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the dental lamina or tooth-band. The dental lamina divides into two plates, a lateral vertical and a medial more horizontal in direction. The medial plate is the portion from which the teeth are formed, and is termed the dental ledge. On the under surface of the dental ledge there soon appears a series of knob-like projections—one for each of the milk teeth (Fig. 971, II.)—which are known as adamant germs or adamant organs. These organs are connected with the epithelium of the dental ledge by a constricted part, and although at first knob-like, they soon become bell-shaped owing to the invagination of the lower surface of the knobs, so that each may now be compared to
an inverted egg-cup.
A, Section of skin showing epithelium e, baseinent membrane b, and connective tissue layer c.
2. As soon as the adamant organs begin to assume a cup-like shape, the cellular connective tissue of the jaw beneath grows up and projects into the cavity of the cup (Fig. 971, III.) in the form of a papilla-the papilla dentis. arrangement, pursuing our simile, may now be compared to an egg fitting into its cup-the papilla representing the egg, and the adamant organ the cup (Fig. 971, III.).
B shows the papilla of the connective tissue layer growing up covered by the epithelial layer.
In C the superficial cells of the papilla z begin to deposit ivory d over the papilla, and at the same time the deepest cells of the epithelium deposit adamant a.
3. The two layers of cells which are thus brought in contact, namely, the epithelial cells lining the concavity of the adamant organ, and the superficial cells of the tooth papilla, become elongated or columnar, and undergo other changes, preliminary to the production of the adamant by the former-which are now called adamant cells or ameloblasts and the ivory by the latter, which are known as odontoblasts. 4. The odontoblasts, that is the layer of columnarshaped connective tissue cells lying on the surface of the dental papilla, begin to form at their outer ends a layer of ivory (Fig. 971, IV.). Similarly, the adamant cells lining the cup begin to form at their inner surface a layer of adamant on the top of the layer of ivory (Fig. 971, IV.), to which it adheres: FIG. 970.-DIAGRAM TO ILLUSTRATE THE
in each case the deposit taking place first at the summit of the tooth.
Ꭰ shows the tooth breaking through the epithelium and reaching the surface.
DEVELOPMENT OF A DERMAL TOOTH IN
all figures-a, adamant; b, basement mem-
5. The formation of these tissues proceeds apace, the ivory increasing at the expense of the papilla, the adamant similarly encroaching on the cup or adamant organ; and in each case the two layers of cells-odontoblasts and adamant cells -which produced the deposits, retiring gradually from one another, as the space between them becomes occupied by the newly formed tissues (Fig. 971, V.).
The remains of the dental papilla persist as the pulp of the tooth, which is covered even in the adult by the odontoblasts, and occupies the tooth cavity, i.e. the central part of the tooth to which the formation of ivory has not extended.
6. Turning now to the jaw itself: The connective tissue of the gum surrounding the tooth germ (as the developing tooth with its adamant organ and dental papilla are called) early becomes condensed and vascular (Fig. 971, V.), and later on forms a membranous bag-the tooth-sac or follicle-which completely shuts off the developing tooth from the surrounding structures. On the floor of the sac the tooth germ sits, the base of its dental papilla being continuous with the tissue of the floor of the sac, and the young tooth being enclosed by the sac, as a kernel is enclosed by its shell.
7. Reverting to the tooth: When the crown is completed the deposit of ivory, but not of adamant, is continued downwards to form the root. The root is composed chiefly of ivory, continuous above with that of the crown, and like it formed by the odontoblasts of the dental papilla. As the ivory is deposited, and the root is being built up, the connective tissue of the tooth-sac comes to surround the root more closely, and deposits on its surface, after the manner of a periosteum, a layer of bone, the substantia ossea. After that layer has been formed, the connective tissue of the sac persists as the alveolar periosteum. The development of the root takes place very slowly, and its lower end is not completed as a rule for some time after the eruption of the tooth has taken place.
8. During the development of the teeth the ossification of the jaw has been going on, and as it grows up on each side, the young teeth, enclosed in their tooth-sacs, come to lie in an open bony groove, which is subsequently divided by septa into compartments-the alveoli-for the individual tooth-sacs. The bone continuing to grow after birth, these
compartments become more perfect, but are never entirely closed in over the crowns of the teeth. During the eruption of the teeth the upper and anterior part of each of the bony cells is absorbed; subsequently, however, it is re-formed around each tooth when it has taken its final position.
9. Eruption. Long before the root is completed, the crown, by some force which is
FIG. 971.-DIAGRAM TO ILLustrate DevELOPMENT
OF A TOOTH.
