Bone or osseous tissue consists of phosphate and
carbonate of lime, salt, and a few other earthy substances. Hollow bones
contain marrow, a fatty substance with delicate connective tissue, except
where it has been driven out by the penetrating air sacs. On the surface
of a bone, covered by a fibrous membrane, the periosteum, there open small,
often microscopic, holes, which as "Haversian Canals" are continued through
the walls of the bone into larger spaces or cancelli, and ultimately into
the marrow cavity. These render possible the entrance of blood-vessels,
air-cells, and nerves.
Bones which have their entire substance or diploe
between the outer and the inner lamella filled with cavities and cancelli
cancellated or spongy bones; this is especially the case
in the bones of the head of owls, and to an enormous extent in the "horn"
of the Hornbills. The bony substance forms consecutive layers around the
Haversian canals. The layers themselves contain numerous irregular lacunae,
formerly but incorrectly called bone-corpuscles, from which radiate numerous
extremely fine canaliculi; these communicate with those of neighbouring
lacunae and with the Haversian canals, securing thus access of blood and
lymph to any part of the bone.
Bone Formation and Development
Bone is not directly formed out of the indifferent
embryonic tissue, it always passes through a stage of connective tissue.
If this tissue ossifies directly, it becomes a primary or membrane bone:
if the tissue is cartilage and finally supplanted by bony tissue,
the latter forms a secondary or cartilage bone. Most of the bones
of a bird's skeleton pass during their development through such a cartilaginous
stage. Membrane bones are principally some of those forming the cranium,
as the parietal, frontal, maxillae, and vomer. Bones which are developed
in tendons by direct ossification are termed sesamoid bones, as
the brachial and the crural patella.
Either kind of bone can ossify from various centres,
but these "centres of ossification" do not necessarily indicate that the
bone in questions is composed of a number of originally separate bones.
In long bones especially the shaft ossifies first, while the ends remain
for a long time cartilaginous as "epiphyses" and eventually ossify often
from a centre of their own, and are only in the adult completely fused
with the shaft, forming the articulating facets, or projecting "processes"
for the attachment and leverage of muscles.