DIGESTION. 



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portion embedded in the alveolar socket; (3) the constricted portion or neck, 

 which is surrounded by the free margin of the gum. The teeth are firmly 

 secured in their sockets by a fibrous membrane, the peridental membrane, 

 which is attached, on the one hand, to the alveolar process, and, on the other, 

 to the cementum. 



A vertical section of a tooth shows that it consists of three distinct 

 solid structures, the enamel, the dentine, and the cementum, which have 

 the anatomic relationship represented in Fig. 61. In the center of the 

 dentine there is a cavity the general shape 

 of which varies in different teeth, and 

 which is occupied during the living condi- 

 tion by the tooth pulp. 



Microscopic examination of the tooth 

 reveals the presence of irregular stellate 

 spaces, the interglobular spaces, between 

 the dentine and the cementum, which are 

 occupied by connective-tissue cells. Clefts 

 of varying size are also observed at the 

 junction of the dentine and the enamel, 

 and extending for some distance into the 

 latter. 



The enamel is composed of dense hard 

 cylinders which, on account of their small 

 size and close relationship, appear to be 

 hexagonal in shape. These cylinders are 

 held together by cement substance. The 

 free border of the enamel is covered by a 

 thin membrane known as the cuticle or 

 membrane of Nasmyth. 



The dentine is somewhat less dense than 

 the enamel. It is composed of connective- 

 tissue fibers embedded in a ground-sub- 

 stance, both of which have undergone 

 calcification in the course of development. 

 The dentine is penetrated by a series of fine 

 canals, the dentine canals or tubules, which 

 begin by open mouths on the pulp side. From this point the tubules pass 

 outward to the cementum and enamel, where their terminal branches com- 

 municate with and terminate in the interglobular spaces and clefts. In 

 their course the tubules give off a series of branches which communicate 

 freely with one another. The dentine bordering the tubule is somewhat 

 more dense than the intertubular portion and constitutes what is known as 

 the dentinal sheath or Neumann's sheath. 



The cementum resembles bone because it contains both lacunae and 

 canaliculi, though it is, as a rule, devoid of Haversian canals. 



The pulp consists of a framework of connective tissue which affords 

 support for blood-vessels and nerves, both of which enter the pulp chamber 

 through a small foramen at the apex of the root. The outer surface of 

 the pulp is covered with a layer of large spheric cells, the odontoblasts. 



FIG. 61. VERTICAL SECTION OF 

 TOOTH IN JAW. E. Enamel. D. 

 Dentine. P. M. Periodontal mem- 

 brane. P.C. Pulp cavity. C. Ce- 

 ment. B. Bone of lower jaw. V. 

 Vein. a. Artery. N. Nerve. (Stir- 

 ling.) 



