308 SPECIAL HISTOLOGY. 



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 merely rest upon the bone, as on 

 the external 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 submucous connective tissue, 

 so that the two cannot be separated, and constitute 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 tax, 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 diaphyses, 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 found to present, almost universally, excepting where 

 muscles arise directly from it, two layers, which, although 

 closely connected, differ, more or less distinctly, in their struc- 

 ture. The outer layer is composed chiefly of connective tissue, 

 with occasional fat-cells, and is the principal seat of the true 

 periosteal vessels and nerves, whilst in the inner layer, elastic 

 fibres, commonly of the finer sort, constitute continuous, and 

 often, very thick networks — true elastic membranes — super- 

 imposed one upon another, the connective tissue forming the 

 less important element. Nerves and vessels occur in this 

 layer also, but they do little more than merely pass through it, 

 being destined for the bone itself. 



