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lamina or tooth-band. The dental lamina divides into two plates, a lateral I a medial more horizontal in direction. The medial plate is the portion the teeth are formed, and is termed the dental ledge. On the under surface ital ledge there soon appears a series of knob-like projections—one for : milk teeth (Fig. 971, II.)—which are known as adamant germs or adamant hese organs are connected with the epithelium of the dental ledge by a conrt, and although at first knob-like, they soon become bell-shaped owing to ation of the lower surface of the knobs, so that each may now be compared to egg-cup. oon as the adamant organs begin to assume a cup-like shape, the cellular tissue of the jaw beneath grows up and to the cavity of the cup (Fig. 971, III.)

A, Section of skin

The. a of a papilla—the papilla dentis.

showing epithelium

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brane b, and connec.

tive tissue layer c. n egg fitting into its cup- the papilla g the egg, and the adamant organ the

cup

B shows the papilla of II.).

layer growing up two layers of cells which are thus brought

covered by the epi.

thelial layer. namely, the epithelial cells lining the conhe adamant organ, and the superficial cells

In Cthe superficial cells h papilla, become elongated or columnar,

of the papilla 2 begin zo other changes, preliminary to the pro

to deposit ivory d

over the papilla, and the adamant by the former — which are

deepest cells of the adamant cells or ameloblasts—and the

epithelium deposit le latter, which are known as odontoblasts. odontoblasts, that is the layer of columnarnective tissue cells lying on the surface of papilla, begin to form at their outer ends

breaking through { ivory (Fig. 971, IV.). Similarly, the

the epithelium and

reaching the surface. ells lining the cup begin to form at their ice a layer of adamant on the top of the Cory (Fig. 971, IV.), to which it adheres :

Fig. 970.-DIAGRAM TO ILLUSTRATE THE

DEVELOPMENT OF A DERMAL TOOTH IN e the deposit taking place first at the sum

THE SHARK. tooth.

In all figures--a, adamant; b, basement mem. forniation of these tissues proceeds apace, brane ; c, connective tissue layer of skin ;

d, ivory ; e, epithelium; and 2, superficial increasing at the expense of the papilla,

cells of papilla. int similarly encroaching on the cup or rgan; and in each case the two layers of cells—odontoblasts and adamar cells roduced the deposits, retiring gradually from one another, as the space between mes occupied by the newly formed tissues (Fig. 971, V.). mains of the dental papilla persist as the pulp of the tooth, which is covered

adult by the odontoblasts, and occupies the tooth cavity, i.e. the central part h to which the formation of ivory has not extended. ning now to the jaw itself: The connective tissue of the gum surrounding the 1 (as the developing tooth with its adamant organ and dental papilla are called) mes condensed and vascular (Fig. 971, V.), and later on forms a membranous tooth-sac or follicle—which completely shuts off the developing tooth from the g structures. On the floor of the sac the tooth germ sits, the base of its illa being continuous with the tissue of the floor of the sac, and the young g enclosed by the sac, as a kernel is enclosed by its shell. erting to the tooth : When the crown is completed the deposit of ivory, but mant, is continued downwards to form the root. The root is composed chiefly ontinuous above with that of the crown, and like it formed by the odontoblasts ntal papilla. As the ivory is deposited, and the root is being built up, the

tissue of the tooth-sac comes to surround the root more closely, and deposits ace, after the manner of a periosteum, a layer of bone, the substantia ossea.

layer has been formed, the connective tissue of the sac persists as the alveolar n. The development of the root takes place very slowly, and its lower end is eted as a rule for some time after the eruption of the tooth has taken place. ring the development of the teeth the ossification of the jaw has been going on, grows up on each side, the young teeth, enclosed in their tooth-sacs, come to lie

bony groove, which is subsequently divided by septa into compartments—the or the individual tooth-sacs. The bone continuing to grow after birth, these

the dental lamina D.L from

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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

not properly understood, but which does

not seem to depend on additions to the I. Shows the downgrowth of

root, is pushed through the top of the the surface epithelium E

tooth-sac, and—the upper and anterior and the beginning of the

wall of the roomy alveolus having been adamant germ E.G.

absorbed at the same time-onwards through the gum until the mouth is reached. Later, when the tooth has

assumed its final position, the alveolus, II. Shows the further growth

as already stated, is re-formed, and of the adamant germ and its invagination.

