1124 



DIGESTIVE SYSTEM 



due chiefly to the great width of the upper central incisor. The lower molars, however, es- 

 pecially the third, are wider (antero-posteriorly) than the upper, so that the two arches are 

 nearly equal in length. The interdental line between the two arches is not straight, but shghtly 

 convex downward (fig. 876). In both arches, the crowns of the incisors and canines are taller 

 than those of the premolars and molars. 



Vessels and nerves. — The vessels and nerves of the teeth are distributed partly to the 

 pulp and partly to the surrounding alveolar periosteum. The arteries are all derived from the 

 internal maxillary. Those for the upper teeth are the posterior superior alveolar and the 

 anterior superior alveolar (from the infraorbital). Similar branches to the lower jaw are given 

 off b}' the inferior alveolar. They give off twigs to the gums (rami gingivales), the alveolar 



Fig. 881.— Hard Palate of a Child of FrvE Years, Showing Deciduous Teeth. 



Gubernacular canal 



Incisive fossa 

 Incisive suture 



Palate bone 



J:- Palate process of Diaxilla 



Greater palatine foramen 

 Lesser palatine foramen 



periosteum (rr. alveolares), and the pulp cavities (rr. dentales). A dental branch enters each 

 root canal through the apical foramen, and breaks up into a rich peripheral capillary plexus 

 under the odontoblast layer. From this plexus, the corresponding veins arise. There is a plexus 

 of peridental lymphatics, which anastomose with those of the surrounding gums, and drain 

 chiefly into the submaxillary nodes. Lymphatics have also recently been demonstrated in the 

 pulp of the tooth (Schweitzer). 



The nerves are sensory branches derived from the trigeminus. Those for the upper teeth 

 are from the anterior, middle, and posterior superior alveolar (fig. 735); while those for the 

 lower teeth are from the inferior alveolar (fig. 736). These nerves give numerous branches to 



Fig. 882. — The Deciduous Teeth, External View. 



Incisors Canine Deciduous molars 



Maxillary or upper set 



Mandibular or lower set 



the gums, alveolar periosteum, and pulp cavities. The latter enter with the corresponding 

 vessels, and their distribution within the tooth is a subject of controversy. They may be 

 followed easily to a i)loxus under the odontoblasts; but whether they end freely, or in connection 

 with the odontoblasts (which by some are considered as peripheral sensory cells), or send fine 

 terminal branches out into tlie dentinal canals is still uncertain. 



Development of the teeth. — '^I'he teeth rei)rosent calcified papillae of the oral mucosa, the 

 enamel being a derivative of tiie cctoderinal ei)it helium, and the remainder of the tooth coming 

 from the umlerlying me.senchynie. Tlie first trace of the teeth appears in the human embryo 

 of about 11 mm., in the form of an ei)ithelial shelf, the dnital ridge, extending into the mesen- 

 chyme corresponding to the future alveolar portions of the jaws (figs. 878, 879). From the 

 dental ridge there is later f)roduccd a series of cup-shaped enlargements, the enamel organs, 

 which become constricted off except for a slender neck attaching each to the common ridge. 

 By the end of the third fci-tal month, the twenty enamel organs of the temporary or deciduous 

 teeth are formed. The concavity of each enamel organ is filled by the dental papilla of mesen- 

 chyme. 



