BOXE. 55 



are attached to the bone, the periosteum is incorporated with them. It consists 

 of two layers closely united together, the outer one formed chiefly of connective 

 tissue, containing occasionally a few fat-cells ; the inner one, of elastic fibres of 

 the finer kind, forming dense membranous networks, which can be again separated 

 into several layers. In young bones the periosteum is thick, and very vascular, 

 and is intimately connected at either end of the bone with the epiphysial cartilage, 

 but less closely with the shaft, from which it is separated by a layer of soft blas- 

 tema, containing a number of granular corpuscles or " osteoblasts." in which 

 ossification proceeds on the exterior of the young bone. Later in life the peri- 

 osteum is thinner, less vascular, and the osteoblasts have become converted into 

 an epithelial layer, which is separated from the rest of the periosteum in many 

 places by cleft-like spaces, which are supposed to serve for the transmission of 

 lymph. The periosteum serves as a nidus for the ramification of the vessels 

 previous to their distribution in the bone ; hence the liability of bone to exfolia- 

 tion or necrosis, when denuded of this membrane by injury or disease. Fine 

 nerves and lymphatics, which generally accompany the arteries, may also be 

 demonstrated in the periosteum. 



The marrow not only fills up the cylindrical cavity in the shafts of the long 

 bones, but also occupies the spaces of the cancellous tissue and extends into the 

 larger bony canals (Haversian canals) which contain the blood-vessels. It differs 

 in composition in different bones. In the shafts of adult long bones the marrow 

 is of a yellow color, and contains, in 100 parts, 96 of fat, 1 of areolar tissue and 

 vessels, and 3 of fluid, with extractive matter, and consists of a matrix of fibrous 

 tissue, supporting numerous blood-vessels and cells, most of which are fat-cells, but 

 some few are "marrow-cells." In the flat and short bones, in the articular ends 

 of the long bones, in the bodies of the vertebrae, in the cranial diploe, and in the 

 sternum and ribs, it is of a red color, and contains, in 100 parts, 75 of water and 

 25 of solid matter, consisting of albumen, fibrin, extractive matter, salts, and a 

 mere trace of fat. The red marrow consists of a small quantity of areolar tissue, 

 blood-vessels, and numerous cells, some few of which are fat-cells, but the great 

 majority roundish nucleated cells, the true "marrow-cells" of Kb'lliker. These 

 marrow-cells resemble in appearance the white corpuscles of the blood, though 

 they are larger and have a relatively larger nucleus and a clearer protoplasm, but, 

 like them, possess amoeboid movements. Amongst them may be seen smaller 

 cells (erythroblasts) which possess a slightly pinkish hue ; and it has been held 

 by Neumann that they are a transitional stage between marrow-cells and red 

 blood-corpuscles, while others believe them to be the direct descendants of the 

 nucleated embryonic blood-cells (see p. 127), and to be transformed into blood- 

 corpuscles by the loss of their nuclei. 



G-iant-cells (myelo-plaques, osteoclasts), large, multinucleated, protoplasmic 

 masses, are also to be found in both sorts of adult marrow, but more particularly 

 in red marrow. They were believed by Kolliker to be concerned in the absorption 

 of bone matrix, and hence the name which he gave to them osteoclasts. They 

 excavate small shallow pits or cavities, which are named Howship's lacunae, in 

 which they are found lying. 



Vessels of Bone. The blood-vessels of bone are very numerous. Those of the 

 compact tissue are derived from a close and dense network of vessels ramifying in 

 the periosteum. From this membrane vessels pass into the minute orifices in the 

 compact tissue, running through the canals which traverse its substance. The 

 cancellous tissue is supplied in a similar way, but by a less numerous set of larger 

 vessels, which, perforating the outer compact tissue, are distributed to the cavities 

 of the spongy portion of the bone. In the long bones numerous apertures may 

 be seen at the ends near the articular surfaces, some of which give passage to the 

 arteries of the larger set of vessels referred to ; but the most numerous and largest 

 apertures are for the veins of the cancellous tissue, which run separately from the 

 arteries. The medullary canal in the shafts of the long bones is supplied by one 

 large artery (or sometimes more), which enters the bone at the nutrient foramen 



