12 



OSTEOLOGY. 



to support the marrow, the nutrient or medullary arteries, enter- 

 ing the bones by the so-called nutrient foramina, being chiefly 

 distributed in it. The periosteum covering the bones of the 

 cranial vault is called the pericranium. 



CONTENTS OF BONE. 



Bones contain Marrow, Connective Tissue, Blood-vessels, 

 Lymphatics, and Nerves. 



Marroio, as found in the shafts of long bones, is a soft 

 yellowish adipose or fatty material, which is contained in a 

 delicate layer of connective tissue, and supported by inflected 

 processes of the same. A few cells are found in yellow marrow. 

 Cancellated tissue contains what is termed the red marrow, a 

 substance consisting for the most part of marroiu-cells, which 

 are round and nucleated, but also containing some smaller objects 

 resembling embryonic blood-corpuscles, and some ver}^ large cells. 

 These latter contain numerous nuclei, and they are known as the 

 giant cells, otherwise iiiyeloplaxes. 



The connective tissue lining th^ medullary canal and cancelli 

 forms the above-mentioned endosteum. The large bones of most 

 birds in adult life contain air instead of marrow, but in the 

 bones of a mammal in perfect health there is a considerable 

 quantity of the latter, Avhich becomes diminished in disease. 



Blood-vessels are numerous in bone tissue ; the arteries rami- 

 fying in the periosteum gain the Haversian canals, the medullary 

 artery enters by the nutrient foramen, and the arteries of the 

 cancellated tissue pass through foramina situated near the 

 articular surfaces. The veins are numerous, and, according to 

 recent observation, do not generally accompany the arteries, but 

 occupy separate canals ; the diploe in the cranial bones contains 

 large dilated veins. Lymphatics exist, but little more is as yet 

 known of them than that lymph spaces are found in the Haversian 

 canals. Nerves likewise exist in osseous tissue, in its coverings, 

 and also in marrow — accompanying the various blood-vessels. 



CLASSES OF BONE. 



Bones are classed as long, fiat, and irregular. Long or 

 cylindrical bones are found in the extremities, where they serve 

 as levers, and pillars of support. Descriptively, a long bone is 

 divisible into a centre or shaft and extremities. The shaft is 



