THE PERIODONTAL MEMBRANE 309 



organisms pass along the course of the Sharpey's fibres 

 suggests that they are not calcified, the organisms penetrating 

 channels in the cement occupied by the fibres exactly as 

 they do the tubes of the dentine. 



It can be easily understood from its mode of development 

 that the blood-vessels and nerves which are supplied to that 

 portion of the papilla which afterwards becomes the dental 

 pulp are common to both pulp and membrane, some branches 

 passing to the pulp, others to the membrane. It is evident 

 that the principal vascular supply comes from the large 

 vessels at the apex of the root, but these also form com- 

 munications with the vessels of the gum and alveolus. 



The rich supply of both blood-vessels and nerves to the 

 membrane would fully account for its great sensibility in 

 inflammatory conditions. If lymphatics are present, as 

 must be expected, in the periodontal membrane, it is not 

 at present known whether they form a perivascular network 

 or exist as distinct lymphatic vessels, but there is, we think, 

 every evidence that the lymphatic system described by 

 Black is in reality the sheath of Hertwig, the remarkable 

 resemblance of these regularly arranged cells to a tubular 

 system easily lending itself to such an interpretation. 



The Gum 



The gum is the name given to that portion of the mucous 

 membrane of the mouth which surrounds the teeth. It is 

 continuous with, and identical in structure with, the mucous 

 membrane of the rest of the oral cavity, but is slightly 

 denser and is blended with the periosteum of the alveolar 

 bone and with the periodontal membrane. 



The microscopic structure is characteristic of all mucous 

 membranes. The surface layer is made up of flattened 

 epithelial cells, beneath which, as in the epidermis, are 

 found the stratum corneum, stratum lucidum, stratum 

 granulosum, and the cylindrical cells of the Malpighian 

 layer. The flattened epithelial cells of the external layer 

 are continually being shed from the surface, and their 

 characteristic forms are seen in all preparations of the fluids 

 of the mouth along with the mouth bacteria shown in all 



