FORMATION OF THE TISSUES TEETH. 495 



wards serve as the itinera dentium, 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 represented in Fig. 130, 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 common trunks, also retire into per- 

 manent dental canals, and gradually become the most direct channels for the blood trans- 

 mitted through the jaw. 



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

 result from Mr. Goodsir's researches. 1. The Milk-leeih are formed on both sides of 

 either jaw in three divisions, a Molar, a Canine, and an Incisive; in each of which, 

 dentition proceeds in an independent manner. 2. The dentition of the whole arch pro- 

 ceeds 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 con- 

 trary direction, the Anterior Molar appearing before the Posterior, the Central Incisor 

 before the Lateral. 4. Two of the subordinate phenomena of nutrition also obey this 

 inverse law; the follicles closing by commencing at the medium 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 com- 

 pletion and appearance. 6. The germs of the Permanent teeth, with the exception of that 

 of the Anterior MoLtr, 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 Follicle 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 thirty-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; sometimes one precedes, and sometimes the other; but 

 in general it may be stated, that, whenever one makes its appearance, the other cannot 

 be far off. The same holds good in regard to the two sides, in which development does 

 not always proceed exactly part passu. 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 irri- 

 tation; producing disorder of the whole system, especially of the Digestive organs, and 

 not unfrequently giving origin to fatal Convulsive affections. These last have been par- 

 ticularly 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 proceeds quite 

 naturally and is not itself a cause of diseased action, it induces an irritable state of the 

 whole constitution, which aggravates the effects of other morbific causes. It is, there- 

 fore, 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 Seventh year; sometimes preceding, and sometimes following, the exchange of the 

 Central Incisors, which takes place about the same time. When the Permanent Teeth 

 have so much enlarged that they can no longer be contained within their own alveoli, 

 they press upon the anterior parietes of those cavities, and cause their absorption ; so 

 that each tooth is allowed to come forwards, in some degree, into the lower part of the 

 socket of the corresponding Temporary tooth. The root of the temporary tooth now 

 begins to be absorbed, generally at the part nearest its successor; and this absorption 

 proceeds as the new tooth advances, until the root of the Milk-tooth is completely 



