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joint. It consists of two layers, an outer fibrous and an inner cellular layer; the cells, which are called osteoblasts, are continuous with those lining the Haversian canals and the medullary cavity.

The arrangement of the blood vessels determines to some extent the incidence of disease in bone. The nutrient artery, after entering the medullary canal through a special foramen in the cortex, bifurcates, and one main division runs towards each of the extremities, and terminates at the ossifying junction in a series of capillary loops projected against the epiphysial cartilage. This arrangement favours the lodgment of any organisms that may be circulating in the blood, and partly accounts for the frequency with which diseases of bacterial origin develop in the region of the ossifying junction. The diaphysis is also nourished by numerous blood vessels from the periosteum, which penetrate the cortex through the Haversian canals and anastomose with those derived from the nutrient artery. The epiphyses are nourished by a separate system of blood vessels, derived from the arteries which supply the adjacent joint. The veins of the marrow are of large calibre and are devoid of valves.

The nerves enter the marrow along with the arteries, and, being derived from the sympathetic system, are probably chiefly concerned with the innervation of the blood vessels, but they are also capable of transmitting sensory impulses, as pain is a prominent feature of many bone affections.

It has long been believed that the function of the periosteum is to form new bone, but this view has been questioned by Sir William Macewen, who maintains that its chief function is to limit the formation of new bone. His experimental observations appear to show that new bone is exclusively formed by the cellular elements or osteoblasts: these are found on the surface of the bone, lining the Haversian canals and in the marrow. We believe that it will avoid confusion in the study of the diseases of bone if the osteoblasts on the surface of the bone are still regarded as forming the deeper layer of the periosteum.

The formation of new bone by the osteoblasts may be defective as a result of physiological conditions, such as old age and disease of a part, and defective formation is often associated with atrophy, or more strictly speaking, absorption, of the existing bone, as is well seen in the edentulous jaw and in the neck of the femur of a person advanced in years. Defective formation associated with atrophy is also illustrated in the bones of the lower limbs of persons who are unable to stand or walk, and in the distal portion of a bone which is the seat of an ununited fracture. The same combination is seen in an exaggerated degree in the bones of limbs that are paralysed; in the case of adults, atrophy of bone predominates; in children and adolescents, defective formation is the more prominent feature, and the affected bones are attenuated, smooth on the surface, and abnormally light.

On the other hand, the formation of new bone may be exaggerated, the osteoblasts being excited to abnormal activity by stimuli of different kinds: for example, the secretion of certain glandular organs, such as the pituitary and thyreoid; the diluted toxins of certain micro-organisms, such as the staphylococcus aureus and the spirochæte of syphilis; a condition of hyperæmia, such as that produced artificially by the application of a Bier's bandage or that which accompanies a chronic leg-ulcer.

The new bone is laid down on the surface, in the Haversian canals, or in the cancellous spaces and medullary canal, or in all three situations. The new bone on the surface sometimes takes the form of a diffuse encrustation of porous or spongy bone as in secondary syphilis, sometimes as a uniform increase in the girth of the bone—hyperostosis, sometimes as a localised heaping up of bone or node, and sometimes in the form of spicules, spoken of as osteophytes. When the new bone is laid down in the Haversian canals, cancellous spaces and medulla, the bone becomes denser and heavier, and is said to be sclerosed; in extreme instances this may result in obliteration of the medullary canal. Hyperostosis and sclerosis are frequently met with in combination, a condition that is well illustrated in the femur and tibia in tertiary syphilis; if the subject of this condition is confined to bed for several months before his death, the sclerosis may be undone, and rarefaction may even proceed beyond the normal, the bone becoming lighter and richer in fat, although retaining its abnormal girth.

The function of the epiphysial cartilage is to provide for the growth of the shaft in length. While all epiphysial cartilages contribute to this result, certain of them functionate more actively and for a longer period than others. Those at the knee, for example, contribute more to the length of limb than do those at the hip or ankle, and they are also the last to unite. In the upper limb the more active epiphyses are at the shoulder and wrist, and these also are the last to unite.

The activity of the epiphysial cartilage may be modified as a result of disease. In rickets, for example, the formation of new bone may take place unequally, and may go on more rapidly in one half of the disc than in the other, with the result that the axis of the shaft comes to deviate from the normal, giving rise to knock-knee or bow-knee. In bacterial diseases originating in the marrow, if the epiphysial junction is directly involved in the destructive process, its bone-forming functions may be retarded or abolished, and the subsequent growth of the bone be seriously interfered with. On the other hand, if it is not directly involved but is merely influenced by the proximity of an infective focus, its bone-forming functions may be stimulated by the diluted toxins and the growth of the bone in length exaggerated. In paralysed limbs the growth from the epiphyses is usually little short of the normal. The result of interference with growth is more injurious in the lower than in the upper limb, because, from the functional point of view, it is essential that the lower extremities should be approximately of equal length. In the forearm or leg, where there are two parallel bones, if the growth of one is arrested the continued growth of the other results in a deviation of the hand or foot to one side.

