tooth enamel dentine pulp permanent cells tissue papilla organ upper
THE TEETH. - The teeth are calcified organs developed in connection with the mucous membrane of the mouth. Their primary use is that of biting and grinding the food ; but in man they serve as aids to speech, and in many animals act as instruments of offence and defence.
Arrangement and Form of the Teeth. - Teeth are present in the greater number of the Mammalia, in which class they are implanted in sockets in the alveolar arches of the bones of the upper and lower jaws, and form only a single row in each arch. In a few mammals, as the toothed whales and the sloths, only one generation of teeth is produced, and when these drop out they are not replaced by successors ; these animals are called Monophyodont.
the majority of the Mammalii, however, there are two generations of teeth, - a temporary or milk set, which are deciduous, and are replaced by a permanent or adult set ; these animals are called Diphyodont. But in speaking of two generations of teeth it is not to be supposed that all the teeth in the adult jaw have had temporary predecessors, for the molar or back teeth have only a single generation. A. few mammals, as the toothed whales, have the teeth uniform in size, shape, and structure, and are named Homodont ; but, in the majority of the Mammalia, the teeth in the same jaw vary in size, form, and structure, and they are therefore called Ileterodont. In every Heterodont mammal, possessing a complete dentition, four groups of teeth are found, which are named incisor, canine, premolar, and molar teeth. Each of these teeth possesses a crown, which projects into the cavity of the mouth, and a fang lodged in the socket in the jaw ; at the junction of the crown and fang there is usually a constriction named the neck of the tooth.
In man the dentition is Diphyodont and Heterodont. The single row of teeth in each alveolar arch of the human jaw is characterized by the crowns of the teeth being of almost equal length, and by the absence of any great interspace, or diastema, between the different teeth, or of irregularities in the size of the interspaces, so that the teeth form an unbroken series in each jaw. The span of the upper dental arch is slightly bigger than that of the lower, so that the lower incisors fit within the upper, and the lower molars, being inclined obliquely upwards and inwards, are somewhat overlapped by the upper molars. The upper and lower dental arches terminate behind in line with each other, and the teeth arc equal in number in the two jaws.
Man possesses 32 teeth in his permanent dentition, arranged in four groups, viz.-8 incisors, 4 canines, 8 pre- molars or bicuspids, and 12 molars. The number and arrangement of the permanent teeth in the two jaws is expressed in the following formula :- Man possesses only 20 teeth in his milk or temporary dentition, and their arrangement is expressed in the following formula : - If the temporary and permanent formula: be compared with each other, it will be seen that, while the incisors and canine teeth correspond in numbers in both dentitions, in the temporary dentition there. is an absence of premolars, and the molar teeth are only eight, instead of twelve, in number. The characters of the permanent teeth will now be considered.
The incisor teeth, eight in number, are lodged in the front of the jaws, two on each side of the mesial plane. The upper incisors project downwards and forwards, the lower are directed almost vertically upwards. The oblique direction of the upper incisors in the Negroes, Kaffres, and Australians adds to the prognathic form of the face possessed by these races. The central pair of upper incisors are larger than the lateral ; whilst the lateral pair of lower incisors are larger than the central pair, which are the smallest incisor teeth. The crowns of the incisor teeth are chisel-shaped, and adapted for biting and cutting the food. When the crown is first erupted the cutting edge ie minutely serrated, but the serrations soon wear down by use. The fangs are long and simple, - being in the upper incisors round and fusiform, in the lower laterally compressed, and sometimes marked by a longitudinal groove. Although the human incisors are, as the name implies, cutting, chisel-shaped teeth, in many mammals the incisors are greatly modified inform, as for example in the tusks of the elephant. The determination of the incisor teeth does not depend, therefore, on their form, but on their position in the jaws. The name incisor is given to all the teeth situated in the pre-maxillary portion of the upper jaw, and in the anterior end of the lower jaw, whatever their shape may be.
The canine or uniruspicl teeth, four in number, one on each side of the mesial plane of each jaw, are placed next the lateral incisors. They are bigger than the incisor teeth, and the upper canines, which are sometimes called the eyeteeth, are larger than the lower ; the fangs of the upper canines are lodged in deep sockets in the superior maxillre, which extend towards the floor of each orbit. The crowns of these teeth are thick and conical ; the fangs are long, single, conical, compressed on the sides where they are marked by a shallow groove. In many Mammals these teeth are developed into large projecting tusks.
