90 THE VERTEBRATE ANIMAL: METABOLISM 



In conclusion, it may be stated that excretion is the ehmination 

 of compounds that have resulted from metaboHc reactions of cells. 

 The materials appearing as excretions have been a part of the 

 protoplasmic system. Excretion, as a process, should not be con- 

 fused with egestion, which is the elimination of undigested or 

 undigestible substances from the digestive tract by way of the 

 anus. Most of the egested materials have never been within 

 the cells of the animal from which they are discharged, but some 

 water and the substances giving odor and color to the faeces may 

 have been. 



Secretions. — It has been pointed out that the protoplasmic 

 constituents are produced, within cells, from the end products of 

 digestion by the reactions of assimilation. Oxidations that 

 break down compounds in the protoplasm liberate energy, furnish 

 heat, and give rise to waste products known as excretions. In addi- 

 tion to these types of reactions, certain substances that are 

 necessary for the performance of bodily functions are formed in 

 protoplasm. These substances are called secretions, and the 

 process of elaborating and passing them out of cells is termed 

 secretion. There is no evidence to indicate whether these com- 

 pounds are made from food materials entering the cells or from 

 intermediate products in dissimilation. It is possible that both 

 types of reactions occur. Secretions, in the nature of enzymes 

 catalyzing protoplasmic reactions, are formed within all cells 

 and used within the cell where they are formed. Certain cells, 

 however, produce secretions that go out of the cells to be used 

 elsewhere in the body. The juices that are secreted into the 

 digestive tract have already been mentioned. These contain 

 materials that serve to soften the food, to render it acid or alkaline, 

 to change its physical state, as in the case of emulsification of fat 

 by bile, and to catalyze the chemical reactions of digestion. These 

 juices are produced in groups of cells composing the digestive 

 glands, and the secretions are emptied into the digestive tract by 

 way of ducts or slender tubes (Fig. 53). The oil glands of the skin 

 likewise discharge their secretions by way of ducts. Sweat glands 

 and kidney tubules are often said to secrete perspiration and urine; 

 but since the substances passed out are not built up in these 

 regions, and also since they are in the nature of waste products, 

 the processes are not functionally comparable. It is, therefore, 

 better to say that the kidney excretes than to say it secretes urine. 



