342 SECRETION. 



As would naturally be supposed, the general mechanism of secretion was very im- 

 perfectly understood early in the history of physiology, when little was known of the 

 circulation, 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 separated or filtered from 

 the blood. Physiologists now regard secretion as the act by which fluids, holding cer- 

 tain solid principles in solution, and sometimes containing liquid nitrogenized princi- 

 ples, but not necessarily possessing formed anatomical elements, are separated from the 

 blood or are manufactured by special organs out of materials furnished by the blood. 

 These organs may be membranes, follicles, or collections of follicles or tubes. In the 

 latter instance they are called glands. The liquids thus formed are called secretions ; 

 and they may be destined to perform some function connected with nutrition or may be 

 simply discharged from the organism. 



It is not strictly correct to speak of formed anatomical elements as the results of 

 secretion, except, perhaps, in the case 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 real, anatomical elements developed in the way in 

 which these structures are ordinarily formed. It has been conclusively demonstrated, 

 for example, that leucocytes, or pus-corpuscles, are developed in a clear blastema, with- 

 out the intervention of any special secreting organ, and that spermatozoids and ova are 

 generated by a true development in the testicles and the ovaries, by a process entirely 

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

 the mechanism by which the secretions are produced. It is true that, in some of the 

 secretions, as the sebaceous matter, a certain quantity of epithelium, more or less disin- 

 tegrated, is found, but this is to be regarded as an accidental admixture of desquamated 

 matter and not as a product of secretion. 



Classification of the Secretions. The secretions are capable of a physiological clas- 

 sification, dependent upon differences in their functions and in the mechanism of their pro- 

 duction. Investigations within the past few years have shown that these differences are 

 very distinct. 



Certain of the fluids are formed by special organs, and have important functions to 

 perform which do not involve their discharge from the organism. These may be classed 

 as the true secretions; and the most striking 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 function ; and they are never produced by any other part. It is the gland 

 which produces the characteristic element or elements of the true secretions out of mate- 

 rials furnished by the blood ; and the principles thus formed never preexist in the circu- 

 lating fluid. The function which these fluids have to perform is generally intermittent ; 

 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 maybe sometimes done in experiments upon living animals, the charac- 

 teristic elements 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 and disturbances consequent upon the loss of its function. 



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

 principles in solution, which result from the physiological waste of the tissues. These 

 principles have no function to perform in the animal economy and are simply separated 

 from the blood to be discharged from the body. These may be classed as excretions, 

 the urine being the type of fluids of this kind. The characteristic principles of the excre- 

 mentitious fluids are formed in, the tissues, as one of the results of the constant changes 

 going on in all organized, living structures. They are not produced in the glands by 

 which they are eliminated but appear in the secretion as the result of a sort of elective 



