CHAPTER NINTH. 



OP THE SECRETIONS. 



261. WHILE the body is assimilating foreign substances 

 for its nutrition and growth, it is also freeing itself from 

 other substances which have become useless (201). The 

 different processes for effecting this latter object are called 

 SECRETIONS. 



262. In this operation the skin is largely concerned. Be- 

 ing the outermost envelop of the body, and designed to pro- 

 tect it from external influences, it is the seat of continual 

 loss and reparation. New membranes and new tissues are 

 constantly forming, while the old ones are removed. This 

 removal is sometimes gradual and continual, in the form of 

 slime, as in the fishes and most of the mollusks, their mucus 

 being in fact a collection of cells detached from the surface 

 of the skin ; and sometimes periodical, when it constitutes 

 moulting. Thus, the mammals cast their hair, the birds 

 their feathers, the serpents their outer skin, the crabs their 

 test, the caterpillars their outer envelop with the hairs arising 

 from it. 



263. We shall hereafter see that the skin presents such 

 varieties of composition in the different groups of the Ani- 

 mal Kingdom as to furnish excellent distinctions for species, 

 genera, and even families. In the vertebrates it is com- 

 posed of three very distinct layers of unequal thickness, as 



