THE MARROW. 265 



The exact knowledge we possess of the minute structure of osseous tissue is largely the 

 result of the careful investigation of the late Professor Sharpey, whose account, published 

 in the fifth edition of this work in 1845, has needed no erasure, and but little addition, 

 even to the present day. His labours in this field have been to a certain extent recognized 

 in the adoption of the name " fibres of Sharpey " for the perforating fibres discovered 

 by him, but it is only of late that the facts which he demonstrated are becoming understood 

 and their significance appreciated by histologists. 



THE PERIOSTEUM. 



The periosteum, as already stated, is a fibrous membrane which covers the 

 bones externally. It adheres to them very firmly, and invests every part of their 

 surface, except where they are covered with cartilage. 



It is composed of two layers ; the outer, consisting chiefly of white fibres, and con- 

 taining occasional fat-cells, is the means of supporting numerous blood-vessels destined 

 for the bone, which ramify in the membrane, and at length send their minute 

 branches into the Haversian canals of the compact substance, accompanied by pro- 

 cesses of filamentous tissue derived from, or at least continuous with, the periosteum. 

 The inner layer is largely made up of elastic fibres, frequently in several distinct 

 strata. Between it, however, and the proper osseous tissue there is a fibrous stratum 

 containing in the young bone a number of granular corpuscles (osteoblasts), while 

 in the adult bone these have become flattened out into an epithelioid layer covering 

 the osseous substance, and are in many places separated by a cleft-like space 

 (serving probably for the passage of lymph) from the rest of the periosteum 

 (Schwalbe). 



By treating the membrane with nitrate of silver, lymphatics are discovered in 

 it accompanying the blood-vessels in the outer layer ; and, as in other aponeurotic 

 structures, extensive epithelioid markings, covering a great part of the surface, are 

 brought into view. 



Fine nerves spread out in the periosteum ; they are chiefly associated with the 

 arteries, and for the most part destined for the subjacent bone ; but some are for 

 the membrane itself, and some of these end in Pacinian corpuscles. 



The chief use of the periosteum is to support the vessels going to the bone, and 

 afford them a bed in which they may subdivide into fine branches, and so enter the 

 dense tissue at numerous points. Hence, \vhen the periosteum is stripped off at 

 any part, there is great risk that the denuded portion of the bone will die and 

 exfoliate. The periosteum also contributes to give firmer hold to the tendons and 

 ligaments where they are fixed to bones. Its relation to the growth and renewal of 

 bone will be referred to later on. 



THE MARROW. 



The marrow (medulla ossmm) is lodged in the interior of the bones ; it fills up 

 the hollow shaft of long bones and occupies the cavities of the cancellated structure ; 

 it extends also into the Haversian canals at least into the larger ones along with 

 the vessels. A fine layer of a highly vascular areolar tissue lines the medullary 

 canal, as well as the smaller cavities which contain marrow ; this has been named 

 the medullary membrane, internal periosteum, or endcstewn ; -but it cannot be 

 detached as a continuous membrane. Its vessels join on the one side those of the 

 osseous substance, and on the other side are continuous with the capillaries of the 

 marrow. 



The marrow differs considerably in different situations. Within the shaft of the 