The premolar or bicuspid teeth, eight in number, two on each side of the mesial plane of each jaw, lie immediately behind the canines, and the upper bicuspids are somewhat larger than the lower. The crown is quadrilateral in form, and convex both on the inner and outer surfaces. It possesses two cusps, of which the outer or labial is larger and more projecting than the inner, palatal, or lingual cusp. The fangs of the upper bicuspids are single and laterally compressed, often bifid at the point into an outer and inner segment ; in the lower bicuspids the fangs are rounded, and taper to a single point.
The molar or multicuspid teeth, twelve in number, arc placed three on each side of the mesial plane of each jaw. They arc the most posterior teeth, are the largest of the series, and as a rule decrease in size from the first to the last ; the crowns of the lower molars are somewhat bigger than those of the upper molars. The last molar tooth does not erupt until the end of puberty, and is called dens sapientice, or wisdom, tooth. The crowns are broad, quadrilateral, and convex both on the inner and outer surfaces. The first and second upper molars have four cusps projecting from the angles of the grinding or masticating surface, and an oblique ridge often connects the large anterior internal cusp with the posterior external cusp; in the upper wisdom teeth, the two inner or palatal cusps are frequently conjoined. The first lower molar has five cusps, the fifth being interposed between the two posterior cusps ; in the second lower molar the fifth cusp is usually absent, or only rudimentary in size, but in the lower wisdom tooth it is often present. The fangs of the first and second upper molars are three in number, and divergent ; two on the outer or buccal side, one on the inner or palatal side ; in the upper wisdom the fangs are frequently partially conjoined, though trifid at the point. The fangs of the first and second lower molars are two in number, an anterior and a posterior, of which the anterior is the larger ; they usually curve backwards in the jaw ; in the lower wisdom the fangs are usually conjoined, but bifid at the point.
The crowns of all the teeth become more or less flattened by use, so that the incisors lose their sharp cutting edge, and the cusps of the premolars and molars are worn away.
The temporary or milk teeth are smaller than the permanent teeth. They are more constricted at the neck, where the crown joins the fang, especially in the milk molars, the fangs of which also diverge more widely than in the permanent set. The second temporary molar is bigger than the first. The crown of the first upper molar has three cusps, two buccal, one palatal; that of the second four cusps. The crown of the first lower molar has four cusps ; that of the second five, three of which are buccal, two lingual. The temporary teeth lie more vertically in the jaws than the permanent.
The alveolus, or socket for the lodgment of the single fanged teeth, is a single socket ; in the multi-fanged teeth, the socket is divided into two or three compartments, according to the number of the fangs. The socket is lined by the alveolo-dental periosteum, which is continuous at the mouth of the socket with the periosteal covering of the jaw, and with the deeper fibrous tissue of the gum, where it embraces the neck of the tooth. The alveolo-dental periosteum is formed of retiform connective tissue, on the one hand connected with the surface of the cement, on the other with the more fibrous periosteum lining the bony wall of the socket (fig. 15), It is vascular, its vessels being continuous with those of the gum, the pulp-vessels, and the bone. It receives nerves from those going to the pulp. The fang fits accurately in the socket, and through a hole at the tip of the fang the blood-vessels and nerves of the tooth pass into the pulp-cavity of the tooth.
Structure of the Teeth. - Each tooth is composed of the following hard structures - dentine, enamel, and cement or crusta petrosa; occasionally other substances, named osteodentine or vasodentine, are present. In a tooth which has been macerated, an empty space exists in its interior, called the pulp-cavity, which opens externally through the hole at the tip of the fang ; but in a living tooth this cavity contains a soft, sensitive substance named the pulp.
The Dentine, or Ivory, makes up the greater part of each tooth ; it is situated both in the crown, where it is covered by the enamel, and in the fang, where it is invested by the crusta petrosa ; whilst the pulp cavity in the centre of the tooth is a cavity in the dentine. The dentine is composed of an intimate admixture of earthy and animal matter in the proportion of 28 of the animal to 72 of the earthy. The animal matter is resolved on boiling into gelatine ; the earthy matter consists mostly of salts of lime.
