THE FRAMEWORK OF THE BODY 



21 



not be increased with enlarged size. The cylindrical portion 

 of a long bone is called the shaft, which is hollow for the 

 accommodation of the marrow, and also for the purpose of 

 obtaining a maximum strength with a minimum weight. 

 This principle is well illustrated in iron construction work, 

 where supporting columns are made hollow. 



6. Histology of Bone. If a thin section of bone be exam- 

 ined under the microscope, we discover that it is pierced by 



.1 



FIG. 3. STRUCTURE OF BONE 

 ENLARGED 



numerous fine tubes, about 

 which layers of bone-sub- 

 stance are arranged. Fig- 

 ure 3 gives a good idea of 

 this microscopic structure 

 of a human long bone. 

 The Haversian canals (a), 

 named in honor of their 

 discoverer, carry blood ; 

 the tiny radiating lines 

 represent the canaliculi (c) 

 (little canals), which carry blood and lymph to the lacunce (b) 



FIG. 4. SECTION PARALLEL TO THE SURFACE FROM 

 THE SHAFT OF THE FEMI it. MAGNIFIED 100 

 DIAMETERS. a, Haversian canals ; 6, c, Lacuna; 



2. Experiment. Obtain one of the long bones of the sheep or calf, 

 scrape it thoroughly clean, but do not detach the cartilage covering the 

 ends. Saw it through crosswise in the middle ; then saw the upper half 

 through lengthwise. (See Fig. 1.) 



6. What are the Haversian canals ? Canaliculi ? Lacuna; ? 



