1120 



SPLANCHNOLOGY 



?3 Cement 



Interglobular 

 spaces 



4 Dentin 



The matrix (intertubular dentiri) is translucent, and contains the chief part of the earthy 

 matter of the dentin. In it are a number of fine fibrils, which are continuous with the fibrils 



of the dental pulp. After the earthy matter has 



.- ^-^, been removed by steeping a tooth in weak acid, 



the animal basis remaining may be torn into laminae 

 which run parallel with the pulp cavity, across the 

 direction of the tubes. A section of dry dentin often 

 displays a series of somewhat parallel lines the 

 incremental lines of Salter. These lines are com- 

 posed of imperfectly calcified dentin arranged in 

 layers. In consequence of the imperfection in the 

 calcifying process, little irregular cavities are left, 

 termed interglobular spaces (Fig. 1008). Normally 

 a series of these spaces is found toward the outer 

 surface of the dentin, where they form a layer which 

 is sometimes known as the granular layer. They 

 have received their name from the fact that they 

 are surrounded by minute nodules or globules of 

 dentin. Other curved lines may be seen parallel to 

 the surface. These are the lines of Schreger, and 

 are due to the optical effect of simultaneous curva- 

 ture of the dentinal fibers. 



Chemical Composition. According to Berzelius 

 and von Bibra, dentin consists of 28 parts of animal 

 and 72 parts of earthy matter. The animal matter 

 is converted by boiling into gelatin. The earthy 

 matter consists of phosphate of lime, carbonate of 

 / V V / f \1 I\J lime, a trace of fluoride of calcium, phosphate of 



' f j [ magnesium, and other salts. 



, ' \ ' s / r i ' |\ V \ The enamel (substantia adamantina) is the hardest 



\ and most compact part of the tooth, and forms a 



1 1 ,, ^ /i I ] 1 ) thin crust over the exposed part of the crown, as far 



l' as the commencement of the root. It is thickest on 



the grinding surface of the crown, until worn away 

 by attrition, and becomes thinner toward the neck. 

 It consists of minute hexagonal rods or columns 



termed enamel fibers or enamel prisms (prismata adamantina). They lie parallel with one 

 another, resting by one extremity upon the dentin, which presents a number of minute depres- 

 sions for their reception; and forming the free surface of the crown by the other extremity. 

 The columns are directed vertically on the summit of the crown, horizontally at the sides; they 

 are about 4ju in diameter, and pursue a more or less wavy course. Each column is a six-sided 

 prism and presents numerous dark transverse shadings; these shadings are probably due to the 

 manner in which the columns are developed in successive stages, producing shallow constric- 

 tions, as will be subsequently explained. Another series of lines, having a brown appearance, 

 the parallel striae or colored lines of Retzius, is seen on section. According to Ebner, they are 

 produced by air in the interprismatic spaces; others believe that they are the result of true 

 pigmentation. 



Numerous minute interstices intervene between the enamel fibers near their dentinal ends, 

 a provision calculated to allow of the permeation of fluids from the dental canaliculi into the 

 substance of the enamel. 



Chemical Composition. According to von Bibra, enamel consists of 96.5 per cent, of earthy 

 matter, and 3.5 per cent, of animal matter. The earthy matter consists of phosphate of lime, 

 with traces of fluoride of calcium, carbonate of lime, phosphate of magnesium, and other salts. 

 According to Tomes, the enamel contains the merest trace of organic matter. 



The crusta petrosa or cement (substantia ossea) is disposed as a thin layer on the roots of the 

 teeth, from the termination of the enamel to the apex of each root, where it is usually very thick. 

 In structure and chemical composition it resembles bone. It contains, sparingly, the lacunae 

 and canaliculi which characterize true bone; the lacunae placed near ,the surface receive the 

 canaliculi radiating from the side of the lacunas toward the periodontal membrane; and those 

 more deeply placed join with the adjacent dental canaliculi. In the thicker portions of the 

 crusta petrosa, the lamellae and Haversian canals peculiar to bone are also found. 



As age advances, the cement increases in thickness, and gives rise to those bony growths or 

 exostoses so common in the teeth of the aged; the pulp cavity also becomes partially filled up by 

 a hard substance, intermediate in structure between dentin and bone (osteodentin, Owen; second- 

 ary dentin, Tomes). It appears to be formed by a slow conversion of the dental pulp, which 

 shrinks, or even disappears. 



Fia. 1008. Transverse section of a portion of the 

 root of a canine tooth. X 300. 