If thin slices through the dentine of a macerated tooth be examined microscopically, it will be seen to consist of a hard, dense, yellowish- white' translucent matrix, penetrated by minute canals, called dentine tubes. The dentine tubes commence at the pulp cavity, on the wall of which they open with distinct orifices. They radiate in a sinuous manner from the pulp cavity through the thickness of the dentine, and terminate by dividing into several minute branches . this division takes place in the crown of the tooth immediately under the enamel, and in the fang of the tooth immediately under the crusta petrosa. lu their course the dentine tubes branch more than once in a dichotomous manner, and give off numbers of extremely minute collateral branches. The transverse diameter of the dentine tubes near the pulp cavity is 4 1„th inch, but that of their terminal branches is much more minute.
If the dentine be examined in a fresh tooth, the tubes will be seen to be occupied by soft, delicate, thread-like prolongations of the pulp. The passage of processes of the pulp into the dentine tubes was first seen by Owen in the examination of the tusk of an elephant ; but the soft contents of the dentine tubes have been made the subject of special investigation by J. Tomes in the human and other mammalian teeth, and have been named the dentinal fibrils.
In sections through the dentine of dried teeth, it is net uncommon to find, near its periphery, irregular, black spaces containing air. These spaces freely communicate with each other. As the dentine which forms their boundary has not unfrequently the appearance of globular contours, they were named by Czermak the interglobular spaces, In a fresh tooth they are not empty, but are occupied by a soft part of the matrix, which is traversed in the usual manner by the dentine tubes. This matrix is apparently imperfectly calcified dentine, which shrinks up in a dried tooth, and occasions an air-containing space. A layer of small irregular spaces situated in the peripheral part of the dentine in the fang, immediately under the crusta petrosa, and sometinies natned the granular layer, is apparently of the same nature as the intergdobular spaces.
The En.amel is the brilliant white layer which fornas a cap on the surface of the crown of a tooth. It is thickest on the cutting edge or grinding surface of the crown, and thins away towards the neck, where it disappears. It is not only the hardest part of a tooth, but the hardest tissue in the body, and consists of 96.5 per cent. of earthy and of 3.5 per cent. of animal matter. The earthy matter consists almost entirely of salts of lime. The great hardness of the enamel admirably adapts it as a covering for the cutting edge, or grind-ing surfaces, of the crowns of the teeth.
The enamel is composed of microscopic rods, - the enamel _fibres, or enctmel prisms. These rods are set side by side in close contact with each other ; one end of each rod rests on the surface of the dentine, the other reaches the free surface of the crown. The rods do not all lie parallel to each other, for whilst some are straight, others are sinuous, and the latter seem to decussate with each other. The rods are marked by faint transverse lines, and are solid structures in the fully fortned enamel. When cut across transversely, they are seen to be hexagonal or pentagonal, and about „1„th inch in diameter.
The free surface of the enamel of an unworn tooth is covered by a thin membrane, named the cuticle of the enanzel, or it'asznyth's membrane. This membrane can be demonstrated by digesting an unworn tooth in a dilute mineral acid, when it separates as a thin flake from the free surface of the crown. It is a horny membrane, which resists the action of acids. Its deep surface is 'pitted for the ends of the enamel rods. As the crown of the tooth comes into use, Nasmyth's membrane is worn off, and the enamel itself by prolonged use is thinned and worn down. In persons who live on hard food, that requires much mastication, it is not uncommon to find the grinding surface of the crowns of the molar teeth worn down quite flat, and the dentine exposed.
The Cenzent, Crusta Petrosa, or Tooth Bone, forms a thin covering for the surface of the fang of a tooth, and extends upwards to the neck. It is of a yellowish colour, and is usually thickest at the point of the fang ; though in the multifanged teeth it sometimes forms a thickish mass at the point of convergence of the fangs. It possesses the structure of bone, and consists of a lamellated matrix with perforating fibres, lacunm, and canaliculi. The lacunm are irregular in size and mode of arrangement, and vary also in the number of the canaliculi proceeding from them. Some-times the canaliculi anastouaose with the branched tertninations of the dentine tubes. In the thin cement situated near the neck of the tooth the lacun are usually absent. If the jaw with its contained teeth be softened in acid, and sections be made so as to show the teeth in situ, there is no difficulty in recognizing the cellular masses of nucleated protoplasm within the lacuna:, NA hich resemble in appearance the corresponding structures in the adjacent bone. Haversian canals arc only found in the cement when it acquires unusual thickness. In old teeth he cement thickens at the tip of the fang, and often closes up the orifice into the pulp cavity ; the passage of the nerves and vessels into the pulp is thus cut off, and the nutrition of the tooth being at an end, it loosens in its socket and drops out.
