h of the epiphyses a disc of actively growing
cartilage--_the epiphysial cartilage_; and at the junction of this
cartilage with the shaft is a zone of young, vascular, spongy bone known
as the _metaphysis_ or _epiphysial junction_. The shaft is a cylinder of
compact bone enclosing the medullary canal, which is filled with yellow
marrow. The extremities, which include the ossifying junctions, consist
of spongy bone, the spaces of which are filled with red marrow. The
articular aspect of the epiphysis is invested with a thick layer of
hyaline cartilage, known as the _articular cartilage_, which would
appear to be mainly nourished from the synovia.

The external investment--the _periosteum_--is thick and vascular during
the period of growth, but becomes thin and less vascular when the
skeleton has attained maturity. Except where muscles are attached it is
easily separated from the bone; at the extremities it is intimately
connected with the epiphysial cartilage and with the epiphysis, and at
the margin of the latter it becomes continuous with the capsule of the
adjacent 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, b