THE PERIOSTEUM. 34:1 



filled either with a fluid or a solid substance, since they do not com- 

 municate with the air. The marrow being a form of adipose tissue, 

 answers the purpose, therefore -first, of mere package ; but, secondly, 

 it also, from the fat it contains being lighter than other animal fluids, 

 renders the bones lighter than they would be, were the latter sub- 

 stituted; and, thirdly, in the emergency of starvation, it is reab- 

 sorbed into the blood, and thus prolongs life its cells becoming 

 at the same time filled with a serous fluid (p. 307). 



In most birds, the cavities of the long bones communicate indi- 

 rectly with the atmosphere, and therefore contain no marrow. 



The Periosteum. 



The periosteum is a more or less transparent, slightly glistening, 

 or whitish-yellow extensible membrane. It is also vascular, and 

 invests the surface of the bones, except where certain muscles and 

 ligaments are inserted, and where the surface is covered by the 

 articular cartilages. 



It is, however, not everywhere constituted alike. When only 

 covered by the skin, or connected with fibrous structures (as liga- 

 ments, tendons, fasciae, and the dura mater), it is opaque, thick, and 

 generally glistening like tendinous structures. On the other hand, 

 it is thin and transparent when muscular fibres rise directly from 

 it, and when the muscles nearly rest upon the bone (as on the ex- 

 ternal surface of the cranium); also in the vertebral canal and in 

 the orbit. When mucous membrane rests on bone, the periosteum 

 is generally very intimately united to it by the submucous areolar 

 tissue ; so that the two cannot be separated, and constitute a single 

 membrane of varying thickness (in the ethmoid cells, maxillary 

 sinus, &c.). 



The connection of the periosteum with the bone is sometimes 

 very loose, it being merely in apposition, or attached by delicate 

 vessels penetrating the bone; and sometimes very firm and intimate 

 by means of larger vessels and nerves, and numerous tendinous 

 filaments. The former occurs more generally where the periosteum 

 is thin, and the osseous tissue more compact, as in the shafts of long 

 bones and in the sinuses of the cranium ; the latter, where the peri- 

 osteum is thicker, and the compact substance thinner, as in the epi- 

 physes, in the short bones, the palate, and at the base of the cranium. 



In its intimate structure, the periosteum presents almost always, 

 except where muscles rise from it directly, two layers, differing 