Osteo-dentine and Iaso-dentine do not oxist as normal structures in human teeth, though they occur in various animals. They may appear, however, as abnormalities in the Inunan teeth, and are found on the inner wall of the pulp cavity. Osteo-dentine consists of dentine structure, intermingled with 'Lenin° and canaliculi. If vascular canals, like the Haversian canals of bone, are formed in it, then the name vaso-dentine The Pulp of the tooth is one of its most important con-stituents. It is a soft substance occupying the cavity in the dentine, or the pulp cavity, and is destroyed in a macerated and dried tooth. It consists of a very delicate gelatinous connective tissue, in which numerous cells are imbedded. Those which lie at the periphery of the pulp are in contact with the dentine wall, and form a layer, named by Kolliker the membrana eboris. As the cells of this •layer play a part in the formation of the dentine similar to that performed by- the osteoblast cells in the formation of bone, Waldeyer has named them odontoblasts. The odontoblasts are elongated in form, and their protoplasm gives off several slender processes ; some enter dentine tubes to form the soft d ntinal fibres already described ; one passes towards the centre of the pip, to become con-nected with more deeply-placed pulp cells ; whilst others are given off laterally to join contiguous cells of the odontoblast layer. The pulp contains the nerves and. blood-vessels of the tooth, which pass into the pulp, through the foramen at the point of the fang. The vessels form a. beautiful plexus of capillaries. The nerves are sensory branches of the fifth cranial nerve. They enter the pulp as medullated fibres, which divide into very fine non-medullated fibres, that form a network in the peripheral portions of the pulp. The pulp of the tooth is the remains of the formative papilla, out of which the dentine or ivory has been produced. In adult teeth changes that lead to the production of osteo-dentine and vaso-dentine may take place in it. Through the dentinal fibres an organic con-nection is preserved between the dentine and the pulp, and the sensitiveness exhibited by the dentine in some states of a tooth is not necessarily clue to the passage of nerves into it, but to its connection with the sensitive dentine pulp.
Development of the 2'eeth. - In studying the development. of the teeth, not only has the mode of formation of the individual teeth to be examined, but the order of succession of the different teeth both in the temporary and permanent series.
The teeth are developed iu the mucous membrane or gum, which covers the edges of the jaws of the young embryo, and their formation is due to a special differentiation in the arrangement and structure of portions of the epithelial and sub-epithelial tissues of that membrane. The enamel is produced from the epithelium, and the dentine, pulp, and cement from the sub-epithelial connective tissue.
The development of the temporary teeth will first be consideied. If a vertical section be made th:ough the mouth of a young human embryo about the sixth or seventh week, its cavity may be seen to be lined by a stratified epithelimn, continuous with the layer of stratified epiblast forming the cuticle of the face. Along the edge of the gum, corresponding in position to that of the future jaws, the epithelium is of seine thickness, and an involution of the epithelium into the subjacent connec-tive tissue has taken place. Owing to this involution a narrow furrow or groove in the connective tissue is. produced, which consti bites the primitive deiztai groove of Goodsir. This. groove is not, however, an empty furrow, but is occu-pied by the involuted epi• thelium. The sub-epithelial connective tissue is softand gelatinous, and abounds in corpuscles, which are espe-cially abundant in the connective tissue at the bottom of the groove, where the dental papillte are pro-duced. These papilhe arc formed,at the bottom of the groove, by an increased development and growth of the corpuscles of the subjacent connective tissue. The base of each papilla is con-tinuous with the-subjacent connective tissue, and the apex projects into the deeper parts of the involuted epithelium. As a papilla increases in breadth and length the groove widens and deepens, and the in-voluted epithelium, in-creasing in quantity, ex-pands over the apex and. sides of the papilla, so as to form a hood-like cover-ing or cap for it. The cap of epithelium consti-tutes the enamel organ, whilst the papilla is the formative pulp for the den-tine and permanent pulp. Whilst these changes are taking place in the epi-thelium and the connective tissue at the bottom of - it a narrow string of epithelial eells, continuous on the one hand with the epithelial lining of the mouth, and on the other with the enamel organ. This epithelial string forms the neck of the enamel organ. Alter a time, however, the growth of the connective tissue formina the lips of the primitive groove causes the neck of the enamel° organ to atrophy, so that all communication between the enamel org:an and the superficial epithelium is eut off ; and the embryo tooth, being now completely inclosed in a cavity or sac, formed by the gelatinous connective tissue of the gum, has entered on what Goodsir termed its &weldor stage of development.
