THE STRUCTURE OF BONE. 



173 



an infinite number of minute canals known as the Haversian canals 

 which are from y 2 oo to Vasooth of an inch in diameter. These canals 

 are parallel to the length of the bone, and frequently communicate 

 together, forming an intricate net-work, which may be seen when 

 a section of the bone made lengthwise 

 is highly magnified, as shown in figure 

 66. In a cross-section of the bone 

 these canals appear as at fig. 67, being 

 surrounded by concentric layers, a, 

 through which radiating tubes, 5, of 

 the minutest size, penetrate. 



The periosteum is a highly vascular 

 and nervous membrane which covers 

 the entire bone, except the cartilagi- 

 nous surfaces which move upon each 

 other at the joints. It connects the 

 outer surface of the bones with the 

 tendons, ligaments, and muscles. 



The marrow is a fatty, pulpy sub- 

 stance, which fills the central canals 

 and the spongy tissue of the bones. It is pink in color, and 

 contains merely a trace of fat in young animals, but as age ad- 

 vances, it becomes yellow and less solid, and contains 96 per cent 

 of fat. The bones are penetrated and lined within and without 

 with numerous arteries, veins, and nerves. Some of these arteries 

 penetrate the bones by appropriate openings, and divide into 

 branches which form a net- 

 work that lines the inner 

 surface of the bones, and 

 another that penetrates the 

 substance of the marrow. 

 Other arteries penetrate the 

 spongy portions of the 

 bones, and others form a 

 net-work which belongs to 

 the periosteum, and which 



Fig. 66. THE HAVERSIAN 



CANALS. 



O 



Fig. 67. CROSS-SECTION OF BONE. 



enter the substance of the 



bones by means of the 



Haversian canals. Veins accompany these arteries, and are 



very numerous where the spongy tissue is abundant. Nerves are 



abundant in the marrow and the spongy tissue, but few in the 



compact tissue. Portions of the vertebra are remarkable for the 



numerous nerves they contain. By means of the arteries and 



