THE DENTAL TISSUES. 109 



At the necks of the teeth the gum becomes continuous 

 with the periosteum of the internal surface of the alveoli, 

 into which it passes without any line of demarcation. 



THE ALVEOLO-DENTAL MEMBRANE. 



The Alveolo-dental 'Periosteum, or Eoot membrane, is a 

 connective tissue of moderate density, devoid of elastic 

 fibres, and richly supplied with nerves and vessels. 



It is thicker near to the neck of the tooth, where it 

 passes by imperceptible gradations into the gum and peri- 

 osteum of the alveolar process, and near to the apex of the 

 root. The general direction of the fibres is transverse ; that 

 is to say, they run across from the alveolus to the cemen- 

 tum, without break of continuity, as do also many capillary 

 vessels ; a mere inspection of the connective tissue bundles, 

 as seen in a transverse section of a decalcified tooth in its 

 socket, will suffice to demonstrate that there is but a single 

 " membrane," and that no such thing as a membrane proper 

 to the root and another proper to the alveolus can be dis- 

 tinguished j and the study of its development alike proves 

 that the soft tissue investing the root, and that lining the 

 socket, are one and the same thing : that there is but one 

 " membrane," namely, the alveolo-dental periosteum. 



At that part which is nearest to the bone the fibres are 

 grouped together into conspicuous bundles ; it is, in fact, 

 much like any ordinary fibrous membrane. On its inner 

 aspect, where it becomes continuous with the cementum, it 

 consists of a fine network of interlacing bands, many of 

 which lose themselves in the surface of the cementum. 



But although there is a marked difference in histological 

 character between the extreme parts of the membrane, yet 

 the markedly fibrous elements of the outer blend and pass 