When inclosed in its sac the embryo tooth, thoug,h perfectly soft, acquires a shape which enables one to recognize to what group of teeth it belongs. After a time it begins to harden and to exhibit the characteristic tooth structure.
The dental papilla is more vascular than the surrounding connee. tive tissue, from the blood-vessels of which its vessels are derived.
The papilla abounds in cells, which are, in the first instance, rounded and ovoid in shape. I Changes then take place in the cells sitnated at its periphery, which become elongated and branched, and form layers of cells (odonto-blasts). Calcification of the protoplasm of these odontoblasts then oc curs, and the peripheral layer of tho dentine is produced. In contact with the inner surface of the thin film of den-tine, a second layer of odontoblast cells is then arranged, which in their turn calcify, and as the process goes on in successive layers of odontoblasts, the entire thickness of the matrix of the dentine and. the dentinal sheaths are produced. But the pro-cess of calcification does not apparently take place throughout the whole thickness of the protoplasm of the odontoblasts, for, as Waldeyer pointed out, the axial part of the cells remains undifferentiated as the soft dentinal fibrils of the dentine tubes. As these changes are goina on in the peripheral layers of the odonto-blasts, the central part of the dental papilla increases in quantity, apparently by a proliferation of its cells ; nerve fibres are developed in it, and it persists as the soft pulp of the tooth. The papilla of the tooth has essentially, therefore, the same relation to the formation of dentine that the cellulo-vascular contents of the medullary spaces, in intra-cartilaginous ossification, have to the formation of bone. In both instances the hard matrix is due to a special differ. entiation of the protoplasm of the formative cells ; the dentinal fibrils are the equivalent structures to the soft contents of the laeunm and canalicnli, and the persistent pulp is equivalent to the cellulo-vaseular contents of the II aversian canals.
Prior to the embryo tooth becoming sacculated, changes had taken place in the enamel organ. Those cells of the enamel organ which lie next the dental papilla are e.ontinnons, through the neck of the enamel organ, with the deepest layer of cells of the oral epithelium, which cells are elongated columns set perpendicularly to the surface on which they rest. Similarly the cells of the deepest layer of the enamel organ are columns set pe,rpendicularly to the surface of the dental papilla. They undergo a greater elongation, and form six-sided prismatic cells, which Kiilliker has pained the internal or enamel epithelium. The cells of the niost superficial layer of the enamel organ lie in contact with the vas-cnlar connective tissue which encloses the embryo tooth. They form the external erithelium of the enamel organ, and slender papillary prolongations of the connective tissue frequently project into this epithelial layer. The cells of the enamel organ, situated. between its external and its internal epithelium, become stellate, and form with each other an anastomosing network of cells like those sometimes seen in the gelatinous connective tissue.
After the tooth has become sacculated, and coincident with t: transformation of the odontoblast cells of the dental papilla in dentine, calcification begins in the elongated prismatic cells of t; internal or enamel epithelium ; their protoplasm becomes calcifie and they become the rods or prisms of the enamel. As the hardening takes place from the periphery to the centre of each cell, the axial portion may, as Tomes pointed out, remain soft for some time in the axis of the enamel rod. With the increase in length, and with the calcification of the cells of the enamel epithelium, the stellate gelatinous cells disappear, and the outer ends of the enamel rods come in contact with the cells of the external enamel epithelium. By some observers the external epithelium is supposed to disappear without undergoing any special differentiation, but by others it is believed to undergo conversion into Nasmyth's membrane.
