MOUTH CAVITY. 161 



enamel prisms are in general arranged in parallel rows, but 

 there may also be bundles of prisms running diagonally and at 

 angles to one another (Fig. 117). 



The surface of the enamel is covered by a very thin (about 

 1 {JL thick) structureless membrane, the cuticula dentis. 



The cement (substantia ossea) (Fig. 116) is a true bony tissue, 

 which in young teeth as a rule possesses Haversian systems and 

 bone lacunae. These lacunae are wanting in the neck of the 

 tooth. The lamellated structure is seldom observed. Large 

 numbers of Sharpey's fibres are present. 



Blood-vessels and nerves reach the tooth through the pulp 

 cavity. Small arteries enter the pulp and break up into 

 numerous branches. These form a network with oblong 

 meshes which extend up to the odontoblast layer as a capil- 

 lary plexus (Lepkowski). 



Lymph-vessels are not known in the pulp of the tooth. 



The nerves enter the pulp in several bundles, which run 

 mainly in the centre, giving out numerous branches. These 

 fibres form a network which runs toward the periphery. The 

 fibres lose their medullary sheath and extend as fine non-medul- 

 lated fibres between the odontoblasts. to end freely in small 

 swellings (Retzitis). 



Development of Teeth. 



In the beginning of the seventh week of foetal life the epi- 

 thelium covering the edge of the jaw grows into the deeper- 

 lying connective tissue in the form of a ridge the so-called 

 dental ridge. In the third month round thickenings occur on 

 the labial side of this ridge, which form the beginnings of the 

 milk teeth (Fig. 118). At the same time certain changes take 

 place in the connective tissue. It projects into the lower side 

 of the thickenings in the dental ridge, and forms in each thick- 

 ening a dental papilla or tooth germ. In consequence of this 

 imagination the epithelium forms a sort of mantle for the 

 dental papilla. The epithelial covering forms the starting-point 

 for the enamel, and is known as the enamel organ. It later 

 separates off from the dental ridge by a gradual narrowing of 



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