A, Inner layer of adamant germ; B, Outer layer; C, Remains of intermediate cells; D, Ivory; D.L, Ivory lamina; E, Epithelium; E.G, Adamant germ; EN, Adamant; F, Dental furrow; L.D, Labio-dental furrow; M, Connective tissue cells; O, Odontoblasts; P, Ivory papilla; R.G, Reserve germ; V,
not properly understood, but which does not seem to depend on additions to the root, is pushed through the top of the tooth-sac, and—the upper and anterior wall of the roomy alveolus having been absorbed at the same time-onwards through the gum until the mouth is reached. Later, when the tooth bas assumed its final position, the alveolus, as already stated, is re-formed, and closely embraces the completed root.
10. After the adamant organs of the deciduous teeth have been formed on the inferior aspect of the dental lamina, as described above, the neck of epithelium by which the lamina is still connected with the surface becomes broken up into a cribriform sheet. Its free posterior border, on the other hand, continues to grow inwards in the tissue of the gum towards the cavity of the mouth (Fig. 971, III. and IV.), and at a later date there appear on its under surface, near the free edge, and behind the several developing milk teeth, the adamant organs-or so-called "reserve germs"for the corresponding permanent teeth, which are developed in exactly the same manner as the deciduous teeth described above.
In connexion with the development of the permanent molars, which have no corresponding teeth in the deciduous set, there takes place a prolongation backwards of the posterior extremity of the dental lamina into the tissue of the jaw, behind the last deciduous molar. On the inferior aspect of this prolongation, which has no direct connexion with the surface epithelium, adamant organs are formed for the permanent molars, and their further development goes on in the manner described for the other teeth.
The dates at which some of the chief events in the development of the teeth occur may be briefly given :-The thickening of the epithelium, the first sign of the future teeth, begins about the sixth week of foetal life, and the dental lamina is completed by the end of the seventh week. The dental papillæ for the eight front teeth appear and become surrounded by their adamant organs about the tenth week, and the papilla for the first permanent molar about
the seventeenth week.
The first traces of calcification, and the formation of the tooth-sacs, take place about the fifth month of foetal life.
Eruption of Deciduous Teeth. The period at which the eruption of the milk teeth takes place is extremely variable, and no two observers seem to agree upon the question.
The following, according to Tomes, may be taken as representing the average. The lower central incisors appear first, usually between the sixth and ninth months; then follows a rest of a few months. Next come the four upper incisors, followed by a rest of a few months. Then the lower lateral incisors and the four first molars erupt, succeeded by a rest of a few months. Next appear the canines, and finally the four second molars, which are all cut by the end of the second year.
The following statement is simple, and perhaps is sufficient for all ordinary purposes. The deciduous teeth usually appear in the following order: Central incisors, lateral incisors, first molars, canines and second molars; the eruption commences between the sixth and the ninth month, and is usually completed by the twenty-fourth-the lower teeth, as a rule, preceding the upper.
Formation of Adamant and Ivory.-Different opinions are held as to the method in which the substantia adamantina is produced by the cells concerned in the process. One view maintains that it is secreted and shed out by the cells (Kölliker). According to the other view, part of the substance of the cells is actually converted or transformed into adamant (Tomes). In connexion with this latter view, which seems to receive more support at present, Tomes has discovered that there projects from the base of each cell, towards or into the most recently formed enamel, a fibrillar process, which has received the name of Tomes' process, and he holds that the substantia adamantina is formed by calcification taking place in or around the process. Similarly, two views are held as to the production of ivory by the odontoblasts; one, that the odontoblasts secrete the matrix ivory, and the other, that their substance is actually converted into the matrix of the ivory. The odontoblasts, when active, are branched columnarshaped cells, and from their outer ends one or more processes extend towards and into the ivory; between these processes a matrix appears-produced probably by the odontoblasts-and soon this matrix becomes calcified. In this way the ivory is formed, and the process is repeated until its full thickness is attained. The branches of the odontoblasts, encased in ivory, just mentioned, are the Tomes' fibrils already described; the canals in which they lie are the dental canaliculi; and the fibrils themselves are concerned in the production of the sheaths of Neumann which line the tubes.
The tooth-sacs, when fully developed, are large and distinct fibrous bags which lie in the alveoli of the maxilla and mandible, and are continuous above with the tissue of the gum. On the lingual side of the sacs of the deciduous teeth are found the germs of the permanent teeth, surrounded by their own sacs. These latter are at first very small, and are partly embedded in the posterior wall of the deciduous tooth-sacs, but subsequently they come to lie in distinct but incomplete bony cavities of their own. The bone surrounding the tooth-sacs, temporary and permanent, is always wanting over the summit of the sac, and the band of connective tissue by which the sac is connected with the overlying gum tissue, through the deficiency, is known as the gubernaculum dentis.
