Bone Development in Birds

Including Formation of Spongy Bones

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.

Spongy Bones

Bones which have their entire substance or diploe between the outer and the inner lamella filled with cavities and cancelli are called 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.


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