It is one of the curiosities of senile atrophy that whilst there is hardening or sclerosis of so many organs, the skeleton, the most solid part of our frame-work, becomes less dense, so that the bones are friable, the condition often leading to serious accidents in old people. The bones become porous, and lose weight. It is difficult to believe that macrophags, although they destroy softer elements such as nerve-cells or muscle fibres, can be able to gnaw through a hard material like bone impregnated with mineral salts. As a matter of fact, the mechanism of bone atrophy must be placed in a different category from the phagocytosis of other organs. It is brought about, however, by the agency of cells very like some of the macrophags. These cells contain many nuclei, and are known as osteoclasts. They form round about the bony lamellæ and lead to their destruction, but are incapable of breaking off fragments of bone and dissolving them in their interiors. Although the intimate mechanism of this destructive action is not thoroughly understood, it seems probable that the cells secrete some acid which softens bone by dissolving the lime salts. The process can be observed in the different varieties of caries of the bone, and in the bony atrophy of old age as is represented in Fig. 10. By the action of the osteoclasts, which themselves are macrophags, part of the lime in the skeleton is dissolved during old age and passes into the general circulation. This is probably a source of the lime which is deposited so readily in the different tissues of old people. Whilst the bones become lighter, the cartilages become bony, the inter-vertebrate discs in particular becoming impregnated with salts, so that the well-known senile malformation of the backbone is produced. [Illustration: FIG. 10.—Destruction by osteoclasts of bony matter in the sternum of a man aged 81 years. (From a preparation made by Dr. Weinberg.)] As a result of this displacement of lime in old age, the blood-vessels become modified in a distinctive fashion. Atheroma of the arteries is not invariable in old people, but it occurs extremely frequently. In this form of degeneration, lime salts are deposited in the walls of the cells, so that they become hard and friable. Several others, among whom I may mention Durand-Fardel and Sauvage, have laid stress on the coincidence of atheromatous lesions of the arteries and senile degeneration of the bones. The relations between the two alterations are very evident in the skull; the meningeal artery becomes sinuous and atheromatous, and the grooves on the inner side of the bones of the skull in which it runs, flatten out, and become larger because of other malformations.[18]