426 POPULAR SCIENCE MONTHLY 



of which the animal as distinguished from the vegetable body is 

 capable. It was thought that animal chemistry was all of a sort 

 which would produce more fixed and stable compounds and convert 

 compounds of greater potential energy into those of little or none. On 

 the other hand, the synthesis of organic compounds was believed to be 

 confined to the vegetable kingdom. This distinction in the character of 

 the chemical processes in the two forms of living things was believed to 

 be one of their fundamental differences. It is still true that the 

 end products of animal metabolism are simple oxidized substances and 

 that plants are largely engaged in synthetic chemistry, but the differ- 

 ence in this regard is one of degree only. The number of known 

 synthetic processes occurring in the animal body is constantly increas- 

 ing, and the formation of the complex fat molecule from the com- 

 paratively simple and partly-oxidized sugar molecule is an instance of a 

 complex synthesis. To build up this fat molecule a number of sugar 

 molecules must be disintegrated and a portion of each must be taken 

 to be combined with others into the large molecule of neutral fat. 

 Another, but more simple, synthesis, to be referred to later, is the 

 synthesis of the neutral fat molecule from the fatty acid absorbed from 

 the small intestine. In this process three molecules of fatty acid are 

 used to make one molecule of neutral fat. 



Our text-books only a few years old tell how fats are absorbed from 

 the intestine by a process entirely different from that by which the 

 sugars and proteids are absorbed. 



The latter substances by an hydrolysis and cleavage are made 

 soluble and diffusible and in this dissolved form are absorbed. We 

 were told a different story of the fats — that while a portion of the fat 

 was really digested, i. e., converted into a fatty acid and glycerine and 

 thus absorbed — that the greater part was simply emulsified and that 

 the finely divided particles of fat were then ' swallowed whole ' by the 

 intestinal epithelium in some such way, to look for an illustration, as 

 the amoeba takes its food. The evidence for this seemed fairly con- 

 vincing; in the first place, the fat could be seen in the emulsified state 

 in the intestine in contact with the epithelial cells lining it. And in 

 the substance of these epithelial cells, as though just devoured from the 

 intestinal contents, were seen similar droplets of fat. This view, how- 

 ever, has given place to the view that all the absorbed fat is first con- 

 verted into fatty acid and absorbed in this form or perhaps partly also 

 as a soap, then reconverted into neutral fat. The older theory was 

 abandoned for the following reasons: No one saw the fat droplets 

 passing into the cell; none were seen in the border of the cell in con- 

 tact with the intestinal contents, but only at the base of the cell farthest 

 removed from the source of supply of fat. The same appearances in 

 the epithelial cells were noted if a dog was fed with no fat, but with 

 fatty acid instead, suggesting in this case certainly that the fat globules 