All the points mentioned are easily demonstrated on the mandible of a child at birth, particularly when the tissues have been allowed to soften a little. If, in such a specimen, the gum and periosteum are reflected upwards from the labial and lingual surfaces of the mandible, and freed as far as the superior border of the jaw, the gum, with the tooth-sacs depending from it like small bags, can be pulled away out of the bony groove of the jaw; and if the operation has been successfully performed, the tooth-sacs of the three front permanent teeth may be seen, varying in size from a small pin's-head to a hemp-seed, hanging down behind the superior part of the corresponding deciduous sacs. As already explained, the tooth-sacs are produced simply by a condensation of the connective tissue around the developing tooth, the condensation going on to the formation of a distinct membranous bag.
Formation of Alveoli and Eruption.-At first the developing teeth lie in an open bony groove or channel between the labial and lingual plates of the young jaw. This groove is subsequently divided up into separate compartments for the sacs of each of the deciduous teeth. As development proceeds these compartments or alveoli surround the sacs more completely, but never actually close over the summit. When the eruption of the deciduous teeth is about to take place, the anterior wall and roof of the alveolus are absorbed; the tooth passes through the sac and appears above the gum, and then the alveolus, which up to this was much too large to give actual support, is re-formed more closely around the tooth. Meanwhile the root, which was only partly formed at the time of the eruption, continues to be added to, possibly for a few years more, and, as it grows, the alveolus is completed around it. When the permanent tooth, or as much of it as is then formed, is about to be erupted, it makes its way from its own bony cell through the posterior wall of the alveolus of its temporary predecessor; the root of the deciduous tooth undergoes absorption at the same time, but quite independently of pressure from the permanent tooth. The alveolus, now occupied by both teeth, is again much enlarged
by absorption, particularly in front; what remains of the temporary tooth is shed; the permanent tooth passes onwards through the enlarged alveolus, and, making its way to the surface, appears above the gum. After some time, when the tooth has taken its final position, the alveolus is again re-formed, first around its neck, and later on, as the root is built up, around it also, and thus the tooth is permanently fixed.
What the force is which causes the eruption, is a question that has not been answered satisfactorily. That the growth of the root pushes up the crown was formerly the favourite explanation. For several reasons, unnecessary to detail, this view is now dis carded, and a theory which attributes the impelling force to the blood pressure is looked upon with more favour, although even this is not altogether satisfactory. (See Tomes Dental Anatomy, 5th Edition, page 211.)
MORPHOLOGY OF THE TEETH.
In most vertebrates below mammals all the teeth are alike in form; such a dentition is said to be homodont. In the majority of mammals, on the other hand, the teeth are arranged in groups of different size and form; such a dentition is heterodont.
Again, mammals have, neglecting exceptional cases, but two functional sets of teeth; they are consequently said to be diphyodont. Most vertebrates below mammals, on the other hand, have a continuous succession of teeth throughout life, and hence are said to be polyphyodont
Seeing that practically all lower vertebrates are provided with simple conical teeth, the evolu tion of the multi-tuberculate mammalian molar has given rise to much speculation. The jaws of the earliest fossil mammals found are furnished with tri-tubercular teeth, the three tubercles being placed in an antero-posterior line; by a rotation of two of the tubercles to the lingual or labial side, as the case may be (a condition found in certain other fossil skulls), we arrive at a tri-tubercular form, from which the transition to an ordinary mammalian molar is not difficult. As to how the tri-tubercular tooth arose from the simple cone, two different views are advanced one, it was formed by the union of several conical teeth as a result of the shortening of the jaw and the crowding of the teeth together; the other, that the single conical tooth developed on its crown two subsidiary tubercles, one in front and the other behind, and that these tuberdes growing larger, the tooth assumed the tri-tubercular form.
The complete or typical mammalian dentition, in its highest development, as in the horse, is represented by the following formula: i., c. t, pm. 1, m. =44. In the dentition of man, there fore, one incisor and two premolars are wanting. Different views are held as to which teeth have been suppressed-most probably they are the second incisors, and the first and second or first and last premolars.
In general it may be said that the dentition of the lower races differs from that of the higher, in that the dental arches are squarer in front, the teeth larger and more regular, the canines stronger, the last molars better developed, and the tubercles on the molars more perfect, in the lower than in the more civilised races. It may be mentioned, however, that the teeth of a savage man, if seen in the mouth of a European, would be looked upon as an "exceedingly perfectly formed set of teeth" (Tomes).
To express the proportion in size of the crowns of the premolars and molars to that of the skull in different races, Flower compared the distance from the front of the first premolar to the back of the last molar, in situ, with the distance from the front of the foramen magnum to the naso-frontal suture (basi-nasal length), in the form of a "dental index ".
and by this means he has divided the various races into microdont (index 42 to 43, Europeans, Egyptians, etc.), mesodont (index 43 to 44, Chinese, American Indians, Negroes, etc.), and macrodont (index 44 and upwards, Australians, Melanesians, etc.).