In this manner the crown of a tooth is formed, and it is lodged in a membranous sac formed by the differentiation into a fibrovascular membrane of the surrounding connective tissue. Whilst within its sac, the crown of the tooth possesses the characteristic form of the group of teeth to which it belongs. After the calcification of the enamel rods is completed, it can undergo no further change either in shape or in increase of size.
Whilst the crown of the tooth is being formed, ossification of the jaws has been going on, and the tooth, with its membranous sae, has become lodged in an alveolus or socket in the jaw, which alveolus is closed in by the gum.
In order that the crown of the tooth may come into use as a masticatory organ, it has to be elevated to the level of the gum, which is absorbed by the pressure, and the crown then erupts into the cavity of the mouth. The process of eruption is due to the development of the fang, which, as it grows in length, elevates the crown of the tooth and forces it outward. The dentine of the fang is developed from the odontoblast cells of the pulp in a manner similar to that already described for the development of the dentine of the crown. The cement or crusty petrosa is developed from the connective tissue lining the alveolus, which forms the alveolo-dental periosteum. It is therefore an ossification in membrane.
As the temporary or milk teeth precede the permanent teeth, their papillae are naturally the first to form. The series of milkpapilhe are not, however, - simultaneously produced. From the observations of Goodsir, it has been shown that the milk-papilla of the anterior molar in the upper jaw appears about the seventh week ; then the canine papilla, the two incisor papilla, and the posterior molar papilla are sucessively formed, the last making its appearance about the end of the tenth week. The dental papilla. in the upper jaw immediately precede the papilla: of the corresponding teeth in the lower jaw.
The eruption of the milk teeth into the mouth does not begin to take place until the latter half of the first year of extra-uterine life, and is not completed until betweeen the second and third year. Though variations occur in the date of eruption of each tooth in different children, it may be stated that the incisors usually appear from the seventh to the ninth month, the anterior molars from the twelfth to the sixteenth month, the canines during the seventeenth or eighteenth month, the posterior milk molars from two to two and a half years. The milk teeth begin to be shed about the sixth year by the dropping out of the incisors. The last to be shed are the canines, which do not fall out till the tenth or eleventh year. The shedding of the milk teeth is preceded by the absorption of the fangs. This is effected, as was satisfactorily shown by J.
Tomes, by the agency of a group of cells situated at the bottom of the sockets. As these cells occasion absorption of the tooth tissue, similar to that occurring in the bone tissue from the action of the large multi-nucleated osteo-klast cells, they may appropriately be called odonto-klasts.
The development of the permanent teeth will now be considered. In the description of the arrangement of the teeth it has been pcinted out that the number of teeth in the permanent set exceeds that of the temporary set. The permanent incisors and canines come into the place of the temporary incisors and canines, and the permanent bicuspids succeed the temporary molars, but the permanent molars have no milk predecessors, and are superadded at the back of the dental series.
The development of the successional permanent teeth., which are the ten anterior teeth in each jaw, will first be examined. Prior to the period when the lips of the primitive dental groove meet, to produce the saccular stage of dentition of the severe,' temporary teeth, an indentation, or fu 1TOW, takes place in the connec• five tissue adjoining the string of epithelial cells which form the neck of the enamel organ. This furrow constitutes what Goodsir termed the cavity of reserve, and it is filled up by epithelial cells continuous with the epithelium of the neck of the enamel organ. As a cavity of reserve is formed immediately behind (i.e. , on the lingual side of) each milk tooth, they are ten in number in each jaw, and, except that for the anterior molar, are formed successively from before backwards.
The cavities of reserve are concerned in the production of the permanent successional teeth, and each temporary tooth is replaced by the permanent tooth formed in connection with the cavity of reserve situated immediately behind it (fig 21). The cavities of reserve become elongated, and widened, and pass above the temporary teeth in the upper jaw, and below those in the lower jaw. At the bottom of each a dental papilla forms, the apex of which indentates and becomes covered by the epithelium contained in the cavity, which forms a cap for the papilla, and constitutes the enamel organ for the permanent tooth. The cavity becomes completely elosed by the growth of the surrounding connective tissue, and the embryo permanent tooth becomes saceulated. The process of calcification then goes on, in both the enamel organ and dental papilla, in a manner similar to that already described in the temporary teeth. The permanent teeth then become lodged in sockets in the jaw distinct from those of the temporary teeth. The sac of each permanent tooth remains connected with the fibrous tissue of the gum by a slender fibrous band, or gubcrnaeulum, which passes through a hole in the jaw immediately behind the corresponding milk tooth. Before the successional permanent tooth erupts, not only should the temporary tooth be shed, but the bony partition between their respective sockets must be absorbed.
