12 OSTEOLOGY. 



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 BONES. 



Bones contain Marrow, Connective Tissue, Blood-vessels, 

 Lymphatics, and Nerves. 



Marrow, 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 tibsue, and supported by inflective 

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

 row. Cancellated tissue contains what is termed the red 

 marrow, a substance consisting for the most part of marroio- 

 cells, which are round and nucleated, but also containing some 

 smaller objects resembling embryonic blood-corpuscies, and 

 some very large cells. These latter contain numerous nuclei, 

 and they are known as the giant cells, otherwise myeloplaxes. 



The connective tissue lining the 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, which becomes diminished in disease. 



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

 fying in the periosteum gain the Haversian canals, the medul- 

 lary 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 BONES. 



Bones are classed as long, flat, 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 cylindrical, and consists of a shell of compact tissue of 

 varying thickness, which encloses the cancellated tissue and 

 medullary canal, and is pierced by the medullary or nutrient 



