Ulcerative lesions of the rumen with papillary epithelial hyperplasia occur in a large proportion of rats maintained on a deficient diet, and are rarely, if ever, present when the diet is complete.

It has not been possible to ascribe the lesions to lack of any of the known dietary components.

The fact that rats on improper diets tend to eat hair, that hair fragments have been found in a number of instances embedded in the ulcers, and surrounded by a definite inflammatory reaction, and finally, that the ulcers have been experimentally produced in rats on a complete diet, when chopped hair was added to the food, lead us to believe that the ingested hair is an important factor in the causation of the lesions. Whether the hair splinters act merely as mechanical irritants, or as the carriers of an infective agent, has not been determined. It is probable, moreover, that the deficient diets in some way that we do not as yet understand, intensify the injury caused by the hair. However produced, the lesions are so frequently found that they should be taken into account in experimental work on rats, especially in judging of the effects of dietary influences by the weight curves. That the presence of severe ulcerative lesions in the prostomach must unfavorably affect the general nutrition of an experimental animal, seems beyond question.

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