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, III. The adamant germ is

as described above, the neck of epithelium more invaginated, and its inner layer of cells becomes

by which the lamina is still connected A, the dental lamina, grows thinner, but

with the surface becomes broken up into near its posterior or lingual

a cribriform sheet. Its free posterior edge there is an enlarge- III

border, on the other hand, continues to serve germ for a permanent tooth. The superficial cells

grow inwards in the tissue of the gum of the ivory papilla P are

towards the cavity of the mouth (Fig. becoming columnar.

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 of the adamant germ (called

organs—or so-called “reserve germs"adamant cells). A formed a capofadamant En,

for the corresponding permanent teeth, inside which the superficial IV

which are developed in exactly the cells of the papilla, the odontoblasts

same manner as the deciduous teeth formed a layer of ivory D.

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 V. Shows a more advanced

of the dental lamina into the tissue of stage still. The deposit of

the jaw, behind the last deciduous molar. ivory is extending down. wards, and enclosing the

On the inferior aspect of this prolongapapilla to form the future pulp, in which a vessel V

tion, 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. Fig. 971.-DIAGRAM TO ILLUSTRATE DEVELOPMENT

The dates at which some of the OF A TOOTH.

chief events in the development of the A, Inner layer of adamant germ; B, Outer layer; C, Remains

of interinediate cells ; D, Ivory'; D.L, Ivory lamina ; E, teeth occur may be briefly given :- The furrow ; L. D, Labio-dental furrow;' M, Connective tissue

cells; thickening of the epithelium, the first 01 Odontoblasts ; P, Ivory papilla ; 'R.G, Reserve germ; v, sign of the future teeth, begins about

the sixth week of fætal 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 fætal 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.

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%, according to Tomes, may be taken as representing the average. The

incisors appear first, usually between the sixth and ninth months; then of a few months. Next come the four upper incisors, followed by a rest of 5. Then the lower lateral incisors and the four first molars erupt, succeeded

few months. Next appear the canines, and finally the four second molars, cut by the end of the second year. wing statement is simple, and perhaps is sufficient for all ordinary purposes. us teeth usually appear in the following order :-Central incisors, lateral

molars, canines and second molars; the eruption commences between the e ninth month, and is usually completed by the twenty-fourth—the lower ale, preceding the upper. n of Adamant and Ivory.-Different opinions are held as to the method in which a adamantina is produced by the cells concerned in the process. One view mainis secreted and shed out by the cells (Kölliker). According to the other view, substance of the cells is actually converted or transformed into adamant (Tomes). 1 with this latter view, which seems to receive more support at present, Tomes d that there projects from the base of each cell, towards or into the most recently el, a fibrillar process, which has received the name of Tomes' process, and he holds -tantia adamantina is formed by calcification taking place in or around the process. , two views are held as to the production of ivory by the odontoblasts ; one, that asts secrete the matrix ivory, and the other, that their substance is actually conthe matrix of the ivory. The odontoblasts, when active, are branched columnar

and from their outer ends one or more processes extend towards and into the ivory ; se processes a matrix appears-produced probably by the odontoblasts—and soon this mes calcified. In this way the ivory is formed, and the process is repeated until ness is attained. The branches of the odontoblasts, encased in ivory, just mentioned, es' fibrils already described ; the canals in which they lie are the dental canaliculi; ils themselves are concerned in the production of the sheaths of Neumann which

es.

th-sacs, when fully developed, are large and distinct fibrous bags which lie oli of the maxilla and mandible, and are continuous above with the tissue of On the lingual side of the sacs of the deciduous teeth are found the germs of the teeth, surrounded by their own sacs. These latter are at first very small, and embedded in the posterior wall of the deciduous tooth-sacs, but subsequently to lie in distinct but incomplete bony cavities of their own. Tho bone surhe tooth-sacs, temporary and permanent, is always wanting over the summit of d the band of connective tissue by which the sac is connected with the overtissue, through the deficiency, is known as the gubernaculum dentis. points mentioned are easily demonstrated on the mandible of a child at birth, y when the tissues have been allowed to soften a little. If, in such a specimen, nd periosteum are reflected upwards from the labial and lingual surfaces of the and freed as far as the superior border of the jaw, the gum, with the tooth-sacs from it like small bags, can be pulled away out of the bony groove of the jaw; operation has been successfully performed, the tooth-sacs of the three front

teeth may be seen, varying in size from a small pin's head to a hemp-seed, own behind the superior part of the corresponding deciduous sacs.

