=Caries.= Caries, a process of molecular disintegration, may occur in all bones, and in every part of their structure, though it generally affects the cancellous tissue. The carious bone is porous and fragile, of a gray, brown, or blackish color, partly broken down in softened masses, and partly hollowed out into cells, which contain a reddish-brown and oily fluid. Small portions of dead bone lie detached in the carious cavity. The periosteal and medullary membranes, and the bone around the carious portion, will be found extremely vascular, and in many cases, compact masses of osseous tissue are deposited around the carious cavity. Caries of bone occurs as a result of inflammation, and corresponds to ulceration of the soft tissues. It frequently results from chronic suppurative arthritis, when, from destruction of the articular cartilages, the disease attacks the cancellated structure of the extremity of the bone. =Necrosis.= The death of a portion of osseous tissue, or necrosis, although frequently accompanied with caries, is entirely distinct from it. It attacks principally the compact tissue, and is met with, therefore, most frequently in the shafts of long bones. The necrosed portion is of a dirty, yellowish-white color, and has a dull, opaque look; after exposure to the air, it gradually becomes of a green, deep brown, or black tint. Its boundaries are usually distinct, but sometimes are so imperceptibly lost in the healthy tissue, that it becomes difficult in the dead body to determine its exact limits. Necrosis results from causes which interfere with the nutrition of bone, as from suppurative periostitis, traumatic destruction of the periosteum, or osteitis. Ulcerative destruction of the surrounding soft parts, or the diminished vitality attending certain general diseases, as typhus, etc., may also result in necrosis. The death of a portion of bone is followed by inflammation at the dividing line, which finally results in the separation of the dead portion or sequestrum. This change is soon followed by the production of new bone, in which process the periosteum and medulla may take part. =Rachitis or Rickets=, is essentially a disease of mal-nutrition, most frequently affecting children between the first and third years, although it does also occasionally occur later. The lower extremities are the first to show the effects of the disease, by a curvature commonly referred to too early attempts at walking. A contortion of the bones of the pelvis, of the spine, the thorax, the upper extremities, and malformations of the skull, may follow in the course of the disease. The bone on analysis, shows a decided diminution in the quantity of phosphate of lime, and a uniform increase of fatty matter; fluoride of calcium always present in healthy born, is also wanting.