If the morphological appearances of fatty infiltration and of fatty degeneration were constant, there would obviously be little or no difficulty in determining the nature of the process manifested by the presence of fat. The exceptions occur both in fatty infiltration and fatty degeneration. In the cells of the liver of an animal poisoned with phosphorus fat makes its appearance in large drops, while in the heart and kidneys of the same animal the fat is present in a granular form. During absorption from the intestine in the process of digestion fat is present in the epithelium in a finely granular form. When digestion is completed fat is no longer met with in these cells. The presence of large or small drops, therefore, cannot be regarded as a sufficient test of the origin of the fat. It is of equal, if not greater, importance to bear in mind the organ concerned. In the heart, liver, kidneys, and gastric glands, as well as elsewhere, with the exception, perhaps, of the mammary gland, the presence of many small fat-drops in the cells indicates a degeneration of its protoplasm. The presence of large fat-drops, on the contrary, in the organs and tissues, with the exception of the liver, indicates an infiltration. Large fat-drops, then, may be present in the cells of the liver as the result of an infiltration or of a degeneration. In order to form a satisfactory opinion of the {77} nature of the appearances in the liver in doubtful cases, it is important to note the condition of those organs which may be simultaneously in a state of fatty degeneration. The accumulation of fat under physiological conditions is obviously brought about, on the one hand, by those causes which permit a free introduction, absorption, and deposition, and, on the other, by those which check its oxidation or elimination with the secretions of the body, as the bile, in which it may be present to a considerable extent. A diet rich in fat, or in albuminates readily converted into fat, offers a favorable element for the absorption of fat by the healthy individual. If the organism demands but little of this fat for oxidation, as in the case of the sedentary person, an accumulation is likely to occur. This may become so considerable that obesity results. Tissues in which normally but little fat is accumulated may become infiltrated to a large extent. The intermuscular fibrous tissue thus becomes loaded, and the activity, as well as the nutrition, of the muscles is impaired. This accumulation may be manifested not only in the voluntary muscles, but in the heart as well, which may present abundant sub-pericardial and sub-endocardial fat, the myocardium also being interlarded with streaks of fat, the so-called fatty infiltration of the heart. The abdominal walls may become thickened to the extent of a couple of inches, and the mesentery, omentum, perinephritic tissue, and liver may become enormously increased in weight from the mass of accumulated fat.