These include veal, chicken, beef, pork, lamb, etc. They must be differentiated into two groups, on the basis of whether they are glandular organs such as the liver and the kidney or highly specialized tissue such as the muscles or connective structures. It seems certain that we are justified in generalizing to the extent of saying that those organs which are the seat of metabolic activity of the type concerned with the elaboration of secretions or intermediate or end products of metabolism are more complete foods than are those characterized by inertness such as are the supporting and the contractile tissues. One would expect, in the light of the marked differences in the dietary properties of the vegetative parts of plants as contrasted with the storage tissues to find comparable differences in the nutritive value of animal tissues having different functions. Experiment has shown this to be warranted.

Watson (35) restricted birds to an exclusive diet of muscle meat, and observed profound pathological changes, especially nervous disturbances and paralysis (polyneuritis), hypertrophy of the thyroid gland to the extent of ten times its normal size, and hypertrophy of the parathyroids. He also studied the effects on rats of an exclusive diet of horse and of beef muscle (36) and found that not only did young animals die but that the older ones were distinctly inferior in their ability to produce and rear young.