14 SECRETION. 



ments, which themselves undergo the processes of molecular 

 destruction and regeneration. These characters are not pos- 

 sessed by the secretions. As a rule, the latter are homoge- 

 neous fluids, without formed anatomical elements, except as 

 accidental constituents; such as the desquamated epithe- 

 lium in mucous or sebaceous matter. The secretions are not 

 permanent, self-regenerating fluids, except when they per- 

 form simply a mechanical function, as the humors of the 

 eye, or the liquids in serous and synovial cavities. They 

 are either discharged from the body, when they are called 

 excretions, or, after having performed their proper function 

 as secretions, are taken up again in a more or less modified 

 form by the blood. 



With the exception of those fluids which have a function 

 almost entirely mechanical, the relations of the secretions to 

 nutrition are so close, that the production of many of them 

 forms almost a part of this great function. It is impossible, 

 for example, to conceive of nutrition without the formation 

 of the characteristic constituents of the urine, the bile, and 

 the perspiration ; and it is impossible, indeed, to study satis- 

 factorily the phenomena of nutrition without considering 

 fully the various excrementitious principles, such as urea, 

 cholesterine, creatine, creatinine, etc. ; for the constant forma- 

 tion and discharge of these principles by disassimilation 

 create the necessity for the deposition of new matter in 

 nutrition. Again, the most important of the secretions, as 

 contradistinguished from the excretions, are concerned in the 

 preparation of food by digestion, for the regeneration of the 

 great nutritive fluid. 



As would naturally be supposed, the general mechanism 

 of secretion was very imperfectly understood early in the 

 history of physiology, when little was known of the circula- 

 tion, the functions of the digestive fluids, and particularly of 

 nutrition. From its etymology, the term should signify 

 separation ; but it is now known that many of the secreted 

 fluids are formed in the glands, and are not simply sepa- 