In certain diseases, such as rickets and inherited syphilis, and in developmental anomalies such as achondroplasia, dwarfing of the skeleton results from defective growth of bone at the ossifying junctions. Conversely, excessive growth of bone at the ossifying junctions results in abnormal height of the skeleton or giantism as a result, for example, of increased activity of the pituitary in adolescents, and in eunuchs who have been castrated in childhood or adolescence; in the latter, union of the epiphyses at the ends of the long bones is delayed beyond the usual period at which the skeleton attains maturity.

Regeneration of Bone.—When bone has been lost or destroyed as a result of injury or disease, it is capable of being reproduced, the extent to which regeneration takes place varying under different conditions. The chief part in the regeneration of bone is played by the osteoblasts in the adjacent marrow and in the deeper layer of the periosteum. The shaft of a long bone may be reproduced after having been destroyed by disease or removed by operation. The flat bones of the skull and the bones of the face, which are primarily developed in membrane, have little capacity of regeneration; hence, when bone has been lost or removed in these situations, there results a permanent defect.

Wounds or defects in articular cartilage are repaired by fibrous or osseous tissue derived from the subjacent cancellous spaces.

Transplantation of Bone—Bone-grafting.—Clinical experience is conclusive that a portion of bone which has been completely detached from its surroundings—for example, a trephine circle, or a flap of bone detached with the saw, or the loose fragments in a compound fracture—may become, if replaced in position, firmly and permanently incorporated with the surrounding bone. Embedded foreign bodies, on the other hand, such as ivory pegs or decalcified bone, exhibit, on removal after a sufficient interval, evidence of having been eroded, in the shape of worm-eaten depressions and perforations, and do not become united or fused to the surrounding bone. It follows from this that the implanting of living bone is to be preferred to the implanting of dead bone or of foreign material. We believe that transplanted living bone when placed under favourable conditions survives and becomes incorporated with the bone with which it is in contact, and does not merely act as a scaffolding. We believe also that the retention of the periosteum on the graft is not essential, but, by favouring the establishment of vascular connections, it contributes to the survival of the graft and the success of the transplantation. Macewen maintains that bone grafts “take” better if broken up into small fragments; we regard this as unnecessary. Bone grafts yield better functional results when they are immovably fixed to the adjacent bone by suture, pegs, or plates. As in all grafting procedures, asepsis is essential.

Transplanted bone retains its vitality when embedded in the soft parts, but is gradually absorbed and replaced by fibrous tissue.

DISEASES OF BONE

The morbid processes met with in bone originate in the same way and lead to the same results as do similar processes in other tissues. The structural peculiarities of bone, however, and the important changes which take place in the skeleton during the period of growth, modify certain of the clinical and pathological features.

Definition of Terms.—Any diseased process that affects the periosteum is spoken of as periostitis; the term osteomyelitis is employed when it is located in the marrow. The term epiphysitis has been applied to an inflammatory process in two distinct situations—namely, the ossifying nucleus in the epiphysis, and the ossifying junction or metaphysis between the epiphysial cartilage and the diaphysis. We shall restrict the term to inflammation in the first of these situations. Inflammation at the ossifying junction is included under the term osteomyelitis.

The term rarefying ostitis is applied to any process that is attended with excessive absorption of the framework of a bone, whereby it becomes more porous or spongy than it was before, a condition known as osteoporosis.

The term caries is employed to indicate any diseased process associated with crumbling away of the trabecular framework of a bone. It may be considered as the equivalent of ulceration or molecular destruction in the soft parts. The carious process is preceded by the formation of granulation tissue in the marrow or periosteum, which eats away and replaces the bone in contact with it. The subsequent degeneration and death of the granulation tissue under the necrotic influence of bacterial toxins results in disintegration and crumbling away of the trabecular framework of the portion of bone affected. Clinically, carious bone yields a soft grating sensation under the pressure of the probe. The macerated bone presents a rough, eroded surface.

The term dry caries (caries sicca) is applied to that variety which is unattended with suppuration.

Necrosis is the term applied to the death of a tangible portion of bone, and the dead portion when separated is called a sequestrum. The term exfoliation is sometimes employed to indicate the separation or throwing off of a superficial sequestrum. The edges and deep surface of the sequestrum present a serrated or worm-eaten appearance due to the process of erosion by which the dead bone has been separated from the living.

Bacterial Diseases

The most important diseases in this group are the pyogenic, the tuberculous, and the syphilitic.

Pyogenic Diseases of Bone.—These diseases result from infection with pyogenic organisms, and two varieties or types are recognised according to whether the organisms concerned reach their seat of action by way of the blood-stream, or through an infection