THE TEETH 1127 



in the time at which the calcification of the various teeth begins. As a rule, the calcification 

 of the roots is not completed at the apices until some time after the crowns are exposed in 

 eruption. 



Eruption of the teeth. — On account of pressure due to growth and expansion at the root of 

 the tooth (and probably other obscure factors), the crowns are pushed toward the surface. The 

 overlying portion of the tooth-sac, together with corresponding portions of the temporary 

 alveolar bone, are absorbed, and the crown is "cut," i. e., breaks through the surface of the gum 

 in eruption. In the case of the permanent teeth, this is normally preceded by a shedding of 

 the deciduous teeth. The latter have been loosened by the absorption of their roots, which is 

 perhaps due largely to the activity of certain odontoclasts (like the osteoclasts of bone) which 

 are found in the region of absorption. 



Time and order of eruption. — The time of the eruption of the various teeth is subject to 

 great variation, so that no two investigators agree upon it. Aside from the wisdom teeth, the 

 time of eruption is most variable in the canines and premolars, and least variable in the first 

 permanent molars (Rose). The eruption averages four and one-half months earliei in the 

 male, and is also earlier in well-to-do and city children (Rose). The order in which the teeth 

 appear is less variable. The average time at which the various deciduous and permanent 

 teeth appear is indicated approximately in the following table. 



A. Deciduous Teeth 



Months after Birth 

 (Average) 



Lower central incisors 7 (6-8) 



Upper central incisors 8-9 



Upper lateral incisors 9-10 



Lower lateral incisors 12-14 



First molars 14 



Canines 18 



Second molars 22-24 



B. Permanent Teeth 



The average time at which the teeth in the lower jaw undergo eruption is shown in the table 

 below. The corresponding teeth in the upper jaw appear a little later : — 



Years 



First molars 6-7 



Central incisors 7 



Lateral incisors 8 



First premolars 9-10 



Second premolars 9-10 



Canines 11 



Second molars 12 



Third molars (wisdom teeth) 17-25 



Variations. — The great variability of the teeth has already been emphasized, and numerous 

 variations described in connection with the various individual teeth and their development. 

 In number, the teeth may be reduced, due to absence (oftenest of the third molar) or incom- 

 plete development with failure of eruption. An increase in the normal number is less common ' 

 It may be only apparent, due to the retention of a deciduous tooth. There may rarely, however, 

 be a true extra third incisor or premolar, or a fourth molar. Aberrant teeth may occur either on 

 the labial or palatal side of the dental arch. A third dentition appears rarely in old age. In 

 form, there is much greater variation as before mentioned. All intermediate forms between 

 rudimentary and fully developed teeth may occur. Fusion between neighbouring teeth is 

 sometimes found, and deformities in the dental arches necessarily accompany palatal defects 

 involving the alveolar arches. 



Comparative. — As the oral mucosa represents an invagination of the integument, so the 

 teeth are morphologically equivalent to dermal papillie. The close relationship between the 

 teeth and the dermal appendages is clearly shown among many of the lower vertebrates, but 

 most clearly in the Selachians (which include sharks and allied forms). In fig. 887, which 

 illustrates a sagittal section through the lower jaw of a young dogfish, it is clearly evident that 

 the external placoid scales or 'dermal teeth' are continuous with the equivalent oral teeth 

 at the oral margin of the jaw. Both the dermal teeth and the oral teeth are composed of dentine 

 which presents an enlarged base and a somewhat conical apex. The base is embedded in the 

 fibrous lamina propria (often in bony plates) while the apex projects tlirough the epithelium 

 and is covered with a thin cuticular layer the "enamel membrane." True enamel is usually 

 rudimentary or absent in the primitive teeth of lower vertebrates, and represents a secondary 

 acquisition. The dentine is in aU cases derived from the connective tissue, and the enamel 

 from the epithelium . 



The process of development of the primitive oral teeth is also illustrated in fig. 887. Just 

 within the oral margin there is a shelf-like downgrowth of the ectodermal epithehum, forming 

 a primitive germinal ridge. Along this ridge may be seen the anlages of several rows of teeth 

 in various stages of development. As fast as the mature teeth at the oral margin are worn off, 

 new teeth pass up from below to replace them. Thus the primitive form of dentition is poly- 

 phyodont, with many sets of teeth developed successively throughout life. As we pass up the 

 vertebrate scale there is a tendency to a reduction in the number of sets, although there is a 



