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 spirochaete of syphilis; a condition of hyperaemia, such as that produced artificially by the application of a Bier’s bandage or that which accompanies a chronic leg-ulcer.