As already the tooth-sacs are produced simply by a condensation of the connective tissue e developing tooth, the condensation going on to the formation of a distinct us bag. ation of Alveoli and Eruption.—At first the developing teeth lie in an open ve or channel between the labial and lingual plates of the young jaw. This subsequently divided up into separate compartments for the sacs of each of the teeth. As development proceeds these compartments or alveoli surround the completely, but never actually close over the summit. When the eruption of nous teeth is about to take place, the anterior wall and roof of the alveolus are ; the tooth passes through the sac and appears above the gum, and then the which up to this was much too large to give actual support, is re-formed more ound the tooth. Meanwhile the root, which was only partly formed at the time aption, continues to be added to, possibly for a few years more, and, as it grows, -lus is completed around it. When the permanent tooth, or as much of it as ormed, is about to be erupted, it makes its way from its own bony cell through rior wall of the alveolus of its temporary predecessor ; the root of the deciduous dergoes absorption at the same time, but quite independently of pressure from anent 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 finai 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 arrangeri 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 evolution 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, that 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 tubercles 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. 1 = 44. In the dentition of man, therefore, 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 fint 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

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".

Length of teeth x 100
Thus :

=Dental index,

Basi-nasal length and by this means he has divided the various races into microdont (index 42 to 43, Europeany 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

MOUTH.

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 stomodeun.

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.

-The tongue is formed in two portions, anterior and posterior, in the floor of

ior portion, forming the anterior two-thirds of the organ, is formed from the :h side of the tuberculum impar, which grow up and enclose that elevation,

tuberculum itself. Evidence of this bilateral origin of the tongue is found 3 of bifid tongue which occur, though rarely (see p. 45). er portion, forming the posterior third of the tongue, is formed from the ing the inner ends of the second pair of arches (see p. 45). ches, as has been pointed out, meet and fuse in a common mass in front of in the floor of the sinus arcuatus. the ridge which they form and the tuberculum impar in front, a slight ession is found, from which the median thyreoid diverticulum is formed, and ts as the foramen cæcum of the tongue. On each side of this depression a obliquely laterally and upwards (the sulcus terminalis) immediately behind of the vallate papillæ, and marks the union of the anterior and posterior he tongue. çue mass formed by the union of these different parts increases in size, rises m the floor of the pharynx, and projects forwards. ue forming its interior becomes transformed into the muscular substance of and is derived largely from the first branchial region, and not from the of the visceral arches. esting epithelium of the anterior two-thirds gives rise to the papillæ and the while that covering the posterior portion remains smooth. The papillæ appear aird month as elevations of the corium, covered with epithelium. ate papillæ are formed by ingrowths of the epithelium in rings, around a . The superficial layers of the epithelium desquamate and form the trench s the papilla. illary and Sublingual Glands.— These glands are formed in the alveoloove in the floor of the primitive pharynx, immediately behind the first arch, ths somewhat similar to those described in connexion with the parotid (q.v.). maxillary outgrowth occurs about the fifth week, and the sublingual, several on the outer side of it at the ninth week. e Tonsil. —The glosso-palatine arch arises in the position occupied earlier by

visceral arch, behind which, in the embryo, lies the pharyngeal portion of visceral cleft. The dorsal extremity of that cleft enlarges, and forms a recess sinus tonsillaris. From the lower and greater part of the sinus tonsillaris the asil is developed ; the upper part of the sinus persists, however, as the suprassa. The palatine tonsil at first is a smooth depression of the mucous membrane. fourth month of fætal life downgrowths of the epithelium take place, which are converted into the tonsillar crypts. Subsequently lymph cells accumulate downgrowths and form the lymph tissue, which constitutes the mass of the ne plica triangularis is formed from a tubercle, which becomes flattened, and d on the anterior and medial aspect of the inferior part of the palatine tonsil.

DEVELOPMENT OF ESOPHAGUS, STOMACH, AND INTESTINES.

agus.-The æsophagus is formed from the foregut. The lengthening of the egion of the trunk, which occurs with the growth and development of the lungs, causes this portion of the alimentary tube to become greatly lengthened. ermal lining forms the epithelial layer, and the mesoderm the other coats of agus. ch.--As early as the fourth week, the foregut exhibits a fusiform enlargement gion of the developing heart, which is the first evidence of the differentiation of ch: this enlargement takes the form first of an outgrowth on the dorsal

form the fundus. Soon, however, as the diaphragm is being formed, the passes into the abdomen, and its dorsal wall—the future greater curvature to grow more rapidly than the ventral wall. As a result the whole organ somewhat curved, and its inferior end is carried forwards from the posterior I wall, giving rise to the curvature of the duodenum. The excessive growth terior wall causes the stomach to turn over on to its right side, which now posterior or dorsal. In this rotation the upper or cardiac portion moves to f the median plane, and the whole organ assumes an oblique direction across

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