The Body-Machine and Its Work 355 



to say something about the last. We have already spoken of the 

 myriads of tiny tubular glands which line the alimentary canal. 

 Another mass of tubular glands make up the essential part of the 

 kidneys. They really form a filter of a remarkable pattern. 

 Arteries bring the blood to the kidney tubules, which are stimu- 

 lated to action by the blood. Each by vital action, not a mere 

 physical process takes out of the blood the fluid nitrogenous 

 waste substances and a certain amount of water, and the tiny 

 ducts connected with the tubules run together and carry the waste 

 or urine to the bladder. 



But the main interest to-day is in what scientific men call 

 the "ductless" glands: glands which extract substances from the 

 blood but do not pour their secretion into special channels. We 

 have already mentioned a most interesting example in the supra- 

 renal glands: two little bodies near the kidneys, each about the 

 size of a segment of a small orange, which pour into the blood a 

 "chemical messenger" (or hormone) for regulating the supply of 

 blood to the various organs. 



It is one of the most remarkable discoveries of recent years, 

 that there are numbers of little glands entirely devoted to the 

 manufacture of hormones. "If all the glands of internal secre- 

 tion were rolled together they would form a parcel small enough 

 to go into a waistcoat pocket, yet such a small mass can influence 

 the working and growth of the whole body." In his interesting 

 book, which we have already mentioned, Sir Arthur Keith, refer- 

 ring to the dispatch of secretion to the stomach, uses the following 

 suggestive words: The secretin, or hormone, which acts as a 

 missive " is posted in the nearest letter-boxes or capillaries in the 

 duodenal wall and is carried away in the general blood circulation, 

 which serves for all kinds of postal traffic. In a postal system 

 where there are no sorters and which must be conducted by an 

 automatic mechanism, letters or missives cannot be addressed in 

 the usual way. Their destination is indicated, not by their in- 

 scription, but by their shape. The molecules of secretin may be 



