BONE. 43 



corpuscles, and canaliculi form a Haversian system. In a section 

 of bone several of these systems may be seen, the spaces between 

 them being occupied by interstitial lamellae. Lamellae which are 

 on the surface of the bone, parallel with its circumference, are 

 circumferential lamellce. A longitudinal section of bone shows 

 the Haversian canals to be what their name indicates, channels 

 running through the bone. Their communication with one another 

 is also seen. In each canal are an artery and a vein. 



If a piece of bone is treated with dilute nitric acid, so as to 

 dissolve the lime salts which it contains, or by some other method 

 of decalcification, a small portion may be torn off, which upon 

 examination shows the fibrous structure of the lamellae. Such 

 specimens also show the perforating fibers of Sharpey, which 

 hold the lamellae together; elastic fibers may also be observed. 



Lacuna. - 

 Canaliculi. 



Haversian canal. - 



FIG. 34. Portion of a transversely ground disk from the shaft of a human femur ; 

 X 400 (Bohm and Davidoff). 



Periosteum. This is a fibrous membrane which encloses the 

 bones except where covered by cartilage. It is made up of an 

 outer layer of connective tissue, in which there are blood-vessels 

 which give off branches that go to the Haversian canals ; and an 

 inner layer, in which elastic fibers are present. Between the peri- 

 osteum and the bone in young animals are nucleated cells, the 

 osteoblasts or bone-forming cells. 



Bone-marrow. Marrow is of two kinds, yellow and red. The 

 yellow marrow is found in the interior of the shafts of long bones, 

 in the medullary canal, and consists of fibrous tissue in which are 

 blood-vessels and cells, fat-cells principally, although somemamwr- 

 cells and myeloplaxes also occur. The composition of yellow mar- 

 row is : fat, 96 per cent, (no other structure of the body containing 

 so much, adipose tissue containing but 82.7 per cent.) ; areolar 

 tissue, 1 per cent. ; and 3 per cent, of fluid. Eed marrow (Fig. 35) 

 occurs in flat and short bones, the articular ends of long bones, 

 bodies of vertebrae, cranial diploe, sternum, and ribs. In structure 

 it resembles yellow marrow, except that fat-cells are few, while 

 marrow-cells are very abundant. Chemically it is composed of 



