The human skin is capable of great extension. It may be distended to four times its size, yet is not endowed with much elasticity. On this account we may notice, in very fat persons, rolls of fat about the neck, back, buttocks, arms and pubis. The epidermis, which constitutes the external layer of the skin, is but slightly capable of extension. When distended beyond a certain point, it tears, and produces those white streaks which are to be seen on the abdomen of pregnant women, or of those who have borne children, and also of those who have laboured under severe dropsical ascites. These white streaks may be formed upon all parts of the body, when the skin is considerably distended: thus they have been seen in a young woman twenty-eight years of age, who weighed three hundred and four pounds. In her case these white streaks were to be seen upon the arms, the shoulders, the breasts, &c. The skin of the abdomen would not be sufficient to retain the abdominal viscera _in situ_, were it not that between these organs and the integument there exists a fibrous or muscular layer, in some places double, consisting of a stronger and less extensible tissue than the skin, in order to strengthen the abdominal walls. It sometimes happens that this fleshy layer, having yielded to a certain amount of distension, occasioned by the volume of the intestines, and of their surrounding fat, and being thereby unduly stretched, permits the passing between its fibres of a certain portion of intestine or of fat, which, lying immediately under and pushing the skin before it, constitutes what is termed a hernia. Dropsy or pregnancy are frequently the primary cause of the various descriptions of hernia, termed inguinal, crural, &c. Umbilical hernia is that which is usually produced by a too great development of fatty tissue in the abdomen. The umbilicus is that part of the abdomen which is the least susceptible of dilatation. When the belly becomes enlarged to a moderate extent, the navel becomes depressed, shewing that this part does not easily yield to the pressure from within; but it is supported by the recti muscles, those two bands of fleshy fibres lying immediately beneath the skin, and passing from above downwards, on each side and close to the navel. In extreme development of the abdomen, these muscles are displaced from their normal position near the umbilicus, and no longer lend it support. The fibres of the umbilical ring are thus separated by the pressure exerted by the abdominal fat, and a portion passing through the fibres pushes the skin before it. A small protrusion takes place, which is not yet outwardly apparent, because the remaining fibres of the umbilical ring still afford considerable resistance, and retain the ring concealed in the deep hollow which is observable in the navel of fat persons. In order to determine the existence of umbilical hernia at this early stage, the patient should be placed in the recumbent position.