LECTURE XVI. 



INORGANIC FOODS. 



L 



IMPORTANCE OF INORGANIC SUBSTANCES AS BUILDING MATERIAL OP 

 THE CELLS AND TISSUE. WATER, SALTS. 



ALL of the food-stuffs which we have studied up to the present, are those 

 by means of which the animal may obtain chemical energy. The con- 

 ception of a food is, in fact, closely related to this property. We recognize, 

 however, a group of compounds indispensable to the organism which it 

 always receives with its food, but from which it can obtain no chemical 

 energy. We refer to water and inorganic salts. Thus far we have con- 

 sidered the foods solely with regard to their value as sources of kinetic 

 energy. We must not forget, however, that the organism is constantly 

 wearing out its cell-material; in fact, individual cells may even be entirely 

 thrown off, only to be regenerated and built up anew. Such processes 

 are particularly noticeable in the case of growing organisms. In such 

 cases the new cells formed in place of the old ones are often of larger 

 dimensions. Yet, it must not be thought that the fully developed 

 organism retains its cellular condition unchanged. At present we are not 

 in a position to explain fully the metabolism within the individual cells. 

 We have no means of knowing how long a single cell may live; we do not 

 know how long it can exercise its function with the same material. All 

 that we can say is that there are certain processes visible to the eye which 

 give indication of a continuous breaking down and building up of cells. 

 We know that hair, feathers, scales, etc., undergo constant changes, 

 processes which take place in some species of animals very slowly, but 

 continuously; while in other cases, as with the shedding of feathers in the 

 case of birds, and the changing of skins with reptiles and amphibia, such 

 processes take place within a relatively short time. Again, we know of 

 the constant change in the cells of the epidermis, and in the cells of the 

 mucous membrane. Similarly we know that there is a continual loss of 

 material involved in the exercise of the function of numerous glands. In 

 this connection we need merely mention the glands of the skin, the 

 sebaceous and sweat glands, the salivary glands, and the numerous little 

 mucous glands of the respiratory and digestive membranes. The same 



349 



