624 EXCRETING ORGANS. 



a limited time have formed part of their system, and this 

 endowment is essential to their development and growth, and 

 constitutes the principal phenomenon of their nutrition and 

 of their life. The effete materials of their nutrition are pouring 

 incessantly in a gaseous or in a fluid state, from the exterior 

 cutaneous surface of the body, or from the internal mucous 

 surface of the alimentary canal and its appendages, in all 

 classes of animals, from the polygastrica to the mammalia, 

 however inappreciable those products may often appear. Re- 

 pair and decay, nutrition and excretion, may alike be re- 

 garded as secerning processes, where dissolved materials 

 permeate the parietes of capillary tubes ; and where those 

 secreted materials do not appear subservient to some useful 

 purpose in the maintenance of the individual or of the spe- 

 cies, they are regarded as excretions, and the parts which 

 form them excrdlny organs, whether they communicate with 

 the internal mucous lining of the alimentary cavity or the 

 exterior cutaneous covering of the body. 



All excreting organs, by extracting their products from 

 capillary blood-vessels spread on their surface, have a se- 

 cerning function, whether performed by simple membranes, 

 follicles, tubuli, or other forms of glands, and a precise 

 limit can scarcely be assigned to the excrementitious cha- 

 racter of glands and secretions. The luminous, the elec- 

 trical, and the stinging materials evolved by many of the in- 

 ferior tribes, are but little connected with the processes of 

 nutrition or of generation, as are also the numerous poiso- 

 nous fluids, inky secretions, and odorous materials formed 

 by animals for self-defence, but unequivocally less are the 

 gaseous and aqueous materials evolved by the respiratory 

 organs, the fluids perspired by the skin, and the heteroge- 

 neous product of the kidneys. As in all other glands con- 

 nected with organic life, the most important and the most 

 developed forms of excreting glands originate from the in- 

 terior of the alimentary canal, and are safely protected in 

 the deeper cavities of the body, while the more simple and 

 minute forms of these organs, and the most numerous, are 

 developed from the skin and spread over the whole exterior 

 of the body. 



