BONE. 



59 



shows the periosteum on its outer surface. In favorable specimens it is 

 seen to include an outer vascular, rather loose connective tissue layer, 

 and an inner dense fibre-elastic layer, in which elastic elements predom- 

 inate. Into this layer the tendons are inserted, which means that they 

 blend with it and may contribute to the fibers penetrating the bone. The 

 innermost cellular layer of the periosteum has become reduced to oc- 

 casional osteoblasts. These may multiply after an injury; in young 

 individuals, if the periosteum is slit and the shaft of bone shelled out, they 

 may produce a new bone. The cross section further shows the contents 

 of the Haversian canals, which include one or two blood vessels, and a 

 few connective tissue or fat cells. 

 Nerve fibers which are found in 

 the periosteum, where they some- 

 times terminate in lamellar cor- 

 puscles (page 107), have been de- 

 scribed as extending into the 

 Haversian canals. They are not 

 easily detected there. Lamellae 

 may be observed as indistinct 

 layers. They are said to be due 

 to the differences in direction of 

 the fibrils which they contain, 

 as shown under polarized light. 

 They may also represent differ- 

 ences in texture, from variations 

 in the food supply at the time of 

 their formation. The lacunae 

 may appear either in or between 

 the lamellae. They are nearly 

 filled by the bone cells, which, 

 however, are seldom well pre- 

 served. The cells are generally flattened, parallel with the lamellae, and 

 are provided with processes extending into the canaliculi. They do not 

 fill them and it is supposed that tissue fluids may circulate through the 

 lacunae and canaliculi. Wandering blood cells are too large to enter them. 

 The lymphatic vessels are limited to the superficial layer of the periosteum. 

 The blood supply of bone is abundant. One or more nutrient arteries 

 enter a bone through its periosteum and break into branches which run 

 in the Haversian canals, thus extending through to the marrow cavity 

 in which they ramify freely. The blood vessels and Haversian systems 

 are parallel with the long axis of the bone, so that they are cut across in 



FIG. 64. CROSS SECTION OP COMPACT BONE, FROM 

 THE SHAFT OP THE HUMERUS, SHOWING THREE 

 HAVERSIAN SYSTEMS AND PART OF A FOURTH. 

 (Sharpey, from Bailey's " Text-book of Histol- 

 ogy.") 



