METABOLISM 37 



fluids. Examples of such substances are the tears, which when poured 

 out upon the surface of the eyeball serve to keep it moist; other fluids, 

 which also serve to moisten or lubricate internal surfaces; the digestive 

 secretions, which when passed into the alimentary canal assist in the 

 digestion of food; and also substances known as internal secretions. 

 These internal secretions are carried over the body and perform various 

 functions in connection with the carrying on of life activities, such as the 

 regulation of metabolism and the control of growth processes. 



56. Excretion. — Some products of dissimilation, such as urea, water, 

 and carbon dioxide, seem to be of no use to the organism and are termed 

 excretions. The process by which they are passed out of the ceU which 

 forms them is termed excretion. In many cases the excretions are poured 

 out directly upon the surface and are immediately disposed of; in other 

 cases, however, they are formed in the organism at some distance from the 

 external surface and have to be transported to some particular part of 

 the body before they can be passed out. Here again the circulation 

 comes into play, it being as necessary for the carrying of waste matters 

 to the point where they are passed out of the organism as for the trans- 

 portation of food and oxygen to the cells. 



57. Expiration. — The carbon dioxide formed in dissimilation is carried 

 by the circulation to some particular part of the organism where it 

 is passed out. This part may be in the lower animals the general body 

 surface or some particular structure within the body; in the higher ani- 

 mals it is the gills or lungs. This process is expiration. Here also a 

 distinction may be made between internal expiration, which is the passage 

 of carbon dioxide out of the tissues into the blood, and external expiration, 

 which is its passage from the body (Fig. 12). Expiration relieves the 

 organism of its gaseous waste. 



58. Elimination. — ^Liquid waste may be eUminated from any point 

 on the body surface, or it may be passed out by some particular structure. 

 In the higher animals the kidneys and skin are the principal organs of 

 elimination, though some elimination may occur through the walls of 

 the alimentary canal toward its posterior end. In this last case ehmina- 

 tion should not be confused with egestion. As an example of the differ- 

 ence between excretion and ehmination may be mentioned the fact that 

 urea is produced in the body in the liver, where excretion proper takes 

 place, but it is very largely eliminated by the kidneys. 



59. Egestion. — Egestion is the passing from the body of indigestible 

 materials contained in the food, which are known collectively as feces. 

 Feces might be referred to as soHd waste, but they have not, properly 

 speaking, been involved in the process of metabolism as have the sub- 

 stances which are expired or eliminated. The material egested has been 

 passed through the body but has at no time been a part of it. Egestion, 



