Osborne and Mendel (37) have shown that muscle tissue is very deficient in the anti-neuritic factor, water-soluble B, and in the anti-ophthalmic factor, fat-soluble A and that it contains but very little of the anti-scorbutic factor, water-soluble C. Meat extracts were also deficient in these respects.

In marked contrast to the results with muscle tissues are those obtained with the glandular organs. McCollum and Davis (38) showed that an ether extract of pig kidney or of codfish testicles is a good source of fat-soluble A. They also showed that heart muscle is decidedly deficient in this substance as compared with kidney. These results for heart and kidney were verified by Osborne and Mendel (37). These authors also showed that liver is a good source of fat-soluble A, and water-soluble B. The liver, when fresh, is rich in anti-scorbutic properties (39). Osborne and Mendel have found that the proteins of muscle, liver, kidney and brain are complete as sources of amino acids for the support of growth when each serves as the sole source of protein in the diet.

These findings are in harmony with the studies of Cooper (40), who made a comparative study of the relative values of a series of food-stuffs for the prevention of polyneuritis in birds. The results were expressed in terms of grams of fresh substance per day necessary to prevent the development of the disease. These values are as follows: Ox muscle, 20; ox cardiac muscle, 5; ox cerebrum, 6; ox cerebellum, 12; ox liver, 3; sheep cerebrum, 8-15; fish muscle, more than 10; egg yolk, 3; cow's milk, more than 35; cheese, more than 8.

As a source of protein for the support of growth the kidney is greatly superior to muscle tissue. Liver appears to fall between these in its value as a source of amino acids.