In Chapter II (A Biological Method For The Analysis Of A Foodstuff. 17. Mccollum'S Experiments Not Verified By Osborne And Mendel) it was shown that it is impossible to secure satisfactory nutrition in the rat without including in the diet certain fats containing a substance indispensable for the normal processes of growth or for prolonged maintenance of well-being. In the light of the history of science the researches relating to the necessity of certain lipins in nutrition are of great interest. The more significant investigations in this field will now be considered from the standpoint of the present day knowledge of what constitutes the normal diet, and emphasis will be laid upon the relation of deficient diets to certain states of malnutrition.

Stepp (1) while a student in the laboratory of Hofmeister at Strassburg, performed the first classic experiment in this field. Since every animal and plant cell contains certain substances having the physical properties of fats, in addition to proteins, salts, carbohydrates and water, to name only the substances of primary importance, Stepp believed that these fat-like bodies, the lipins, must be considered to be primary and indispensable cell components. Because most food-stuffs of animal or vegetable origin contain these lipins they occur regularly in our diet. What would happen if they were eliminated from an otherwise adequate food mixture? The answer to this question Stepp sought to obtain by a series of experiments on mice.

The group of substances classified as lipins includes fats and certain other compounds which have more or less similar physical properties. They have a greasy feel, and are soluble in one or more of such solvents as ether, alcohol, benzene, chloroform and acetone. The best known of these lipins aside from ordinary fats are lecithin, cholesterol, phytosterol, cephalin and the cerebrosids. Lecithin contains combined in its molecule glycerin, fatty acid, phosphoric acid and an organic base called cholin. Cephalin closely resembles lecithin, but yields on hydrolysis a different organic base, amino ethyl alcohol. Cholesterol, chemically an alcohol, has certain physical resemblances to the fats. Cerebrosids are compounds containing in their molecules a fatty acid, a sugar (galactose) and an organic base (sphingosine). All of these are soluble in alcohol, but they differ in some extent in their solubility in the other solvents named. All of these substances play a prominent part in the composition of brain and nerve tissue.