ORGANS OF SECRETION. 5/3 



CHAPTER FIFTH. 



ORGANS OP SECRETION. 

 FIRST SECTION. 



General Observations on the Secreting Organs. 



THE blood, replenished with fresh materials from the 

 lacteals, and oxygenated by respiration, is fitted to be sent 

 through the system for the nourishment of all the organs, 

 and to afford the materials of all the secretions. But every 

 living membrane in contact with a fluid, whether in the 

 interior or on the surface of the body, exhales its own pecu- 

 liar secretions ; and in the lowest tribes of animals, all the 

 requisite secretions are furnished from the fluids of the body, 

 often without even the presence of a distinct sanguiferous 

 system. The materials thus transuded through the porous 

 texture of membranes, or the parietes of capillary vessels, 

 are sometimes destined to form directly a part of the living 

 system, sometimes to assist in the complicated process of 

 the assimilation of foreign matter, and sometimes to form 

 excretions to be discharged from the body. The secretions, 

 however, are not mere transudations of the constituent 

 materials of the fluids which afforded them, unchanged in 

 composition and properties ; they are generally altered, both 

 in chemical and physical properties, during their transmis- 

 sion by nervous influence, or by the peculiar texture and 

 form of the secreting membranes, or by the mode of distri- 

 bution of the vessels. They are poured out alike by the 

 external cutaneous surface of animals, and by its internal 

 continuation the mucous lining of the digestive canal, or into 

 the shut cavities of serous membranes, to lubricate the 



