18 PHYSIOLOGY AND TEMPERANCE. 



The spongy portion forms the bulk of the shaft, and at 

 either extremity makes an expansion to form joint surface. 

 This spongy tissue consists of elongated bony cells, and the 

 walls of the cells give lightness and strength, while the tiny 

 channels afford a passage for the nutrient fluid to pass. In 

 the interior of the bone is lodged the marrow, composed of 

 fat and numerous blood-vessels, for supplying the central por- 

 tion of the bone with nourishment. The blood-vessels within 

 the bone are a continuation and subdivision of an artery, 

 which enters the bone through a small opening seen in all 

 bones, and called the nutrient foramen. 



FIG. 4. A thin slice of bone highly magnified, showing little central canals for the 

 blood-vessels, and tiny tubes for the passage of nutriment to the bone tissue. The 

 black specks are little cavities in the bone. 



If we examine a thin slice of bone under the microscope, 

 we find it is mapped out into a number of circular districts, 

 and in the centre of each district is a small canal for the 

 passage of a minute blood-vessel, These canals do not run 

 with 4ach ether, or with the ahftft of the bone. 



