ALIMENTATION AND DIGESTION 93 



foods belong to a comparatively small number of chemical 

 groups. These classes, or groups, may be called nutrients; 

 and as all the members of the same group undergo practi- 

 cally the same processes of digestion and perform similar 

 functions in nourishing the body, it will be equally accurate 

 and more convenient, in treating of this part of physiology, 

 to speak of the different nutrients, and not of beef, mutton, 

 fish, eggs, bread, milk, butter, etc. 



From the point of view of digestion the most important 

 nutrients are the proteins, the carbohydrates, the fats, the 

 inorganic salts, and water-, and the student must at this 

 point become thoroughly familiar with what is meant by 

 these fundamental terms. 



5. The group of proteins. We may obtain a working idea 

 of what a protein is by recalling some of the foods in which 

 protein preponderates or is easily seen. Such foods are the 

 white of egg, the lean of tender meat (muscle fibers), the 

 curd of milk, the tenacious gluten of wheat. Proteins also 

 exist in relatively large quantities, though not so readily 

 seen, in yolk of egg, beans, peas, oats, and other grains. 



Proteins contain carbon, hydrogen, nitrogen, oxygen, and 

 sulphur. Some contain phosphorus and some contain iron. 

 Chemically they are exceedingly complex substances. It 

 should be noted that the proteins are the most important 

 nutrients which contain nitrogen and sulphur. 



Many proteins readily become insoluble. Examples of this 

 are the hardening of the white of egg or the lean of meat 

 by cooking and of the casein or curd of milk by rennet or 

 " junket tablets." This change is known as coagulation, and 

 most of our protein food is eaten after having been coagulated 

 in the process of cooking. 



Proteins occur only within the living cells of plants and 

 animals or as the products of these living cells. They form, 

 as we shall more clearly see later, an essential part of the 

 basis of the living cell and are constantly disintegrating 