The superadded permanent teeth, or permanent molars, three in number on each side, lie behind the successional teeth. Their mode of origin is similar to that of the temporary teeth. The primitive groove, occupied by an involution of the epithelial covering of the gum, is prolonged backwards. Three dental papilhe successively appear at the bottom of this groove, and the epithelium covering ench papilla forms its enamel organ. Legros and Magitot, however, state that the second permanent molar arises in connection with a diverticulum (cavity of reserve) proceeding from the epithelial string of the enamel organ of the first permanent molar, and that the wisdom tooth is formed in connection with a similar diverticulum from the second permanent molar. The embryo tooth becomes sacculated, and goes through the process of calcification similar to what has been described in the other teeth The germ of the first permanent molar appears about the sixteenth week of embryo life ; that of the second permanent molar not until about the seventh month after birth ; whilst that of the wisdom tooth is not formed until about the sixth year The crown of the first molar is the first of the permanent teeth to erupt into the mouth, which it usually does in the sixth year. The incisors appear when the child is seven or eight ; the bicuspids when it is nme or ten ; the canines about twelve ; the second molars about thirteen ; and the wisdom teeth from seventeen to twenty five.
In his dentition man is diphyodout as regards his incisor, canine, and premolar teeth, but nionophyodont in the molar series.
From the description of the development of the teeth, it will have been seen that a tooth is made up of three hard tissues - enamel, dentine, and, cement - and of the soft vascular and nervous pulp. These tissues are not developed from one layer only of the blastoderm. The enamel is of epiblast origin, whilst the dentine, cement, and pulp are derived from the mesoblast. A tooth in its fundamental development, as was long ago pointed out by Goodsir, must be referred to the same class of organs as the hairs and feathers. The enamel of the tooth, like the hair, is produced by a differentiation of the involuted epithelium of the epiblast, whilst the dentine and pulp resemble the papilla of the hair, in proceeding from the mesoblast. The tooth-sae, like the hair-follicle, is also of mesoblast origin. Whether the cement, as Robin and Magitot have described, be developed by means of a special cement organ, in the interior of the tooth-sac, or be ftirmed, as has been stated in this description, by the alveolo-dental periosteum, it is on either view derived front the mesoblast. As to the origin of Nasmyth's membrane, there is a difference of opinion ; some regard it as a special cornification of the external cells of the enamel organ, in which case it would be from the epiblast ; whilst others consider it to be continuous with though structurally different from, the cement - homologous, therefore, with the layer of cement, which in the horse, ruminants, and some other mammals covers the surface of the crowns of the teeth.
The tissues of a tooth have not all the same importance in the structure of a tooth. The dentine is apparently always present, but the enamel, or the enamel and cement, may be absent in the teeth of some animals. For example, the tusks of the elephant and narwhal, and the teeth of the Edentata, are without enamel, and in the Rodentia enamel is present on only the anterior surface of the incisors. But though the enamel is not developed, or forms only an imperfect covering for the crowns of these teeth, yet an enamel organ is formed in the embryo jaws. In 1872 W. Turner described a structure homologous with the enamel organ in relation with each of the dental papillae in the lower jaw of a foetal narwhal ; but this organ did not exhibit a differentiation into the three epithelial layers, such as occurs in those teeth in which enamel is developed. Since then C. S. Tomes has seen an enamel organ in the embryo armadillo, and has also pointed out that, in teeth generally, enamel organs exist, quite irrespective of whether enamel subsequently does or does not form.
But further, the involution of the oral epithelium, and the coincident formation of a primitive groove, take place not only where the teeth subsequently arise, but along the whole curvature of the future jaws ; whilst the production of dental papillae is restricted to the sots where the teeth are formed. Hence it would seem that the Inflection of the oral epithelium is not so essential to the development of a tooth as the formation of a papilla. The inflected epithelium marks only a preliminary stage, and it may or may not be transformed into tooth structure. But that which is essential to the formation of a tooth is the production of the papilla which appears at the bottom of the primitive groove. (W. T.)