characterized by them is a living 

 creature. We have now to consider some of these life- 

 processes — the sum of which we may term the process of 

 life — a little more fully and closely. 



The substance that exhibits these life-processes is 

 protoplasm, which exists in minute separate masses 

 termed cells. It seems probable, however, that these 

 cells, separate as they seem, are in some cases united to 

 each other by minute protoplasmic filaments. In the 

 higher animals the cells in different parts of the body 

 take on different forms and perform different functions. 

 Like cells with like functions are also aggregated together 

 into tissues. Thus the surfaces of the body, external and 

 internal, are bounded by or lined with epithelial tissue ; 

 the bones and framework of the body are composed of 

 skeletal tissue ; nervous tissue goes to form the brain 

 and nerves ; contractile tissue is found in the muscles ; 

 while the blood and lymph form a peculiar nutritive tissue. 

 The organs of the body are distinct parts performing 

 definite functions, such as the heart, stomach, or liver. 

 An organ may be composed of several tissues. Thus the 

 heart has contractile tissue in its muscular walls, 

 epithelial tissue lining its cavities, and skeletal tissue 

 forming its framework. Still, notwithstanding their aggre- 



