146 PERIOSTEUM AND MEDULLARY MEMBRANE. 



]ias not been satisfactorily explained. They are to be 

 found by pulling asunder the sections of lamellee of a 

 decalcified cylindrical or cranial bone. From the 

 circumstance that some of these fibres have escaped 

 calcification, the organic matter has shrunk in the 

 dried bone, and thus has resulted a tube, which has 

 been referred to by Tomes and De Morgan, and other 

 observers. 



sol. Periosteum and iiiertiillary membrane. — The 

 compact external surface of bone (except where it helps 

 to form a joint, is covered by a firm tough membrane, 

 termed the periosteum, which, like the perichondrium 

 investing cartilage, consists of white fibrous tissue, 

 densely interwoven in all dii'ections, § 212. The 

 cancelli are filled with fat, or medulla, the marrow of 

 bone. They are lined by a delicate membrane, called 

 the medullary onenihrane, which serves to suppoi't the 

 fat. In the shaft of the long bones the medulla is con- 

 tained, not in ordinary cells, but in one great canal, 

 which occupies the centre of the shaft, the medullary 

 canal. Here the m.edullary membrane lines the com- 

 pact tissue that forms the wall of the cavity. 



Both the periosteum and the medullary membrane 

 adhere intimately to the bone. Both are abundantly 

 supplied with blood-vessels, which, after ramifying 

 upon them, send numerous branches into the bone. 

 These membranes are of great importance to the 

 nutrition of the bone, inasmuch as they support its 

 nutrient vessels ; and, if either of them be destroyed 

 to any great extent, the part in contact with them 

 necessarily perishes : and they not only cover the 

 outer and inner surfaces of the bone, but also send 

 processes, along with the vessels, into mimite canals 

 traversing the comjoact tissue, and are, through the 

 medium of these, rendered continuous with one 

 another. When the periosteum or medullary mem- 

 brane is torn away from the surface of a fresh bone, 

 the vessels may be seen very readily passing from 



