THE OSSEOUS SYSTEM. 281 



92. The Periosteum. Among the soft tissues appertaining to 

 bone, the periosteum is one of the most important. It is a more or less 

 transparent, slightly glistening or whitish yellow, vascular, extensible 

 membrane, investing a great part of the surface of bones, and contribut- 

 ing most importantly to their nutrition, by the numerous vessels which 

 it sends into their substance. 



The periosteum is not, everywhere, constituted alike. Opaque, thick, 

 and for the most part with the glistening aspect of tendinous structures 

 where it is covered only by the skin, or is connected -with fibrous parts, 

 such as ligaments, tendons, fascice, and the dura mater cerebri, it is, on 

 the other hand, thin and transparent in situations where muscular fibres 

 arise directly from it without the intervention of tendon, and also on the 

 diaphyses, where the muscles nearly rest upon the bone, as on the ex- 

 ternal surface of the cranium (pericranium), in the vertebral canal, and 

 in the orbit (periorbita). Where mucous membrane rests upon bone, 

 the periosteum is, in most cases, very intimately united to it by the sub- 

 mucous connective tissue, so that the two cannot be separated, and con- 

 stitute a single membrane, which, as in the palate, alveolar processes, 

 nares, &c., is of greater, or, as in the maxillary sinus, tympanum, ethmoid 

 cells, &c., of less thickness. 



The connection of the periosteum with the bone itself is either more 

 lax, consisting in simple apposition, and by more delicate vessels which 

 penetrate the bone, or more intimate, taking place by means of larger 

 vessels and nerves, and by numerous tendinous filaments. The former 

 mode of connection is found especially where the periosteum is thin, and 

 the osseous substance more compact, as in the diapJiyses, on the inner 

 and outer surfaces, and in the sinuses of the cranium ; the latter, where 

 the periosteum is thicker, and the compact substance thinner, as, for 

 instance, in the apophyses, in the short bones, palate, and at the basis 

 of the cranium. 



With respect to the intimate structure of the periosteum, it will be 



young bone. In old bone we have frequently been unable to discover them. Tomes and 

 De Morgan, however, state that the nuclei are visible in sections of a fossil bone (supposed 

 of a Pterodactyle) in their possession. 



Another peculiar condition of the " lacunal cells," described by these authors, is their ossi- 

 fication. They found the light and spongy bones of old people to yield, if broken, a white 

 powder, which was composed of large cells detached or united into masses. They are 

 spherical, and contain a dark granular nucleus, which is surrounded by a thick transparent 

 wall. Similar cells may be found adherent to the walls of the Haversian canals and can- 

 celli; and in this case their nuclei have assumed the form of lacuna, and the canaliculi of 

 adjacent lacuna advance into them. Similar cells may be found in most preparations of 

 adult bone (1. c., p. 12). 



We must confess that we doubt the assumption of a lacunal form by the "nucleus" in 

 these cases. We have repeatedly examined these bodies, but if the nucleus was visible at 

 all, we found it unchanged, and often adhering to one side of the lacimce. Again, it is ques- 

 tionable whether they may not rather be compared to the globules of dentine than to cells. 

 TRS.] 



