CLASSIFICATION OF THE SECEETIONS. 307 



not possessed by the secretions. As a rule, the latter are homogeneous fluids, 

 without formed anatomical elements, except as accidental constituents, such 

 as the desquamated epithelium in mucus or in sebaceous matter. The secre- 

 tions are either discharged from the body, when they are called excretions, 

 or after having performed their proper office as secretions, are absorbed in a 

 more or less modified form by the blood. 



Physiologists now regard secretion as the act by which fluids, holding 

 certain substances in solution, and sometimes containing peculiar ferments 

 but not necessarily possessing formed anatomical elements, are separated 

 from the blood or are formed by special organs out of materials furnished by 

 the blood. These organs may be membranes, follicles or collections of folli- 

 cles, or tubes. In the latter instances they are called glands. The liquids 

 thus formed are called secretions; and they may be destined to perform 

 some office connected with nutrition or may be simply discharged from the 

 organism. 



It is riot strictly correct to speak of formed anatomical elements as prod- 

 ucts of secretion, except in the instance of the fatty particles in the milk. 

 The leucocytes found in pus, the spermatozoids of the seminal fluid, and the 

 ovum, which are sometimes spoken of as products of secretion, are anatomi- 

 cal elements developed in the way in which such structures are ordinarily 

 formed. For example, leucocytes, or pus-corpuscles, may be developed with- 

 out the intervention of any special secreting organ ; and spermatozoids and 

 ova are generated in the testicles and the ovaries, by a process entirely differ- 

 ent from ordinary secretion. It is important to recognize these facts in 

 studying the mechanism by which the secretions are produced. 



Classification of the Secretions. Certain secretions are formed by special 

 organs and have important uses which do not involve their discharge from 

 the body. These may be classed as the true secretions ; and the most strik- 

 ing examples of such are the digestive fluids. Each one of these fluids is 

 formed by a special gland or set of glands, which generally has no other 

 office ; and they are never produced by any other part. It is the gland which 

 produces the characteristic constituent or constituents of the true secretions, 

 out of materials furnished by the blood ; and the matters thus formed never 

 pre-exist in the circulating fluid. The office which these fluids have to per- 

 form is generally 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 which produce one of the true secretions are destroyed, as is some- 

 times done in experiments upon living animals, the characteristic constituents 

 of this particular secretion never accumulate in the blood nor are they formed 

 vicariously by other organs. The simple effect of such an experiment is 

 absence of the secretion, with the disturbances consequent upon the loss of 

 its physiological action. 



Certain other of the fluids are composed of water, holding one or more 

 characteristic constituents in solution, which result from the physiological 

 wear of the tissues. These matters have no office to perform in the animal 

 economy and are simply separated from the blood to be discharged from the 



