

CHEMISTRY AND DEVELOPMENT OF A TOOTH. 22 5 



The dentinal tubules, as well as the fibres of Tomes, anastomose throughout their entire extent 

 by means of fine processes. As the fibres approach the enamel, which they do not penetrate, 

 some of them bend on themselves, and form a loop (fig. 166, c), whilst others pass into the 

 "interglobular spaces " (fig. 165) which are so abundant in the outer part of the dentine. The 

 interglobular spaces are small spaces bounded by curved surfaces. Certain curved lines, 

 " Schreger's lines," may be detected with the naked eye in the dentine (e.g., of the elephant's 

 tusk) running parallel with the contour of the tooth. They are caused by the fact that at these 

 parts all the chief curves in the dentinal tubules follow a similar course. 



The enamel, the hardest substance in the body (resembling apatite), covers the crown of the 

 teeth. It consists of hexagonal flattened prisms arranged side by side like a palisade (fig. 166, 

 B and C). They are 3 to 5 [x (sVtftt inch) broad, not quite uniform in thickness, curved 

 slightly in different directions, and, owing to inequalities of thickness, they exhibit transverse 

 markings. They are elongated, calcified, cylindrical, epithelial cells. Retzius described dark 

 brown lines running parallel with the outer boundary of the enamel, due to the presence of 

 pigment (fig. 163). The fully formed enamel is negatively doubly refractive and uniaxial, while 

 the developing enamel is positively doubly refractive (Hoppe-Seyler). 



The cuticula or Nasmyth's membrane covers the free surface of the enamel as a completely 

 structureless membrane 1 to 2 (x thick, but in quite young teeth it exhibits an epithelial struc- 

 ture, and is derived from the outer epithelial layer of the enamel organ. 



The cement or crusta petrosa is a thin layer of bone covering the fang (fig. 167, a). The 

 bone lacuna? communicate directly with the dental tubules of the fang. Haversian canals and 

 lamellae are only found where the layer of cement is thick, and the former may communicate 

 with the pulp-cavity. Very thin layers of cement may be devoid of bone-corpuscles. Sharpey's 

 fibres occur in the cement of the dog's tooth ; while in the horse's tooth single bone-corpuscles 

 are developed by a capsule. In the periodontal membrane, which is just the periosteum of the 

 alveolus, coils of blood-vessels similar to the renal glomeruli occur. They anastomose with each 

 other, and are surrounded by a delicate capsule of connective-tissue. 



Chemistry of a Tooth. The teeth consist of a gelatine-yielding matrix infiltrated with cal- 

 cium phosphate and carbonate (like bone). (1) The dentine contains organic matter, 27 "70 ; 

 calcium phosphate and carbonate, 72*06 ; magnesium phosphate, 0'75 ; with traces of iron, 

 fluorine, and sulphuric acid. 



(2) The enamel contains an organic proteid matrix allied to the substance of epithelium. It 

 consists of 3 "60 organic matter and 96*00 of calcium phosphate and carbonate, 1*05 magnesium 

 phosphate, with traces of calcium fluoride and an insoluble chlorine compound. 



(3) The cement is identical with bone. 



The pulp in a fully-grown tooth represents the remainder of the dental papilla around which 

 the dentine was deposited. It consists of a very vascular indistinctly fibrillar connective-tissue, 

 laden with cells. The layers of cells, resembling epithelium, which lie in direct contact with 

 the dentine, are called odontoblasts, i.e., those cells which build up the dentine. These cells 

 send off long branched processes into the dentinal tubules, whilst their nucleated bodies lie 

 on the surface of the pulp, and form connections by processes with other cells of the pulp and 

 with neighbouring odontoblasts. Numerous non-medullated nerve-fibres (sensory from the 

 trigeminus), whose mode of termination is unknown, occur in the pulp. 



The periosteum or periodontal membrane of the fang is, at the same time, the alveolar peri- 

 osteum, and consists of connective-tissue with elastic fibres and many nerves. 



The gums are devoid of mucous glands, very vascular, and often provided with long vascular 

 papillae, which are sometimes compound. 



Development of a Tooth. It begins at the end of the second month of foetal life. Along the 

 whole length of the foetal gum is a thick projecting bridge (fig. 168, a) composed of many layers 

 of epithelium. A depression, the dental groove, also filled with epithelium, occurs in the gum, 

 and runs along under the ridge. The dental groove becomes deeper throughout its entire length, 

 and on transverse section presents the appearance of a dilated flask (b), while at the same time 

 it is filled with elongated' epithelial cells, which form the "enamel organ." A conical papilla, 

 the "dentine germ," grows up from the mucous tissue, of which the gum consists, towards the 

 enamel organ (fig. 169, c), so that the apex of the papilla comes to have the enamel organ resting 

 upon it like a double cap. Afterwards, owing to the development of connective-tissue, the parts 

 of the enamel organ lying between and uniting the individual dentine germs, disappear, and 

 gradually the connective-tissue forms a tooth-sac enclosing ihe papilla and its enamel organ (d). 



Those epithelial cells (fig. 169, 3) of the enamel organ, which lie next the top of the papilla, 

 are cylindrical, and become calcified to form enamel prisms. The layer of cells of the 

 double cap, which is directed towards the tooth-sac (1), becomes flattened, fuses, undergoes a 

 horny transformation, and becomes the cuticula, whilst the cells which lie between both layers 

 undergo an intermediate metamorphosis, so that they come to resemble the branched stellate 

 cells of the mucous tissue (2), and gradually disappear altogether. 



The dentine is formed in the most superficial layer of the projecting connective- tissue of the 

 dental papilla, owing to the calcification of the continuous layer of odontoblasts which occur 

 there (figs. 169 and 170, Tc). During the process, fibres or branches of these cells are left 



P 



