DIVISION OF SECRETIONS. 15 



rated, or filtered from the blood. Physiologists now regard 

 secretion as the act by which fluids, holding certain solid 

 principles in solution, and sometimes containing liquid nitro- 

 genized principles, 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, when they are called 

 glands. The liquids thus formed are called secretions; 

 and they may be destined to perform some function con- 

 nected 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, without 

 the intervention of any special secreting organ ; l and that 

 spermatozoids and ova are generated by a true development 

 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 secre- 

 tions are produced. It is true that in some of the secretions, 

 as the sebaceous matter, a certain quantity of epithelium, 

 more or less disintegrated, is found, but this is to be regarded 

 as an accidental admixture of desquamated matter, and not 

 as a product of secretion. 



Division of Secretions. The secretions are capable of a 

 physiological division, dependent upon differences in their 

 functions and the mechanism of their production. Investi 



1 See vol. i., Blood, p. 124, and voL ii., Absorption, p. 523. 



