A group of 50 normal male rabbits kept under conditions of constant light that had none of the shorter ultra-violet rays and another group kept in constant darkness for 2 to 12 weeks were observed clinically and subjected to postmortem examination for the purpose of determining the effect of these environmental conditions upon general body health and the weights of organs. A similar group of 50 rabbits caged in an ordinary animal room for the same period, and two groups of 40 and 20 rabbits respectively, which had recently been brought into the laboratory, served as controls.

It was found that the general health of the rabbits was not impaired by the artificial light or the exclusion of light. The gain in body weight which occurred in all groups was especially marked in the case of those kept under conditions of constant light.

The incidence of spontaneous disease recognizable clinically during the experiment was extremely low and of a mild character and did not obviously disturb the health of the animal. It was found at postmortem examination, on the other hand, that 59.3 per cent of the rabbits caged indoors, that is in the light, dark, or unaltered rooms, and 58.3 per cent of those recently brought to the laboratory had visible lesions of some kind. The great majority of these lesions, however, were of a slight grade, and none appeared to have any deleterious effects upon the general physical state of the animals.

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