CHAPTER I. 

 REGENERATION. 



DEFINITION. — Regeneration is the process which 

 leads to the replacement of lost tissue. It takes place under 

 normal and physiological conditions and in all pathological 

 processes. In both cases the reactive phenomena are the 

 same. 



The annual moulting of the feathers of the bird, the pe- 

 riodic loss and growth of the horns of the stag, and the 

 breaking down of cells in different parts of the body after 

 they have been active for a time, and their replacement by 

 new cells are examples of the phenomena of physiological 

 regeneration. As a general rule, the higher the tissue stands 

 in the scale of differentiation, the smaller the tendency of its 

 cells to proliferate. The power of regeneration in mammals 

 is limited as compared with that of many of the lower ani- 

 mals. The upper part of the hydra, if separated, will repro- 

 duce the rest of the body; if the lower part is cut off, it will 

 add the rest. Certain worms may be cut into several pieces, 

 and each part will regain what is needed to complete the 

 mangled organism. The starfish can reproduce its arms; 

 the holothurian, its stomach; the snail, its tentacles; the 

 lobster, its claws; the spider, its legs; the fish, its fins; the 

 lizard, its tail. 



In the domestic animals, nature facilitates the struggle 

 for existence in another way. If a leg is cut off, an animal 

 can make three do; if one kidney is destroyed, the other will 

 enlarge, and together with the skin make it possible for the 

 animal to live and work in comfort; if the spleen is removed, 

 the lymphatic glands will perform its functions almost as 



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