194 QUABTEKLY JOURNAL. 



tissue, but this organized tissue is itself exactly replaced] by the 

 food, so that the result is the same as though the food itself 

 were at once converted into the excretions* 



In the above exposition, I have taken the simplest form of ani- 

 mal nutrition, that of carnivorous animals, and in order to pre- 

 vent complication, I have represented this as more simple than it 

 is in reality, for I have omitted one essential ingredient of their 

 food, the fat. 



What becomes of the fat contained in the food of these ani- 

 mals 1 What is its use ? It is not found as fat in the excretions ; 

 it does not make part of the body of the animal, for in general, 

 carnivorous animals are remarkably destitute of fat. 



Nor can it contribute to the formation of the organized tissues, 

 for they all contain nitrogen, and fat is a compound of carbon, 

 oxygen and hydrogen. We cannot suppose that it abstracts nitro- 

 gen from the albuminous principles of the food, for these prin- 

 ciples contain nitrogen precisely in the necessary proportions for 

 the formation of the tissues. 



The fat is absorbed with the albuminous products of digestion, 

 passes into the chyle and ultimately into the blood. A portion is 

 in some cases deposited in the cellular tissue as fat, the rest disap- 

 pears. What becomes of it 1 What form does it assume '? 



The oxygen, introduced by the red disks, is not all consumed by 

 the decomposition of the tissues. A portion of it remains, which 

 combines in the blood with the carbon and hydrogen of the fat 

 and forms carbonic acid and water. The fat is thus burned in the 

 blood by the oxygen, and is thrown off in the shape of the two 

 compounds just named. It thus contributes to generate animal 

 heat, for by this slow combination as much heat is generated as 

 by their rapid combustion, attended with flame and light, when it 

 takes place in the air or in oxygen. Heat is also generated by 

 the union of oxygen with the carbon liberated in the decomposi- 

 tion of the tissues, but this decomposition is not in general suffi- 

 ciently rapid to liberate carbon enough to generate all the heat 

 required. 



Fat, then, is not a supporter of nutrition. It does not contri- 

 bute to the formation of organized tissues. It is a supporter of 

 respiration ; a means of generating heat. 