DEVELOPMENT OF PRIMITIVE PHARYNX AND PHARYNGEAL PORTION OF THE
The anterior blind termination of the foregut in the head region constitutes the primitive pharynx.
Its roof is formed by the tissues covering the under aspect of the mid- and fore-brain, and its floor by the tissues overlying the heart and pericardium. Each side wall is a lamina of tissue extending from the floor to the roof, continuous, in front, with the bucco-pharyngeal membrane, which forms the anterior wall of the pharynx and separates it from the stomodeum.
In the roof are formed the tissues which form the hinder part of the base of the skull. In the side wall and in the floor extensive changes occur, connected with the appear ance of structures known as the visceral arches and pouches, and with the origin of numerous structures from them and the development of the tongue.
Tongue. The tongue is formed in two portions, anterior and posterior, in the floor of the pharynx.
The anterior portion, forming the anterior two-thirds of the organ, is formed from the tissues on each side of the tuberculum impar, which grow up and enclose that elevation, and from the tuberculum itself. Evidence of this bilateral origin of the tongue is found in those cases of bifid tongue which occur, though rarely (see p. 45).
The hinder portion, forming the posterior third of the tongue, is formed from the tissues covering the inner ends of the second pair of arches (see p. 45).
These arches, as has been pointed out, meet and fuse in a common mass in front of the furcula, in the floor of the sinus arcuatus.
Between the ridge which they form and the tuberculum impar in front, a slight median depression is found, from which the median thyreoid diverticulum is formed, and which persists as the foramen cæcum of the tongue. On each side of this depression a groove runs obliquely laterally and upwards (the sulcus terminalis) immediately behind the region of the vallate papillæ, and marks the union of the anterior and posterior portions of the tongue.
The tongue mass formed by the union of these different parts increases in size, rises upwards from the floor of the pharynx, and projects forwards.
The tissue forming its interior becomes transformed into the muscular substance of the tongue, and is derived largely from the first branchial region, and not from the musculature of the visceral arches.
The investing epithelium of the anterior two-thirds gives rise to the papillæ and the taste buds, while that covering the posterior portion remains smooth. The papillæ appear about the third month as elevations of the corium, covered with epithelium.
The vallate papillæ are formed by ingrowths of the epithelium in rings, around a central core. The superficial layers of the epithelium desquamate and form the trench
surrounding the papilla.
Submaxillary and Sublingual Glands.-These glands are formed in the alveololingual groove in the floor of the primitive pharynx, immediately behind the first arch, by outgrowths somewhat similar to those described in connexion with the parotid (q.v.). The submaxillary outgrowth occurs about the fifth week, and the sublingual, several in number, on the outer side of it at the ninth week.
Palatine Tonsil. The glosso-palatine arch arises in the position occupied earlier by the second visceral arch, behind which, in the embryo, lies the pharyngeal portion of the second visceral cleft. The dorsal extremity of that cleft enlarges, and forms a recess termed the sinus tonsillaris. From the lower and greater part of the sinus tonsillaris the palatine tonsil is developed; the upper part of the sinus persists, however, as the supratonsillar fossa. The palatine tonsil at first is a smooth depression of the mucous membrane. About the fourth month of fœtal life downgrowths of the epithelium take place, which are afterwards converted into the tonsillar crypts. Subsequently lymph cells accumulate around the downgrowths and form the lymph tissue, which constitutes the mass of the organ. The plica triangularis is formed from a tubercle, which becomes flattened, and forms a fold on the anterior and medial aspect of the inferior part of the palatine tonsil.
DEVELOPMENT OF ESOPHAGUS, STOMACH, AND INTESTINES.
Esophagus.-The œsophagus is formed from the foregut. The lengthening of the thoracic region of the trunk, which occurs with the growth and development of the heart and lungs, causes this portion of the alimentary tube to become greatly lengthened. The entodermal lining forms the epithelial layer, and the mesoderm the other coats of the œsophagus.
Stomach. As early as the fourth week, the foregut exhibits a fusiform enlargement in the region of the developing heart, which is the first evidence of the differentiation of the stomach: this enlargement takes the form first of an outgrowth on the dorsal border to form the fundus. Soon, however, as the diaphragm is being formed, the stomach passes into the abdomen, and its dorsal wall-the future greater curvature begins to grow more rapidly than the ventral wall. As a result the whole organ becomes somewhat curved, and its inferior end is carried forwards from the posterior abdominal wall, giving rise to the curvature of the duodenum. The excessive growth of its posterior wall causes the stomach to turn over on to its right side, which now becomes posterior or dorsal. In this rotation the upper or cardiac portion moves to the left of the median plane, and the whole organ assumes an oblique direction across