800 



THE TEETH. 



limits. The eruption commences at the age of eeven months, and is com- 

 pleted about the end of the second year. It begins with the central incisors 



Fig. 558. 



Fig. 558. THE DENTAL 

 SACS EXPOSED IN THE JAW 

 OF A CHILD AT BIRTH. 



a, the left half seen from 

 the inner side ; b, the right 

 half seen from the outer 

 side ; part of the hone has 

 been removed so as to ex- 

 pose the dental sacs as they 

 lie below the gum ; the 

 lower figure shows the sacs 

 of the milk-teeth and the 

 first permanent molar, ex- 

 posed by removing the bone 

 from the outside ; the upper 

 figure shows the same from 

 the inside, together with the 

 pediculated sacs of the per- 

 manent incisor and canine 

 teeth adhering to the gum. 



of the lower jaw, which 

 are immediately followed 



by those of the upper jaw ; and, as a general rule, each of the lower range of 



teeth rises through the gum before the corresponding tooth of the upper set. 



The following scheme indicates, in months, the order and time of eruption of 



the milk-teeth. 



MOLARS. 



CANINES. 



INCISORS. 



CANINES. 



Before the teeth protrude through the gum, this undergoes some peculiar 

 changes : its edge at first becomes dense and sharp, but, as the tooth ap- 

 proaches it, the sharp edge disappears, the gum becomes rounded or tumid, 

 and is of a purplish hue ; the summit of the tooth is seen like a white spot 

 or line through the vascular gum, and soon afterwards rises through it. As 

 the crown of the tooth advances to its ultimate position, the elongated fang 

 becomes surrounded by a bony socket or alveolus. Before the eruption, the 

 mucous membrane is studded with a number of small white bodies, which 

 were described by Serres as glands (dental glands), and were supposed by him 

 to secrete the tartar of the teeth. Meckel thought they were small abscesses, 

 because no aperture could be detected in them. In a fcetus of six months, 

 they were found by Sharpey to be small round pearl- like bodies situated hi 

 the corium of the mucous membrane, and having no aperture : they con- 

 sist of small spherical capsules of various sizes, lined with a thick stratum 

 of epithelium, the inner cells of which are flattened or scaly, like those lining 

 the cheek, and are so numerous as almost to fill up the cavity. They are 

 the prominences of the outer epithelial layer of the enamel organ, already 

 referred to. 



Development of the permanent teeth. The preceding description of the 

 structure of the dental sacs and pulps and of the mode of formation of the 



