CLASSIFICATION OF BONES 177 



the hard material that it is, we have already pointed out the provisions 

 which exist in it for an ample supply of blood to circulate in its interior, 

 and repair the waste of tissue that is here as elsewhere constantly taking 

 place. How this is effected will presently be seen. 



If a very thin transverse section of bone (fig. 281) be made, and sub- 

 jected to the scrutiny of the microscope, it will be found to present a 

 definite order of arrangement of its several parts, conspicuous among which 

 are a number of openings -^ - 5 -oo to -^Q inch in diameter. These are the 

 Haversian canals, so called from the name of the person (Havers) who first 

 detected them. The Haversian canals are each surrounded by a group 

 of bony rings arranged concentrically or one outside another. In and 

 between these rings will be noticed a number of small spider-like bodies 

 (lacunae) from which fine dark lines (canaliculi) radiate in all directions. 

 If a similar section be made longitudinally, and inspected under a similar 

 power, what in the first appeared as openings will now come into view as 

 tubes traversing the bone tissue (fig. 282), and dividing and reuniting; the 

 same dark lacunae and canaliculi intervening between them. 



The Haversian canals are so many channels for the accommodation of 

 blood-vessels, by which the circulation in the bone is carried on. 



The lacunse are small corpuscles or spaces containing a mass of living 

 protoplasm, and the fine lines proceeding from them are minute channels 

 which communicate with each other and with the Haversian canals, into 

 which some of them open. These channels serve the purpose of distribut- 

 ing nutritive matters for the support of the bone tissue. 



CLASSIFICATION OF BONES 



Bones are divided into three classes, distinguished as long bones, flat 

 bones, and irregular bones. 



Long bones make up the extremities, where they give support to the 

 body, and act as so many levers in the function of locomotion. Each long 

 bone is composed of a central portion or shaft and two extremities. The 

 former is the more compact and narrow, the latter is chiefly formed out of 

 spongy tissue, and is broad, and yields an articular surface covered with 

 cartilage. 



Flat bones, for the most part, enter into the formation of cavities con- 

 taining important organs, as the cranium, the chest, and the pelvis. 



Irregular bones are distinguished by their many angles and depres- 

 sions, such as the vertebrae, and the bones of the knee and the hock -joint. 

 They are mainly composed of cancellated tissue enclosed in a dense outer 

 layer of compact structure. 



VOL. II. 43 



