COVEBING 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 canceUi ; 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. 



— — _^ COYERING OF BONE. 



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

 fibro-vascular, inelastic membrane, the 'periosteum^ excepting 

 where tendons play over the bone, and its articular surfaces 

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

 firmly adherent to the bone, contains minute blood-vessels, 

 which are thickly distributed before entering the osseous tissue, 

 and it consists of two layers — aa outer one, fibrous and protect- 

 ive, and an inner one, which consists of fine connective tissue, 

 contains bone-producing cells, or osteohlastSj 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 devel- 

 oped 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 con- 

 tact after an oblique fracture ; in the young, it is thicker and 

 more vascular than in the adult. Blood-vessels which ramify 

 in the periosteum pass directly to the bone ; the external 

 surface of a bone is always studded with 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 nest 

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

 medullary cavities are hned by a more delicate vascu& mem- 

 brane, the endosteum. 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 

 to support the marrow, the nutrient or medullary arteries, enter- 



