THE DIGESTIVE TRACT. 



The tubules give off numerous secondary canals along their course, 

 by which means the adjacent tubules communicate ; on approaching 

 the enamel or the cement the tubules undergo repeated division, 

 the resulting smaller secondary channels corresponding in their 

 general direction with the larger canals. 



The marked parallel curves described by the dentinal tubules pro- 

 duce optical effects which are appreciated as a coarse striation con- 

 centric with the outline of the pulp-cavity ; these appearances, known 

 as Schreger's lines, may be seen in sections with the unaided eye. 

 That part of the dentinal matrix immediately surrounding the tubules 

 is especially dense and resistant, and constitutes the so-called den- 

 tinal sheaths which may be isolated by acids. Within the tubules 

 lie the delicate dentinal fibres, which are the modified processes 

 of the connective-tissue cells forming the peripheral layer of pulp- 

 cells. When cut across the tubules appear circular or slightly oval, 

 and contain a minute dot, the dentinal fibre in transverse section. 

 Want of uniformity in the calcification of the outer zone of dentine 

 gives rise to the incremental lines of Salter. 



The interglobular spaces are irregular stellate intercommuni- 

 cating clefts situated at the outer margin of the dentine, traversed 

 by some of the dentinal tubules. The walia,of these spaces pre- 



FIG. 175. 



FIG. 176. 



Interglobular spaces of dentine from dried 

 human tooth : t, t, spaces into which certain 

 dentinal tubules (d) open. 



Section of enamel from dried human 

 tooth : a, b, longitudinal and trans- 

 verse views of enamel rods. 



sent numerous minute spherical projections, the dentinal globules, 

 produced during calcification of the dentine. 



The enamel covers the exposed parts of the softer underlying 

 dentine, and is composed of irregular 4-6-sided columns, the enamel 

 prisms, closely placed and generally vertical to the surface of the 

 dentine. After suitable isolation the enamel prisms appear slightly 

 varicose in outline, the minute concavities producing the irregular 

 dark bands often seen traversing the prisms. The prisms are held 

 together by a delicate layer of cement-substance and grouped into 



