_Skeleton._--The atrophy characteristic of senescence is well shown in the fixed tissues of the bony skeleton. Though the bones do not as a rule alter materially in size or shape, they do so markedly in substance from rarefaction and absorption, the latter taking place mainly from the inside of the bones and especially the cancellous tissue, the medullary cavity and the Haversian canals becoming larger (senile osteoporosis). Hence fractures near the joints, particularly intracapsular fracture of the neck of the femur, are favoured. The absorption of the alveolar border of the jaw is intimately connected with the loss of the teeth, and brings the mental foramen to the top of the edentulous mandible. The angle of the jaw at the junction of the body and the ramus now opens out and comes to resemble that of an infant. It is often stated that the angle of junction of the neck and shaft of the femur becomes less, more of a right angle, but Humphry regarded this as exceptional.
From muscular weakness the back becomes bent and as a result the vertebrae become altered in shape. Ossification of the anterior common spinous ligament--a change analogous to rheumatoid osteophytes around the more movable joints--is an added and not uncommon change but, like calcification of the costal and laryngeal cartilages, it is a morbid process, and when advanced constitutes spondylitis deformans. The intervertebral discs undergo some loss of elasticity and atrophy, thus contributing to the loss of height. The cranial bones are usually thinned, and the parietal bones may show symmetrical or nearly symmetrical oval areas of excessive absorption of the outer and even of the inner table, so that the epicranium and the dura mater may be in contact. These areas which are close to the longitudinal fissure must not in the case of ancient skulls be mistaken for examples of early trephining. The bony sutures tend to become obliterated. Instead of thinning and loss of weight the skull, especially the vault, may show thickening and be heavier than normal--a change involving chiefly the inner surface and ascribed by Sir George Humphry to shrinkage of the brain.
Calcification of the costal and laryngeal cartilages, which have a yellow tint from fatty change, is, like a similar change in the arteries, pathological and not part of the process of senescence. Thus among 10 recorded necropsies on centenarians the costal cartilages were calcified in 2 only. Calcification of the costal cartilages interferes with the respiratory movements and was regarded by Sir George Humphry, who tested for it by estimating the elasticity perceived when gentle pressure is exerted on the lower part of the sternum, as a bad omen for the future.