III. NUTRITIVE FUNCTIONS. 



DIGESTION. 



HAVING considered the composition of the body and food, there 

 may now be taken up the study of the nutritive functions. 



As has been noted already, the body is constantly producing 

 energy and undergoing waste, both of which require the taking of 

 food. But food is of absolutely no use to the body until it reaches 

 the blood and by this fluid is conveyed to the tissues. So long as 

 the food remains within the alimentary canal it is as much out- 

 side the body, so far as nutrition is concerned, as if it had never 

 been taken inside. To be of any service the food must enter the 

 blood, and it does this by being absorbed. 



In some forms of animal life the food is of such a nature that 

 it readily and without further change undergoes absorption that 

 is, passes through the walls of the absorbing vessels. In other 

 forms of animal life this is not the case : in the latter, unless 

 certain changes take place, the food passes out of the alimentary 

 canal as waste material, without having contributed to the nutri- 

 tion of the body in the slightest degree. Unless, therefore, some 

 provision was made to obviate this, such animals would die of 

 starvation. The provision which has been made consists in the 

 presence of certain organs whose duty is to change the form of 

 the food-substances from that in which they will not, into that in 

 which they will, be absorbed ; and into such forms that when they 

 reach the tissues they can be utilized by them. It is this prepara- 

 tion for absorption which constitutes digestion, and the organs 

 concerned in bringing about the necessary changes in the food are 

 the digestive organs. 



Manifestly, these organs will be simple or complex according 

 to the amount of change which it is necessary to bring about in 

 the food in order that absorption may take place. Thus, if the food 

 on which an animal relies for its sustentation is already in a proper 

 form, no change will be needed, and the animal will therefore have 

 no digestive organs. If the requisite change is a slight one, the 

 number of the digestive organs will be few and their structure 



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