474 THE METABOLIC PROCESSES OF THE BODY. 



olism. And as a matter of fact not only in adipose tissue, but in every 

 part of the body, living substance is continuously giving rise to and tempo- 

 rarily depositing in itself some amount of fat, and in what is known as fatty 

 degeneration there seems to be evidence of the formation of fat out of 

 proteid material. 



On the other hand, we have traced the fats taken as food, and found that 

 they pass with comparatively little change from the alimentary canal, chiefly 

 through the intermediate passage of the lacteals,into the blood, from which 

 they rapidly disappear after a meal. We might infer from this that an 

 excess of fat thus entering the blood would naturally be disposed of by 

 being simply stored up in the available adipose tissue without any further 

 change ; we can imagine that the fat not immediately wanted by the 

 economy passes in some way from the blood to the connective tissue (the 

 white blood-corpuscles which appear loaded with fat after a meal possibly 

 acting as intermediaries), and that the connective-tissue corpuscles swallow 

 the fat brought to them after the fashion of an amoeba, not digesting it but 

 simply keeping it in store until it is wanted elsewhere. 



What do experiments teach on this matter? 



In the first place, it is evident that in an animal fattened on ordinary fat- 

 tening food, only a small fraction of the fat stored up in the body can possi- 

 bly come direct from the fat of the food. Long ago in opposition to the 

 views of Dumas and his school, who taught that all construction of organic 

 material, that all actual manufacture of living substance or even of its 

 organic constituents, was confined to vegetables and unknown in animals, 

 Liebig showed that the butter present in the milk of a cow was much greater 

 than could be accounted for by the scanty fat present in the grass or other 

 fodder she consumed. He also urged as an argument in the same direction, 

 that the wax produced by bees, which though having a different composition 

 from fat may be used as an analogy, is out of all proportion to the wax or 

 allied bodies contained in their food, consisting as this does chiefly of sugar. 

 And it has since been shown in many ways that, in fattening animals, the 

 fat accumulated in the body cannot be accounted for by the fat which has 

 been taken in the food. It has been proved by direct analysis. Thus of 

 two young pigs, as much alike as possible, of the same litter, one was killed 

 and analyzed, the amount of fat in the body being among other things deter- 

 mined. The other was fattened for a certain length of time on food whose 

 composition was known, and then killed and analyzed. It was found that 

 for every 100 parts of fat in the food 472 parts of fat were stored up in the 

 body during the fattening period. It is clear that fat may be formed in the 

 body out of something which is not fat. 



421. There are two possible sources of this manufactured fat. The 

 carbohydrates of the food form one source. In treating of digestion ( 243), 

 we referred to the possibility of carbohydrates during digestion in the alimen- 

 tary canal becoming by fermentation converted into butyric acid ; and we 

 suggested that higher and more complex members of the same fatty acid series 

 might be obtained out of carbohydrates by somewhat analogous changes, 

 carried on, however, not in the alimentary canal by means of foreign organ- 

 ized ferments, but in the tissues through the activity of the tissues them- 

 selves. We cannot as yet trace out the steps nor can we definitely point to 

 any particular tissues other than the fat-cells themselves as the seats of any 

 such changes. But there can be no doubt that the carbohydrate material 

 does in some way or other give rise to fat. A carbohydrate diet is the kind 

 of diet most efficacious in producing an accumulation of fat in the body ; 

 sugar or starch, in some form or other, is always a large constituent of 

 ordinary fattening foods. 



