186 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



and is converted by ossification into the Cementum or Crusta Petrosa ( 210). What is 

 commonly denominated the Periosteum of the Tooth, really belongs as much to the Alveolus. 

 It is connected with the tooth by the submucous cellular tissue, which originally intervened 

 between the tooth-sac and the walls of the osseous cavity. It appears from Mr. Nasmyth's 

 researches, that the inner layer of the portion of the capsule which covered the crown of 

 the tooth, remains adherent to it; forming a thin coating of Crusta Petrosa (most of which 

 is, however, soon worn off) over the Enamel. During the period that the Milk-teeth have 

 been advancing, along with their sockets, to their perfect state and ultimate position, the Per- 

 manent sacs have been receding in an opposite direction, and have with their bony crypts 

 been enlarging; and at last they occupy a position almost exactly below the former (wando). 

 They still retain a communication with the gum, however; the channel by which they de- 

 scended not having completely closed up, and the neck of the sac being elongated into a 

 cord which passes through this. The channels may afterwards serve as the itinera ckntium, 

 and the cords as gubernacula ; but it is uncertain whether they really afford any assistance in 

 directing the future rise of the tooth to the surface ; the successive stages of which are repre- 

 sented in Fig. 98, p t. The sacs of the permanent teeth derive their first vessels from the 

 gums ; ultimately they receive their proper dental vessels from the Milk-sacs ; and, as they 

 separate from the latter into their own cells, the newly-formed vessels, conjoining into com- 

 mon trunks, also retire into permanent dental canals, and gradually become the most direct 

 channels for the blood transmitted through the jaw. 



g. The following interesting generalizations respecting the development of the teeth, result 

 from Mr. Goodsir's researches. 1. The Jfi^-teeth are formed on both sides of either jaw 

 in three divisions,' a Molar, a Canine, and an Incisive ; in each of which, dentition pro- 

 ceeds in an independent manner. 2. The dentition of the whole arch proceeds from 

 behind forwards ; the Molar division commencing before the Canine, and the Canine 

 before the Incisive. 3. The dentition of each of the divisions proceeds in a contrary direc- 

 tion, the Anterior Molar appearing before the Posterior, the Central Incisor before the La- 

 teral. 4. Two of the subordinate phenomena of nutrition also obey this inverse law ; 

 the follicles closing by commencing at the median line and proceeding backwards ; and the 

 dental groove disappearing in the same direction. 5. Dentition commences in the Upper 

 Jaw, and continues in advance during the most important period of its progress. The 

 development of the Superior Incisors, however, is retarded by a peculiar cause; so that the 

 Inferior Incisors have the priority in the time of their completion and appearance. 6. The 

 germs of the Permanent teeth, with the exception of that of the Anterior Molar, appear in a 

 direction from the median line backwards. 7. The Milk-teeth originate, or are developed, 

 from mucous membrane. 8. The Permanent teeth, also originating from mucous membrane, 

 are of independent origin, and have no connection with the milk-teeth. 9. A tooth-pulp 

 and its sac must be referred to the same class of organs, as the combined Papilla and Folli- 

 cle from which a hair or feather is developed. 



h. The following is the usual order and period of appearance, of the several pairs of Milk- 

 teeth. The Four Central Incisors first present themselves, usually about the seventh month 

 after birth ; but frequently much earlier or later : those of the Lower Jaw appear first. 

 The Lateral Incisors next show themselves, those of the Lower Jaw coming through before 

 those of the upper ; they usually make their appearance between the seventh and tenth months. 

 After a short interval, the Anterior Molars present themselves, generally soon after the 

 commencement of the Second Year ; and these are followed by the Canines, which usually 

 protrude themselves between the fourteenth and twentieth months. The Posterior Molars are 

 the last, and the most uncertain in regard to their time of appearance; this varying from 

 the eighteenth to the tJtirty-sixth month. In regard to all except the front teeth, there is no 

 settled rule as to the priority of appearance of those in the Upper or Under Jaw : some- 

 times one precedes, and sometimes the other; but in general it may be stated, that, when- 

 ever one makes its appearance, the other cannot be far off. The same holds good in re- 

 gard to the two sides, in which development does not always proceed exactly paripassu. 

 The period of Dentition is one of considerable risk to the Infant's life. The pressure upon 

 the nerves of the gum, which necessarily precedes the opening of the sac and the eruption 

 of the tooth, is a fruitful source of irritation; producing disorder of the whole system, espe- 

 cially of the Digestive organs, and not unfrequently giving origin to fatal Convulsive affec- 

 tions. These last have been particularly studied by Dr. M. Hall, who recommends the free 

 use of the gum-lancet, as a most important means of prevention and cure. Even where 

 Dentition proeeeds quite naturally and is not itself a cause of diseased action, it induces an 

 irritable state of the whole constitution, which aggravates the elferfs of other morbific causes. 

 It is, therefore, of the greatest consequence that the infant should be withdrawn during this 

 period, from all injurious influences; and that no irregularity of diet, or deficiency of fresh 

 air and exercise, should operate to its disadvantage. 



i. After the lapse of a few years, the further elongation of the jaw permits the appear- 

 ance of the First True Molar; which, as already remarked, is really a Milk-tooth, so far as 

 its formation is concerned. This commonly presents itself about the middle or end of the 



