974 THE DIGESTIVE SYSTEM. 
cuticular layer which covers the enamel of Scone cut on and is very ae ferinis 
resisting almost all re- -agents. Two chief views are held as to its origin. One that it is 
the last formed layer of enamel, which has not yet been calcified, and therefore the 
final product of the enamel cells. The other that it is produced by the outer layer of cells 
of the enamel organ. This latter seems to be the more probable view. 
Dentine (substantia eburnea) is the hard and highly elastic substance, yellowish 
white in colour, which forms the greater part of the mass of every tooth (Fig. 657). 
Like the enamel it is highly calcified, but it differs from enamel in containing a very con- 
siderable amount of organic matter and water incorporated with its salts, which are | 
chiefly phosphate and carbonate of lime. . 
Fresh human dentine contains 10 per cent of water, 28 per cent of organic and 62 per cent of 
inorganic material. The organic matter is composed chiefly of collagen, and to a less extent of 
elastin. The inorganic matter consists of (1) caleium phosphate (with a trace of fluoride), (2) 
calcium carbonate, and (3) magnesium phosphate, the percentages present in dried dentine being 
66°72, 3°36, 1:08 respectively. 
Dentine consists of a highly calcified organic matrix, which is itself Prcaoe 
structureless, although everywhere traverse 
to this tissue a finely striated appearance, the striz usually running in wavy lines. 
The dentinal tubes begin by open mouths on 
the wall of the pulp cavity, whence they run an 
undulating, and at the same time a somewhat 
Enamel spiral course, towards the periphery of the 
dentine. They give off fine anastomosing 
branches, and occasionally divide into two. 
Somewhat reduced in size, they usually end in 
the outer part of the dentine. 
The tubules are generally described as being 
cavity lined by special pheathe (dentinal sheaths of 
Neumann) which are composed of a most re- 
sistant material, and possibly are calcified. It 
should be mentioned that the presence of these 
sheaths as separate structures is doubted by 
some authorities, who hold that the part de- 
scribed as the sheath is only a modified portion 
of the dentinal matrix surrounding the tubules. 
The dentinal tubules are occupied by pro- 
cesses, prolonged from the outermost cells of 
the pulp—the odontoblasts. These processes 
are called after their discoverer, Tomes’ fibrils 
(dentinal fibrils), and they are probably sensory 
in function. | 
Crown 
Gum 
Neck 
BASSAS OPE 
The concentric lines of Schreger, frequently seen 
in the dentine, are due to bends in successive 
dentinal tubes taking place along regular lines 
parallel to the periphery of the dentine. Other 
lines (the incremental lines of Salter), due to 
imperfect calcification, are found arching across the 
substance of the dentine, chiefly im ‘the crown. 
There must also be mentioned the interglobular 
spaces, intervals left in the dentine, as a result of 
AA } imperfect calcification, bounded by the fully calcified 
Fic. 657.—VERTICAL SECTION OF CANINE T'OOTH, surrounding dentine, he contour of which is in the 
to illustrate its various parts, and its structure. form of a number of small projecting globules 
of dentine. These interglobular spaces are very 
numerous in the outer or “granular layer” of the dentine, particularly beneath the cementum 
(see Fig. 649). 
Cement or crusta petrosa 
Alveolar periosteum or root-membrane 
The crusta petrosa or cementum (substantia ossea) is a layer of modified bone 
which encases the whole of the tooth except its crown. It begins as a very thin stratum, 
slightly overlapping the enamel at the neck. From this it is continued, increasing in 
amount, towards the apex, which latter is formed entirely of this substance. It is relatively 
less in amount in the child, and increases during life. In places the dentine seems to 
pass imperceptibly into the crusta petrosa (the “granular layer” of dentine marking the 
