HISTOGENETIC AND CALORIFACIENT FOOD. 27 



The supply of food to animals requires a more complicated provision 

 than it does to plants, in which the elaborating organs, the Sourcesoffood 

 leaves, presenting themselves superficially, are always in for animals and 

 contact with the air, from which much of their nutrition is p a 

 derived. And as one portion after another becomes exhausted, it is re- 

 newed by simple mechanical agencies, such as the trembling of the leaf, 

 the warmth of the sun, or the winds. 



Food, therefore, comes spontaneously to plants, which need no powers 

 of locomotion. And though, as we shall hereafter find, muscular move- 

 ment requires as its essential condition the waste of tissue, it is not nec- 

 essary for their nutrition that plants should destroy organized substance. 

 But an animal must seek its food, and for this purpose is endowed with 

 locomotion, involving the destruction of tissue. In a chemical point of 

 view, plants are organizing, and animals destroying machines. Nor is 

 this general assertion controverted by the apparent exceptions which are 

 here and there presented, as, for example, that the herbivora can form 

 sugar and fat from food in which those substances did not pre-exist, and 

 the salts of the biliary acids, which are never found in plants. 



To obtain for animals the necessary supply of nutriment, the resources 

 of nature are displayed in the most wonderful contrivances. According 

 as their modes of life may be, one takes its food with its teeth, another 

 with its lips, another with its fore member, another winds around it its 

 whole body. The geometrical spider weaves a net, and lies in wait for 

 his prey ; the ant lion digs a pit in the sand. Some rely upon labor, 

 some upon force, some upon fraud. Man depends upon all. 



Viewed as regards its physiological distinction, the food is generally 

 considered as of two kinds : Histogenetic or tissue-making, and classification 

 Calorifacient or heat-making. Histogenetic food furnishes the f food int . 

 chemical substances carbon, hydrogen, oxygen, nitrogen, sul- a ndcaiorifa- 

 phur, chlorine, phosphorus, iron, potash, soda, lime, &c. Ca- cient - 

 lorifacient food furnishes carbon and hydrogen mainly. In consequence 

 of this chemical constitution, tissue-making food is sometimes called ni- 

 trogenized, and heat -making non-nitrogenized food. The former is also 

 sometimes designated nutritive, and the latter respiratory. 



It is, however, to be distinctly understood that these divisions are only 

 adopted for the sake of convenience, and that they have no natural foun- 

 dation. Thus it will be found, when we examine the functions which 

 the fats discharge, that though they are non-nitrogenized bodies, and are, 

 therefore, considered as belonging to the class of respiratory food, there 

 is every reason to believe that they are essentially necessary to tissue 

 development, and that the metamorphoses of nitrogenized bodies can 

 only go on in their presence. They are, therefore, as truly essential to 

 nutrition as are the latter substances. 



