

STKUCTUKE OF THE TEETH. 1123 



very considerable amount of organic matter and water incorporated with its salts, 

 which are chiefly phosphate and carbonate of lime. 



Fresh human ivory 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 organic matter consists of (1) calcium phosphate (with a trace of fluoride), 

 (2) calcium carbonate, and (3) magnesium phosphate, the percentages present in dried dentine 

 being 6672, 3 '36, 1*08, respectively. 



Ivory consists of a highly calcined organic matrix, which is itself practically 

 structureless, although everywhere traversed by tubes the canaliculi dentales, or 

 dentinal canaliculi which give to this tissue a finely striated appearance, the striae 

 usually running in wavy lines. The canaliculi begin by open mouths on the wall of 

 the pulp cavity, whence they run an undulating, and at the same time a somewhat 

 spiral course, towards the periphery of the ivory. They give off fine anastomosing 

 branches, and occasionally divide into two. Somewhat reduced in size, they usually 

 end in the outer part of the ivory. 



The canaliculi dentales are generally described as being lined by special sheaths 



(dentinal sheaths of Neumann) which are composed of a most resistant 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 described 



as the sheath is only a modified portion of the ivory which forms the tubules. 



The canaliculi dentales are occupied by processes, 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. 



The concentric lines of Schreger, frequently seen in the ivory, are due to bends in successive 



, canaliculi along regular lines running parallel to the periphery of the ivory. Other lines 



(the incremental lines of Salter), due to imperfect calcification, are found arching across the 



substance of the ivory, chiefly in the crown. There must also be mentioned the interglobular 



spaces, intervals left in the ivory, as a result of imperfect calcification, bounded by the fully 



calcified surrounding tissue, the contour of which is in the form of a number of small projecting 



globules. These interglobular spaces are very numerous in the outer or " granular layer " of the 



, ivory, particularly beneath the osseous substance. 



The substantia OSSea (O.T. cement) 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 adamant at the neck. From there it is continued, increasing in 



. amount, towards the apex, which is formed entirely of this substance. It is relatively 

 less in amount in the child, and increases during life. In places the ivory seems to 

 pass imperceptibly into the substantia ossea, the "granular layer" marking the junction 

 of the two, and some of the canaliculi dentales are continuous with the lacunae of the 



. substantia ossea. Like true bone, it is laminated, it possesses lacunae, canaliculi, and, 



, when in large masses, it may even contain a few Haversian canals. 



The pulpa dentis occupies the tooth cavity and the root-canals of the teeth. It 

 is composed of a number of branched connective tissue cells, the anastomosing processes 



( of which form a fine network, containing in its meshes a jelly-like material, in addition 

 to numerous vessels and nerves, but no lymph-vessels. The most superficial of these 

 cells are arranged in the young tooth as a continuous layer of columnar, epithelium- 



, like cells, lying on the surface of the tooth pulp against the ivory ; they are known 



I as odontoblasts, for they are the active agents in the formation of the ivory. From the 

 outer ends of these odontoblasts processes are continued into the canaliculi dentales, 



. where they have been already referred to as Tomes' fibrils. The vessels of the tooth 

 pulp are numerous, and form a capillary plexus immediately within the odontoblasts. 

 The nerves form rich plexuses throughout the pulp, but their exact mode of ending 

 is unknown. 



The periosteum alveolare is a layer of connective tissue free from elastic fibres, 

 but well supplied both with blood-vessels and nerves, which fixes the root of the tooth in 

 the alveolus, being firmly united by perforating fibres of Sharpey, to the substantia 

 ossea on the one hand, and to the bone of the alveolus on the other. It estab- 

 lishes a communication between the bone of the jaw and the substantia ossea, and it 

 is continuous with the tissue of the gum. Its blood comes chiefly from the arteries, 

 which subsequently enter the apical foramina for the supply of the tooth pulp, but in 

 part also from the vessels of the surrounding bone and of the gum (hence the relief 



: obtained in dental periostitis by lancing the gum). 



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