BONE. 301 



they having merged into canaliculi, which are often of con- 

 siderable size. 



Marrow of Bones. 



The medullary cavity of adult long bones, the medullary 

 cells, and the larger medullary canals, all contain a loose cel- 

 lular tissue, in the meshes of which a greater or less amount 

 of marrow or fat cells is enclosed. (See Plate XXX. Jigs. 

 3,4.) 



In foetal and very young bones the fat vesicles are wanting 



in the three situations named, the place of fat being supplied 



by immense numbers of the small granular nucleated cells, 



which have already been referred to. (See Plate XXXIII. 



Jig. 5.) 



It has been stated that the medullary cavity, cells, and 

 canals all communicate freely ; the marrow therefore and its 

 enclosing cellular tissue are everywhere continuous. 



Periosteum. 



The external surface of all bones, with the exception of 

 their articular extremities, is covered with a dense membrane 

 composed of fibrous tissue, which is very rich in blood-vessels, 

 and which is called the periosteum. 



The internal surface of the medullary cavity, cells, and 

 larger Haversian canals, is also lined by a vascular membrane 

 much more delicate in structure, which may be regarded as 

 an internal periosteum. . 



It is by means of the vessels which ramify through these 

 membranes that the nourishment of the bone is secured. 



Vessels of Bone. 



Bones are richly supplied with blood-vessels, which pene- 

 trate every part of their structure. 



Thus externally, branches, principally arterial, proceed from 

 the periosteum, enter the numerous apertures of the Haver- 



