TEXT-BOOK OF PHYSIOLOGY. 



A thin, transverse section of a decalcified bone, when examined micro- 

 scopically, reveals a number of small, round, or oval openings, which repre- 

 sent transverse sections of canals which run through the bone, for the most 

 part in a longitudinal direction, though frequently anastomosing with one 

 another. These so-called Haversian canals in the living state contain blood- 

 vessels and lymphatics. (See Fig. n.) 



Around each Haversian canal is a series of concentric laminae, composed 

 of white fibers. Between every two laminae are found small cavities (lacunae) , 

 from which radiate in all directions small canals (canaliculi) , which com- 

 municate freely with one another. The Haversian canals, with their associ- 

 ated lacunae and canaliculi, form a system of intercommunicating passages, 

 which circulate lymph destined for the nourishment of bone. Each lacuna 

 contains the bone corpuscle, which bears a close resemblance to the usual 

 branched connective-tissue corpuscle, and whose function appears to be the 

 maintenance of the nutrition of the bone. 



Periosteum. 



Outer ground lamellae. 



Haversian canals. 



Haversian lamellae. 



Interstitial lamellae. 

 Inner ground lamellae. 



Marrow. 



FIG. ii. FROM A CROSS-SECTION OF A METACARP OF MAN. X 50. The Haversian canals 

 contain a little marrow (fat-cells) . Respiration line at h. (Stohr) . 



The surface of every bone in the living state is invested with a fibrous 

 membrane, the periosteum, except where it is covered with cartilage. The 

 inner surface of this membrane is loose in texture, and supports a fine plexus 

 of capillary blood-vessels and numerous protoplasmic cells the osteoblasts. 

 As this layer is directly concerned in the formation of bone, it is spoken of 

 as the osteogenetic layer. 



A section of a bone shows that it is composed of two kinds of tissue 

 compact and cancellated. The compact is dense, resembling ivory, and is 

 found on the outer portion of the bone; the cancellated is spongy, and appears 

 to be made up of thin, bony plates, which intersect one another in all direc- 

 tions, and is found in greatest abundance in the interior of the bones. The 

 shaft of a long bone is hollow. This central cavity, which extends from one 

 end of the bone to the other, as well as the interstices of the cancellated tissue, 

 is filled in the living state with marrow. The marrow or medulla is composed 

 of a connective-tissue framework supporting blood-vessels. In its meshes 

 are to be found characteristic bone cells or osteoblasts, the function of which 

 is supposed to be the formation of bone. In the long bones the marrow is 

 yellow, from the presence in the connective-tissue corpuscle of fat globules, 



