THE TEETH 1117 



submaxillary gland. It has no distinct capsule, thus differing from the submaxil- 

 lary and parotid glands. In structure, it is a racemose mixed gland, but predomi- 

 inantly mucous. 



Ducts. — The minor sublingual duds [ductus sublinguales minores], ducts of Rivinus, vary 

 from five to fifteen or more in number, and open on minute papillae along the crest of the plica 

 sublingualis (fig. 858). The anterior portion of the gland often forms a larger (Bartholin's) 

 duct [ductus sublingualis major] which opens alongside the submaxillary duct on the caruncula 

 sublingualis (figs. 858, 866). 



Vessels and nerves. — The arteries are derived from the sublingual and submental, with 

 corresponding reins. The lymphatics are tributaries of the superior deep cervical nodes. 



Nerves. — The sublingual glands receive their secretory fibres from the submaxillary and 

 associated sympathetic ganglia, conveying impulses from the chorda tympani; sympathetic 

 branches come from the cervical sympathetic and sensory fibres probably fi'om the geniculate 

 ganglion, although this question needs further investigation. 



Development of the salivary glands. — The salivary glands appear early as buds from the 

 ectodermal epithehum extending into the adjacent mesenchyme of the mouth cavity. The 

 parotid appears first on the side of the mouth cavity in an embryo of 8 mm., as a groove which 

 becomes tubular and pushes back over the masseter to the ear region, developing branches (at 

 first solid). Around the gland and between the branches is mesenchyme which becomes 

 condensed to form the peripheral capsule. The submaxillary gland appears in the 13 mm. 

 embryo as a ridge in the epithelium of the alveolo-lingual groove. The solid cord (lumen 

 appearing later) grows forward to the region of its adult orifice. Its posterior end extends 

 backward and gives off solid branches which later form the acini and duct system of the mature 

 gland. The sublingual glands appear somewhat later (24 mm. embryo) as a series of separate 

 anlages of variable number, budding off in the positions where the adult ducts empty. The 

 major sublingual gland, if present, appears first. The histogenetic development of the sahvary 

 glands is not completed until some time after birth, probably about the time of weaning. 

 However, mucin cells appear in the sublingual glands in the foetus of four months and serous 

 cells in the parotid of five months. 



Variations. — The duct of Bartholin is present in about half of the cases, and the corre- 

 sponding anterior part of the gland may be more or less separate [gl. sublingualis major]. The 

 number of ducts may reach thirty (Tillaux). Rarely processes from the gland may penetrate 

 the mylohyoid, appearing on its lower surface in one or more places (Moustin). Most of the 

 variations in this and the other salivary glands are due to developmental irregularities. 



Comparative. — Oral glands are not found in the lower aquatic vertebrates. INIucous 

 glands occur in all terrestrial vertebrates, but true sahvary (digestive) glands appear only in 

 mammals. Although great variations occur in the different species of mammals, those in man 

 (excepting the anterior Lingual) are typical for the order. The sublingual gland, however, 

 often occurs as two separate glands, corresponding to the sublinguahs major and minor. The 

 parotid gland apparently has no representative in forms below mammals. In some mammals 

 (e. g., monkey) it has two main lobes — a larger superficial and a smaller deeper lobe between 

 which lies the facial nerve (Gregoire). Other oral glands (e. g., orbital, zygomatic) appear in 

 some mammals. 



THE TEETH 



The teeth [dentes] are highly specialized structures developed in the oral 

 mucosa as organs of mastication and also (in man) of speech. The adult indi- 

 vidual with perfect dentition has thirty-two teeth, arranged arch-like in the sock- 

 ets (alveoli) of the maxilla and the mandible. Sixteen belong to the upper or 

 maxillary arch; and sixteen to the lower or mandibular. The four central teeth 

 in each dental arch are the incisors; the tooth next to these on each side is the 

 canine; behind these are the two premolars (bicuspids); and lastly the three 

 molars. This relation of teeth is expressed by the following dental formula: 



.21 23^^ 



1 2, cp pm 2, m ^ = 32. 



Form. — Each tooth [dens] has a crown [corona dentis], the portion exposed 

 beyond the gum, and covered with enamel (figs. 871, 872). The root [radix den- 

 tis] is the portion covered with cementum and embedded in the bony socket. At 

 the line of union of crown and root is the slightly constricted 7ieck [collum dentis]. 

 The surface of the tooth directed tov/ard the lip (or cheek) is termed the labial 

 (or buccal) surface [facies labialis; f. buccalis]; while that toward the tongue is 

 the lingual surface [f. lingualis]. The crowns of the opposite arches meet at the 

 masticating surface [f. masticatoria]. The surfaces in contact with the adjacent 

 teeth of the same arch [facies contactus] are, for the incisors and canines, termed 

 medial and lateral, while those for the premolars and molars are termed anterior 

 and posterior. 



