CHAPTER XVI. 

 MUTILATION AND REGENERATION. 



Regeneration is the function of repair. It embraces a 

 number of dissimilar processes. Thus certain used-up 

 or worn-out elements are constantly renewed and in this 

 sense repaired. The human skin is subject to attrition 

 by which its superficial cells are being rubbed off, but 

 new cells are always forming to take their place; the 

 nails are always wearing away, but are always growing 

 from the matrix; the hairs are broken or shed, but con- 

 tinue to grow or new hairs to take their place. Among the 

 birds there are periodical moults, when the old feathers 

 that may have become broken or useless are shed and 

 replaced by new ones. Reptiles, lizards, and snakes 

 periodically shed the entire skin beneath which a new 

 one has formed. Stags annually shed their horns, 

 new ones of slightly different form being produced to 

 take their places. 



When the entire thickness of the cuticular covering is 

 accidentally penetrated and the subjacent tissues ex- 

 posed, almost every living organism is capable of effect- 

 ing a repair by which the exposed surface, if not too large, 

 becomes covered by a new protective skin. When the 

 damage is of a more serious nature and a part of the 

 organism torn away, the reaction that follows varies in 

 different cases. Sometimes the injury simply heals; that 

 is, is covered by a new protection and the mutilation per- 

 sists; sometimes, as among certain ccelenterates and 

 worms, there is a rearrangement of the remaining tissues 

 by which the symmetry of the organism is restored 

 though its size is diminished; and sometimes the loss of 

 the part is followed by a new growth which gradually 



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