ut 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.

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 o