THE ALIMENTARY CANAL 1S5 



pears like a filter, adapted to remove something from 

 the blood while keeping other constituents back. Secre- 

 tion, however, is much more than filtration. . The 

 material derived from the blood is often greatly changed 

 during its stay in the gland cells and so we find many 

 bodies in the product which are not to be found in the 

 blood. 



It is necessary now to show that the custom of apply- 

 ing the word gland both to minute developments of the 

 alimentary mucous membrane and to massive organs 

 like the liver can be justified. The fact is that an 

 organ like the liver or the pancreas is a vast aggregate 

 of secreting recesses which individually are much like 

 the simple glands of the stomach and intestine. The 

 branching ducts provide for the gathering of the com- 

 bined secretions from all these units. A large gland 

 may be expected to have a supporting capsule and par- 

 titions of connective tissue subdividing it into lobes and 

 lobules. It is to be borne in mind that many glands are 

 as distinctly under nervous control as are the contractile 

 tissues. 



Classification of Foods. In Chapter II the contrast be- 

 tween proteins and non- protein organic foods is briefly 

 indicated. The former contain carbon, oxygen, nitrogen, 

 hydrogen, and sulphur, sometimes phosphorus. The 

 latter are non-nitrogenous. They may be grouped as 

 below: 



Carbohydrates, including starches and sugars. 



Fats (or oils). 



Alcohol. 



Starches are incompletely soluble, of large molecule, 

 and tasteless. They are easily converted to sugars 

 which are freely soluble, of small molecule, and sweet. 

 Fats have familiar physical characters: low melting- 

 points and insolubility in water. They contain the same 

 elements as carbohydrates : carbon, hydrogen, and oxy- 

 gen (much more of the first, much less of the last). 

 Water and mineral salts are reckoned as inorganic foods. 



