INTEODIJCTION. 



§ 1. Everything that lives is subject to perpetual destruction 

 and renewal of its constituent elements. These changes are 

 inferred from observation; we mark the continual adoption of 

 certain substances into the organism, and the excretion of others 

 in proportionate quantity ; these others being demonstrably pro- 

 duced by the metamorphosis of the living substance. 



§ 2. The human eye, even when armed with the highest 

 magnifying powers, cannot detect this molecular activity of 

 matter. It becomes apparent to our senses only when it is dis- 

 turbed, whether in a plus or minus direction. Who can see the 

 nutritive processes taking place in the fibres of striped muscle 

 in the tail of a living tadpole, or note the silent coming and 

 going of matter in a neighbouring connective tissue corpuscle ? 

 So long as these structures continue to exhibit, even to the 

 smallest dot and line, a certain definite and familiar aspect, we 

 hold this "morphological permanence" to be a proof that the 

 metamor2:»hoses associated with nutrition are progressing undis- 

 turbed. It is only when we discover some qualitative change in 

 the cells or other elementary parts of the organism, that we are 

 made aware of some past or present change in their constitution, 

 and incline with reason to the belief that such chanoje is due to 

 some alteration in the nutritive process. 



§ 3. Changes of this kind occur even in the normal course 

 of life. The age of the body, its gradual growth and decay, are 

 to some extent reflected in the condition of the tissues. We 

 must distinguish vi limine between two leading tendencies, under 

 which all tissue changes may be grouped ; these are, development 

 and retrograde metamorphosis. The former teaches us to trace 

 the origin and growth of the entire organism from the repeated 

 multiplication and manifold differentiation of the cell ; the latter 

 shows us that the infirmities and the frailty of the body as it 



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