THR DKIKSTIVE SYSTEM 



239 



The Peridental Membrane. — SurrouiKling the root of the tooth and filling 

 in the space between it and the wall of I lie alveolus is a layer of connective 

 tissue which is known as the peridental membrane (Fig. 13S). It attaches the 

 tooth to the alveolus, attaches the teeth to each other, supports the free margin 

 of the gum and holds it to the tooth, and serves for the transmission of vessels 

 and nerves. It consists of dense white fibrous tissue with few or no elastic 

 fibres. In general it resembles periosteum and has been described as a reflec- 

 tion of the alveolar periosteum upon the root of the tooth. The fibres fall into 

 two classes, long fibres which pass from the ccmcntum to the coarser connective 



.^A 



Bk 



■k 



s 



Fig. 141. — From Longitudinal Section of Crown of Human Premolar (X200) 

 (Sobotta), showing junction of enamel and dentine. S, Enamel; D, dentine; Sp, enamel 

 prisms; Dk, dental canals; Jg, interglobular spaces. A few dentinal fibres are seen 

 passing beyond the limits of the dentine into the enamel. The oblique dark bands in the 

 enamel are the lines of Retzius. 



tissue of the gum, to the alveolar wall or to the cementum of an adjacent tooth, 

 and short fibres which fill in the interstices between the long fibres. IMany of 

 the long fibres are continued as calcified fibres into the cementum on the one hand 

 and into the bone of the alveolus on the other. They are analogous to Sharpey's 

 fibres of bone (p. 197). As they enter the cementum or bone, the fibres are 

 grouped in bundles but in the central portion of the membrane these bundles 

 break up and their fibres interlace in all directions. The long fibres differ in 

 direction in different parts of the membrane. Those which spring from the 

 cementum near its junction with enamel, pass out at right angles and then turn 

 sharply toward the free margin of the gum which they support. Passing toward 

 the apex, the next fibres instead of bending toward the free margin of the gum 

 pass out at right angles and blend with the gum connective tissue. xAll fibres 



