CLASSIFICATION OF THE SECRETIONS 269 



spoken of as products of secretion, are anatomical elements developed 

 in the way in which such structures are ordinarily formed. 



Classification of the Secretions. Certain secretions are formed by 

 special organs and have important uses that do not involve their dis- 

 charge from the body. These may be classed as the true secretions ; 

 and the most striking examples are the digestive liquids. Each one of 

 these is formed by a special gland or set of glands, which usually has 

 no other office ; and they are never produced by any other part. It is 

 the gland that produces the characteristic constituent or constituents of 

 the true secretions ; and the matters thus formed do not preexist either 

 in the blood or in the lymph. The office which the true secretions have 

 to perform usually is not continuous; and when this is the case, the 

 flow of the secretion is intermittent, taking place only when its action is 

 required. When the parts that produce one of the true secretions are 

 destroyed, the characteristic constituents of this secretion do not accu- 

 mulate in the blood nor are they formed vicariously by other organs ; the 

 effect is absence of the secretion, with the disturbances consequent to 

 the loss of its physiological action. 



Certain other of the liquids of the body are composed of water, 

 holding one or more characteristic constituents in solution that result 

 from the physiological wear of the tissues. These matters have no office 

 to perform in the economy and are separated from the blood to be dis- 

 charged from the body. Such products may be classed as excretions, 

 the urine being the type of liquids of this kind. The characteristic 

 constituents of the excretions have their origin in the tissues and are 

 products of the changes going on in all organized living structures. 

 They preexist in the blood or lymph and may be eliminated, either con- 

 stantly or occasionally, by a number of organs. As they are produced 

 continually in the substance of the tissues or organs and are taken up 

 by the blood, they are constantly separated from the blood by the 

 proper eliminating organs. When the glands which thus eliminate these 

 substances are destroyed or when their efficiency is seriously impaired, 

 the excrementitious matters may accumulate in the blood and give rise, 

 directly or indirectly, to toxic phenomena. These effects, however, are 

 often retarded by the vicarious action of other organs. 



There are some liquids, such as the bile, that have important uses as 

 secretions and nevertheless contain excrementitious matters. In these 

 instances, it is only the excrementitious matters that are discharged 

 from the organism. In the sheaths of some tendons and of muscles, in 

 the substance of muscles and in some other situations, liquids are found 

 which simply lubricate the parts and which contain very little organic 

 matter, with but a small proportion of inorganic salts. 



