1 8 ANATOMY AND PHYSIOLOGY FOR NURSES 



as round or oval holes, which vary in diameter from 

 to 5~J-Q inch. Each canal, as a rule, contains two blood- 

 vessels, a small artery, and vein ; the larger ones also con- 

 tain a small quantity of connective tissue as well as bone 

 cells. The canals near the surface open upon it by minute 

 orifices, and those near the medullary cavity open in the 

 same way into the spaces, so that the whole of the 

 bone is permeated by a system of blood-vessels running 

 through the bony canals in the center of the Haversian 

 system. 



Periosteum. The periosteum is a fibrous vascular 

 membrane which adheres to the surface of the bones in 

 nearly every part, except their cartilaginous extremities. 

 Where strong tendons or ligaments are attached to the 

 bone, the periosteum is incorporated with them. It is 

 composed of two layers closely united together. In 

 young bones the periosteum is thick and very vascular, and 

 is intimately connected at either end of the bone with the 

 epiphyseal cartilages, but less closely with the shaft, from 

 which it is separated by a layer of soft blastema, containing 

 a number of granular corpuscles or "osteoblasts," in 

 which ossification proceeds on the exterior of the young 

 bone. Later in life the periosteum is thinner, less vascu- 

 lar, and the osteoblasts have become converted into a 

 connective-tissue layer, which is separated from the rest 

 of the periosteum in many places by cleft-like spaces, 

 which serve as a nidus for the ramification of the vessels 

 previous to their distribution in the bone; hence the 

 liability of bone to exfoliation or necrosis when denuded 

 of this membrane by injury or disease. Nerves and 

 lymphatics generally accompany the arteries in the peri- 

 osteum. 



Endosteum. The endosteum is a delicate connective- 

 tissue membrane lining the medullary and cancellous 

 cavities of the bone which contains numerous bone- 



