MEDULLA, AND ITS MEMBRANE. 65 



manifests a great similitude to other fibrous membranes, as the 

 dura mater, the sclerotica, and the tendons. 



The use of the periosteum is to conduct the blood vessels to 

 the bones, to protect the latter from the impression of the mus- 

 cles, and other organs, which come in contact with them, to 

 keep the ossification of the bones within its proper boundaries, 

 to give shape to them, and to secrete bone in the growing state 

 or in fractures; and, finally, as has been suggested by the late 

 Dr. Physick, it exerts a very happy influence in turning from 

 the bones suppurations in their vicinity, which would otherwise 

 be pernicious to them. 



SECT. II. OF THE MEDULLA, AND ITS MEMBRANE, CALLED THE IV- 



TERNAL PERIOSTEUM. 



A greasy substance, as already stated, fills the cells of the 

 bones : it does not, in its composition, differ from common fat : 

 its granules, however, seem to be somewhat finer. From its re- 

 semblance in position to the pith of vegetables, it has obtained 

 the name of medulla or marrow. It is contained in a very fine 

 cellular and vascular membrane, lining the internal cavities of 

 the bones, and sending into their compact substance very deli- 

 cate filaments. The existence of this membrane has been de- 

 nied, but it may be established by sawing a bone in two, and 

 approaching the cut end to the fire, so as to melt out the mar- 

 row, or by immersing it in an acid, in which case the mem- 

 brane becomes crisp and distinct. Its delicacy is so extreme, 

 that it can only be compared to a spider's web. In this state it 

 may be traced, lining the whole cylindrical cavity of the long 

 bones, and extending itself to their extremities. It also exists 

 in the diploic or cellular structure of all the other bones ; but it 

 is scarcely possible to demonstrate it there in a very distinct 

 manner, owing to its extreme tenuity. 



The medullary membrane is composed principally of the mi- 

 nute and numerous blood vessels spent upon the internal sur- 

 face of the bones, aided by a very fine, soft, cellular tissue, 

 merely sufficient in quantity to fill up the meshes between the 

 frequent anastomoses of the vessels. From the latter cause, it 

 is compared to the pia mater and to the omentum. It has been 

 stated, that its blood was derived from the nutritious artery, 



6* 



