294 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



ing stage, which has long been known as the saccular, is the one during which 

 the whole formation of the Dentine, and of the Enamel, takes place. It is during 

 this period, also, that the ossification of the jaw is being effected; and that the 

 bony sockets are formed for the teeth, by the consolidation of the anterior and 

 posterior ridges bounding the alveolar groove (in which the dental groove was 

 originally imbedded), and of the interfollicular septa, which are produced by the 

 meeting of transverse projections from these ridges. We have now only to con- 

 sider the fourth or eruptive stage ; that in which the Teeth make their way 

 through the gum. This process chiefly results from the lengthening of the fang, 

 by the addition of new dentinal substance; so that the crown of the tooth is 

 made to press against the closed mouth of the sac (Fig. 84, m). This at last 

 gives way, and the sac then assumes its previous condition of an open follicle. 

 When the edge of the tooth has once made its way through the gum, it advances 

 more rapidly than can well be accounted for by the usual rate of lengthening of 

 its fang; and this appears to be due to the separation of the bottom of the sac 

 from the fundus of the alveolus ; so that the whole tooth apparatus is carried 

 nearer to the surface, leaving a space at the bottom of the alveolar cavity, in 

 which the further lengthening of the root can take place (n). The open portion 

 of the sac remains as the narrow portion of the gum, which forms a vascular 

 border and groove round the neck of the perfected tooth (o). The deeper portion 

 of the sac adheres to the fang of the tooth ; and it is here that the cemental pulp 

 is found, constituting (as it were) its inner layer. What is commonly denomi- 

 nated 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. During 

 the period that the "milk" teeth have been advancing, along with their sockets, 

 to their perfect state and ultimate position, the " permanent" sacs have been 

 receding in an opposite direction, and have with their bony crypts been enlarg- 

 ing; and at last they occupy a position almost exactly below the former (n and 

 o). They still retain a communication with the gum, however; the channel by 

 which they descended not having completely closed up, and the neck of the sac 

 being elongated into a cord which passes through this. The channels may after- 

 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. 84, p L 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 alveoli, the newly-formed 

 vessels, conjoining into common trunks, also retire into permanent dental canals, 

 and gradually become the most direct channels for the blood transmitted through 

 the jaw. The history of development in the Lower Jaw is very nearly the same ; 

 the chief difference being in the origin and situation of the primitive dental 

 groove. 



287. The following interesting generalizations respecting the development of 

 the teeth, result from Prof. Groodsir's researches: 1. The "milk" teeth 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 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 direction, the an- 

 terior Molar appearing before the posterior, the central Incisor before the lateral. 

 4. Two of the subordinate phenomena of nutrition also follow 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 com- 

 mences in the upper jaw, and continues in advance during the most important 



