Rickets is a constitutional disorder in which there are nutritional disturbances involving, to a greater or lesser extent, all the bones of the skeleton. The long bones become shortened, thickened, and twisted, and there is a lack of calcium salts, proper ossification not taking place. It makes its appearance most frequently in the first or second years of life, but may be delayed until several years later. The cause is not known, but it evidently depends upon poor hygiene and malnutrition, consequently it is more common among the poorer classes.

The head has a peculiar square shape and is large in proportion to the body. In infants the anterior fontanel may persist for three or four years, and the bones of the skull may contain localized areas in which there is a lack of mineral salts - craniotabes. The chest is usually prominent, coming to a distinct angle - "pigeon-breast" - and the anterior ends of the ribs at the junction with the cartilage are distinctly enlarged, forming the rachitic rosary. If the disease is severe, there may be involvement of the spinal column. If the curvature is forward, lordosis; if backward, kyphosis; sideways, scoliosis. These curvatures may occur in combination. The pelvis may show changes that have a direct influence upon childbirth; it is distinctly flattened. The anteroposterior diameter is decreased, while the transverse is increased. The tibiae and fibulae are usually much curved outwardly, the femur anteriorly. The deformities are due to the fact that the bones, lacking lime salts, are not stiff enough to resist the pull of the muscles nor outside pressure. The lesions are most marked at the extremities of the long bones. Microscopically there is seen at the extremities an irregular bone-formation with a deficiency of lime salts in many areas, while neighboring portions will show ossification. The normal laminated appearance is absent.

The marrow is of the red variety, similar to that in the fetus, being rich in cells. The increase in size of the heads of the long bones is due to an increase of the cartilage cells and enlarged marrow-spaces. Associated with the lesions of the bones are gastro-intestinal disorders and fibrous hyperplasia in both spleen and liver. There is a decrease in the erythrocytes and an increase in the leukocytes.