1. Atrophy

The bones are liable to various forms of atrophy. One of the commonest forms is that described as Atrophy from pressure, which, however, is not true atrophy, but an absorption of bone. It is illustrated by the case of Aneurysms which in their growth meet with bony structures, such as the vertebrae (see Fig. 253, p. 498), or the sternum and ribs. The bone is eroded, while any related cartilage is preserved. In this way the intervertebral cartilages or the cartilaginous ribs may be rendered unduly prominent by destruction of the bone. A similar erosion of the bone is sometimes produced in the skull by the Pacchionian bodies which may produce deep pouches in the internal table. Tumours also lead by pressure to destruction of bone.

Senile atrophy of the bones is a true atrophy. It consists in a general diminution in the organic matrix of the bone so that the proportion of lime salts increases, but there is at the same time frequently a diminution in thickness or an increased porosity of the bone. In old age the neck of the femur is frequently shortened and forms a wider angle with the shaft. The calvarium in old people is frequently diminished in thickness and very translucent, but this is by no means constant, nor is it limited to old people.

Atrophy from disuse and from nervous lesions occurs chiefly in cases of paralysis. In pseudo-hypertrophic paralysis, as shown by a specimen in the museum of the Western Infirmary, there is great atrophy of the bones of the lower limb. In infantile paralysis and locomotor ataxia there is also frequently atrophy of the bones at the joints. After Fractures which have not united, the bones may atrophy, sometimes from disuse but at other times from interference with the nutrient artery. In cases of atrophy from disuse, from whatever cause, the medullary spaces are enlarged by absorption of the bone. Thus, the shaft is thinned and the spaces in the cancellated tissue are enlarged. The increased bone marrow is a fatty marrow, and the bone as a whole contains more fat than normal, a kind of Fatty infiltration.