2 INTERNAL SECRETION 



rity of the entire organism. On the other hand, they may 

 be of no further use in the economy, and may be cast out as 

 \\a-te products. 



By the term " secretion," applied in its most general sense, 

 is, or has been, understood the separation out of a substance 

 or of substances from or by the agency of the living protoplasm. 

 Tli is. the original conception of the process, has long been 

 < \t< nded to include also the preliminary preparation, or a 

 more or less complete elaboration of the materials which are 

 supplied by the processes of osmosis and diffusion, and in the 

 case of the multicellular organisms by the blood circulating 

 through the organ or tissue. 



Johannes Miiller pointed out that the whole process of 

 secretion consists of two phases the production of certain 

 materials and the casting out of these materials upon a surface 

 either in the interior or upon the exterior of the body. The 

 first phase he called " secretion," the second " excretion." In 

 some cases the material eliminated might be found in the 

 blood-stream, and was simply separated by the cells of the 

 organ and passed out. This applied to the urea of the urine, 

 which was looked upon by Miiller as a pure case of " excretion." 

 The distinction thus set up has, however, not been maintained 

 by physiologists. The term " excretion " has been, and is at 

 the present time, applied sometimes in a vague kind of way, 

 sometimes more definitely and specifically, to denote the 

 process of elimination of waste products from the body. This 

 process is frequently of the nature of secretion. Thus the 

 elimination of urea by the kidney is referred to as a " secretion," 

 although the product itself is an " excretion." This is due to 

 Mic fact, now well established, that, although urea is present 

 in the Mood supplied to the kidneys, yet it is not simply filtered' 

 out of the blood, but is got rid of by a definite " secretory " 

 activity of the cells of the tubules. It can be shown that the 

 cell- arc capable of performing a definite chemical synthesis, 

 and t hi> is regarded as a sign, or one of the signs, of a " secre- 

 tory " activity; so that the term "secretion" includes 

 "ii. When the products of metabolism are of no 

 SWTviofl in the economy, they are called "excretions." 



Among the Iic-t known of the secretions are the enzymes 

 and analogous products (such as will be referred to later as the 

 products of the "internal" secretions). But many skeletal 



