Page:Popular Science Monthly Volume 68.djvu/430

 426 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 difference in this regard is one of degree only. The number of known synthetic processes occurring in the animal body is constantly increasing, and the formation of the complex fat molecule from the comparatively 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 convincing; 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, however, has given place to the view that all the absorbed fat is first converted 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 contact 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