ASSIMILATION 



81 



type of organism are not ordinarily usable as such by another kind 

 of animal. Therefore, food, in addition to containing the neces- 

 sary elements as outlined, must be utilizable by the form 

 eating it. That is, it must be possible for the food to be made 

 available for use in the building of the protoplasmic system of the 

 animal using it. The changes which food undergoes, in the body 

 of a vertebrate, before it is assimilated, are well known and will 

 be discussed under the headings of digestion and absorption. 



Digestion. — Diges- 

 tion may be defined as 

 the series of chemical 

 changes which food 

 undergoes in the diges- 

 tive tract in the pre- 

 sence of digestive en- 

 zymes. It is necessary 

 that the majority of 

 foodstuffs have their 

 complex molecules 

 broken down into their 

 simple structural units 

 in order that they may 

 be absorbed from the 

 digestive tract and 

 distributed to the cells 

 of the body; and also 

 in order that they ma}" 

 be transformed into 

 substances that can be 



Fig. 46. — This picture illustrates the emaciated 

 appearance of a middle-aged rat after being fed 

 about four months on a diet consisting of bolted 

 flour, dcgcrniinated corn meal, rice, sugar, 

 starch, pork fat, molasses, sweet potato, and 

 cabbage. 



This diet affords wide variety and consists of whole- 

 some food products, yet fails to maintain normal nutri- 

 tion because it contains too little of the protective foods, 

 milk, eggs and the leafy vegetables. (From McCollum, 

 "Newer Knowledge of Nutrition," copyright, 1922, by 

 the Macmillan Co., reprinted by permission. J 



assimilated by the protoplasm. Digestion, then, is a chemical 

 process. Closely associated with the chemical phenomena are 

 certain mechanical factors which will be indicated as the discus- 

 sion progresses, but which must not be confused with the essential 

 digestive reactions. 



Food enters the digestive tract at the mouth by a process known 

 as ingestion. In the mouths of the higher vertebrates the food 

 may be torn apart or ground into fine particles by the teeth, but in 

 lower forms the food is merely held by the teeth and no mechanical 

 disintegration occurs. The frog, for instance, merely retains its 

 food with the teeth until it can be swallowed, and no digestive 



