COVERING OF BONE. 11 



Cancellated or spongy tissue is always situated internally. It 

 consists of a large number of fragile bony plates, with spaces 

 between them called the cancelli ; there are lacunae and canali- 

 culi, but no Haversian canals, their place being taken by the 

 cancelli ; in the bones of the cranium this tissue is called the 

 diploe. 



COVERING OF BONE. 



The external surface of every bone is covered by a tough, 

 fibro-vascular, inelastic membrane, the periosteum, excepting 

 Avhere tendons play over the bone, and its articular surfaces, 

 upon both of which there is a layer of cartilage. The periosteum, 

 firmly adherent to the bone, contains minute blood-vessels, v/hich 

 are thickly distributed before entering the osseous tissue, and it 

 consists of two layers — an outer one, fibrous and protective, and 

 an inner one, which consists of fine connective tissue, contains 

 bone-producing cells, or osteoblasts, and has been termed the 

 osteogenic membrane. The inner layer is continued into the 

 Haversian canals, a layer of cells also existing between the canal 

 wall and the contained vessel. It affords support and protection 

 to the bone, and attachment to tendons and ligaments, which 

 frequently become continuous with it. It varies in thickness, 

 being dense and strong on bones nearest the skin, and liable to 

 injury ; in the horse, it is well developed on the inner surface of 

 the tibia, and over the anterior surfaces of the metacarpal and 

 metatarsal bones. By its strength it sometimes retains the 

 fragments of a bone in contact after an oblique fracture ; in the 

 young, it is thicker and more vascular than in the adult. Blood- 

 vessels Avhich ramify in the periosteum pass directly to the bone ; 

 the external surface of a bone is always studded Avith numerous 

 foramina through which these enter. 



The periosteum, owing to its inelasticity, is, when inflamed, 

 the seat of intense pain ; and should any part of it be stripped 

 off, there is every probability of the denuded bone dying and 

 exfoliating. It is most vascular near the joints, where it termi- 

 nates by joining the articular cartilage, or passes to the next 

 bone, but it never covers an articular surface. The internal or 

 medullary cavities are lined by a more delicate vascular mem- 

 brane, the endosteiira, or medullary membrane, which is pro- 

 longed into the cancelli and Haversian canals. It is very thin, 

 consisting of delicate areolar tissue, filaments from which serve 



