638 EXCRETING ORGANS. 



SECOND SECTION. 



External Organs of Excretion. 



As the larger and more complex internal excretory organs 

 are developed from the common mucous lining of the diges- 

 tive canal of animals, the smaller and more numerous exter- 

 nal forms of these organs are developed from the cutaneous 

 covering of the body. The naked surface of the skin in most 

 of the lowest animals, being both respiratory and secerning, 

 may likewise be regarded as a general excretory surface, and 

 the various forms of extravascular scales, shells, and other epi- 

 dermic materials, poured out as nuclei or in a fluid state from 

 its capillaries, and growing or concreting into granules, cells, or 

 cytoblasts, have also a close analogy to excretions. The sub- 

 cutaneous muciparous glands so large and complex in fishes, 

 and so numerously spread over the naked surface of am- 

 phibia, and various other cutaneous glands of higher animals, 

 eliminating materials little subservient to individual nutrition 

 or to the race, are partly excretory in their function. The 

 cutaneous glands most special and distinct in their excretory 

 function, and the product of which is most analogous to the 

 urine of the kidneys, and the carbonic acid and halitus 

 expired from the lungs, are the small, simple, convoluted, 

 sudoriferous follicles, or sweat-glands, perforating the epi- 

 dermic layers, and so numerously spread over the entire 

 surface of the body in the warm-blooded vertebrata. The 

 innumerable minute ramified sebaceous glands, which pour 

 their oily secretion, by distinct ducts, into the wider cuta- 

 neous follicles for the hairs, to lubricate and protect the skin 

 and its epidermic developments, may likewise be considered 

 as partly cutaneous emunctories, eliminating the oleagenous 

 materials of the blood ; so that these external forms of ex- 

 cretory organs become almost essential constituents of the 

 cutaneous or tegumentary parts of animals. 



