GENERAL STRUCTURE AND COMPOSITION. 11 



to be, they are only known in or derived from living beings. 

 Gelatin, obtained from bones and ligaments by boiling, is a 

 typical albuminoid; as is chondrin, which is obtained similarly 

 from gristle. Mucin, which gives their glairy tenacious char- 

 acter to the secretions of the- mouth and nose, is anothei 

 albuminoid. 



Enzymes or Soluble Ferments are a group of substances 

 which seem to be allied in chemical composition to the true 

 proteids, but it is so difficult to be sure of the purity of any 

 specimen that their composition is still in doubt. The 

 enzymes have the power, even when present in very small 

 quantity, of bringing about extensive changes in other sub- 

 stances, and they are not themselves necessarily used up or 

 destroyed in the process. Many enzymes of great physiologi- 

 cal importance exist in the digestive fluids and play a part in 

 fitting food for absorption from the alimentary canal. For 

 example, pepsin found in the gastric juice and trypsin found 

 in the pancreatic secretion convert, under suitable conditions, 

 albuminous substances into peptones; and ptyalin, found in 

 the saliva, converts starch into sugar. Other ferments 

 cause the clotting of various animal liquids: rennin from the 

 gastric juice clots the caseinogen of milk preparatory to its 

 digestion; and a ferment which forms in drawn blood con- 

 verts fibrinogen into fibrin. We shall have occasion later to 

 study several enzymes more in detail in connection with their 

 physiological uses. 



Crystalline Nitrogenous Substances. These are a heter- 

 ogeneous group, the great majority of them being materials 

 which have done their work in the Body and are about to be 

 got rid of. Nitrogen enters the Body in foods for the most 

 part in the chemically complex form of some proteid. In the 

 vital processes these proteids are broken down into simpler 

 substances, their carbon being partly combined with oxygen 

 and passed out through the lungs as carbon dioxide; their 

 hydrogen is similarly in large part combined with oxygen and 

 passed out as water; while their nitrogen, with some carbon 

 and hydrogen and oxygen, is usually passed out in the form 

 of a crystalline compound, containing what chemists call an 

 "ammonium residue/' Of these the most important is urea 

 (Carbamide, 2NH 2 , CO), which is eliminated through the kid- 

 neys. Uric acid is another nitrogenous waste product, and 

 many others, such as kreatin and leucin, seem to be inter- 



