6 MANUAL OF ELEMENTARY ZOOLOGY 



known electric eel ; and there are other processes, such as 

 secretion, its action in which has not yet been certainly 

 compared with any event in the lifeless world and is some- 

 times supposed to be of a kind peculiar to the living body. 

 We shall return to this question in a later chapter. 



It is thus characteristic of the living body to be 

 continually wasting away by disintegration. 



ii. incorpora- Clearly this could not go on indefinitely without 

 tion of Food : ' & . ,„, J . , 



(a) Absorption, some compensating repair. The waste is made 



good by the incorporation of food. Two 

 distinct processes may be recognised in incorporation — 

 absorption and assimilation. Before it can be absorbed 

 the food has generally to undergo a preliminary process 

 of digestion, whereby the solid or indiffusible nutriment 

 which it contains is made soluble and diffusible. The 

 food of all animals must contain the following materials : 

 (i) water, (2) certain inorganic salts, such as the chlorides 

 and phosphates of sodium, potassium, and calcium, (3) the 

 very complex compounds known as proteins. A protein 

 is a colloid substance consisting of carbon, hydrogen, 

 nitrogen, and oxygen, with small quantities of sulphur 

 and sometimes phosphorus. A familiar example is the 

 " albumen " which, mixed with water, forms white of egg. 

 The molecular structure of proteins is not yet fully under- 

 stood, 1 and they have not yet been made in the laboratory. 

 Besides these substances the food usually contains (4) 

 carbohydrates (sugars, starches, and related substances), (5) 

 fats. Proteins, carbohydrates, and fats belong to the class 

 of compounds known as " organic," which, in nature, are 

 found only in the bodies of plants and animals and in 

 their remains ; all animals therefore require for food the 

 bodies of plants or of other animals. The digested 

 materials undergo absorption into the substance of the 

 body, leaving the indigestible matter to be cast away as 

 the dung or faces. 



Incorporation, however, is not brought about simply by 



the absorption of digested matter. The food, 

 t!on!' 8almlla " after as before digestion, is not of the same 



composition as the substance to which it is 

 to be added. The flesh of a dead ox or. sheep differs 

 1 They are probably exceedingly complex linkages of ammo-acids. 



