Gelatin, which results from incipient hydrolysis of connective tissue and the organic matrix of bone, is a protein of peculiar nature which early came to the attention of chemists. Gelatin and the casein of milk were among the proteins first to be separated in a state of approximate purity. Voit attempted to substitute with gelatin a part or all of the meat protein in the diets of his dogs and discovered that gelatin alone as the sole source of nitrogen could not prevent loss of body protein. There was something lacking in its makeup which made it an incomplete food. Gelatin became, therefore, a substance of unique interest to physiological chemists.

In 1849 Millon (3) discovered a reagent which when added to most proteins gives a characteristic brick red color. Gelatin does not exhibit this property. In 1879 Nasse (4) demonstrated that the reaction of Millon is due to the presence in the protein molecule of the amino-acid tyrosin, which is one of the digestion products of many proteins. Gelatin, therefore, does not yield tyrosin when digested. It has still other deficiencies which were later discovered. Tryptophan and the sulphur-containing amino-acid, cystin, are both lacking in its molecule. Gelatin has, therefore, long been known to be an "abnormal" or incomplete protein, and its value in nutrition has been the subject of many investigations.

Orum (5) and Munk (6) confirmed the earlier observations that a part of the protein could be substituted by gelatin, but that when it forms the sole source of nitrogen there is always a deficit.

Kossel discovered the chemical nature of the protein molecule to be a chain-like structure consisting of a number of amino-acids linked together, the amino group of one being united to the carboxyl group of its neighbor in the chain through abstraction of the elements of water. Digestion consists of the addition of the elements of water and the dissolution of the union. The correctness of this view was proven by Curtius and by E. Fischer, through synthesis, under the guidance of Kossel's theory (7).