DENTITION

The mammalian dentition possesses certain characters that in combination, if not individually, are diagnostic of the class. The complement of teeth is limited to a fairly small number, rarely exceeding 44 in the perma­nent dentition, which is determined for each species— although minor variations may occur.

set that is better adapted to the larger jaws and to the more vigorous mastication of the adult. The sequence is known as diphyodonty in contrast to the polyphy- odonty (multiple succession) of most other vertebrates. Finally, the teeth are implanted in sockets set along the margins of the jaws, which is an arrangement described as thecodont.

The number and classification of the teeth in a par­ticular species are conveniently represented by a formula. For the dog, the formula of the permanent dentition may be written

or, more succinctly and no less clearly,

The temporary (milk or deciduous) dentition of the same animal may be represented

without risk of confusion, as molar teeth are always lacking in the milk set. There are various notations for the identification of individual teeth. According to the most convenient, P¹ may stand for the first permanent upper premolar, i₂ for the secondary temporary lower incisor, and so forth, precision being achieved by the use of upper and lower case letters and superscript and subscript numerals.

The term diastema is used for a considerable gap between teeth in the one jaw, most usually for that between the incisors and premolars.

The description of a simple tooth may be considered before returning to the features of the different types of teeth. A tooth (dens) consists of crown and root, and each is easily distinguished. The crown is encased in enamel, a very resistant, calcified, slightly opalescent, white material, while the root is encased in cement, a softer, less shiny, yellowish tissue. The part of the tooth between root and crown is termed the neck (Figure 3-13). Certain variations in structure may occur at the neck: the cement and enamel commonly abut, but the cement may overlie the enamel or sometimes the two tissues fail to meet, exposing a narrow strip of dentine, the third calcified tissue of the tooth. The dentine, which is also known as ivory, provides the greater part of the substance of the tooth and contains a small central cavity that houses the connective tissue pulp. The pulp continues through a canal in the root of the tooth to merge with the connective tissue in the depth of the tooth socket (alveolus).

Figure 3-13 depicts the idealized condition in which both the gum (gingiva) embraces the neck and the

Figure 3-13 Schematic longitudinal section of a simple tooth. 1, Enamel; 2, dentine; 3, cement; 4, pulp; 5, apical foramen; 6, periodontal ligament; 7, socket (alveolus); 8, gum.

crown corresponds to the exposed part of the tooth. The gums may recede with advancing age, exposing the cervical part of the root, which is a condition familiar in many older people who are said, on this account, to be “long in the tooth.” The opposite condition, in which part of the enamel-covered crown is concealed below the gum line, occurs in many mammals; in some a large portion of the crown is initially held in reserve to be extruded gradually in compensation for the attrition at the masticatory surface.

The detailed description of the crown requires some system for indicating its various surfaces. The usual terms of relative position are inadequate for this purpose because the curved line followed by the tooth row

Figure 3-14 Premolar teeth exposed in the upper jaw of a horse. The part protruding above the gum is the clinical crown (1); the whole enamel-covered part is the anatomical crown or body (2) of the tooth.

(arcade) alters the orientation of equivalent surfaces of successive teeth in the series. Less ambiguous terms are vestibular (labial, buccal) and lingual, and mesial and distal; their usage is indicated in Figure 3-18. Where adjacent teeth touch, the appropriate mesial and distal surfaces may both be termed contact surfaces. The working area, if extensive and not a mere cutting edge, is known as the occlusal or masticatory surface.

Enamel is a densely calcified tissue of ectodermal origin. It is acellular and therefore unable to react to injury: it cannot regenerate to patch a hole or repair a fracture. Because it is exposed to rough treatment, it is necessarily very hard, indeed uniquely so for a biologi­cal material. Despite this, the enamel casing may even­tually be breached, and the softer dentine that wears away more rapidly would then be exposed.

Cement is the least hard of the calcified tissues of the tooth and resembles bone in structure, although it lacks so regular an organization. The initial deposit over the root is thin, but as deposition continues throughout life it may eventually form quite a thick crust. Collagen fibers extend from the cement into the periodontal liga­ment or membrane (periodontium), the specialized con­nective tissue that fastens the tooth in its socket. Although broadly comparable to bone in structure and development, cement differs in one important respect: it is relatively immune to pressure erosion. Orthodon­tists make use of this characteristic when they adjust the position of a tooth in the jaw by fitting an appliance that presses the tooth against the alveolar wall. If the adjustment is performed correctly, the pressure pro­duces an erosion of the bone but leaves the tooth unaf­fected and free to shift into the space created. This lack of response to pressure is relative, not absolute, and excessive pressure causes resorption; indeed, the roots of the temporary teeth are resorbed under pressure from their permanent replacements thrusting against them.

Dentine is also similar to bone in having a calcified, collagen-rich matrix. In bone the osteoblasts become imprisoned in the matrix, but the dentine-producing cells (odontoblasts) recede from the newly formed dentine and remain as a continuous layer on the surface lining the dental (pulp) cavity. The odontoblasts retain their productive capacity throughout life, and a slow but continuous production of secondary dentine, with cor­responding reduction of the dental cavity, continues into old age.

The dental cavity reflects the external form of the tooth, sending a branch into each major elevation of the crown and through a narrow passage in the root where it opens at the apical foramen; when more than one root is present, each contains a channel that joins the central cavity.

The pulp that fills this space is a very delicate con­nective tissue margined by the odontoblast layer and richly vascularized. A lymphatic plexus also exists, although this is difficult to demonstrate. Numerous nerves run within the pulp; some are vasomotor, although most are sensory and possess endings that can be stimulated in various ways. Whatever the stimulus, thermal, mechanical, or chemical, the sensation per­ceived is pain; because the pulp is contained within unyielding walls, even a slight inflammatory swelling is quickly appreciated.

Each tooth is implanted in a separate socket in the margin of a jaw. The form of the socket corresponds to that of the root and is therefore often branched and irregular. Where the teeth lie close together the septa between adjacent sockets may be very delicate or even defective. Typically, the socket is lined by a thin lamina of compact bone perforated for the passage of the vessels and nerves that supply both the socket and the tooth. The outer surface of the lamina may be braced by trabeculae of spongy bone extending toward the surface of the jaw or radiating into surrounding parts; where the alveolar margin is narrow, however, the lamina merges with the external compacta of the jaw. The tooth is attached to the socket by means of the tough fibrous periodontal ligament. This is particularly rich in collagen fibers that attach to both the cement and the alveolar bone and are so oriented that the tooth is suspended in a sling; masticatory forces that tend to drive the tooth deeper into the socket are thus trans­formed into tension on the socket wall.

The vessels and nerves that supply the teeth are derived from the major trunks (superior and inferior alveolar arteries, veins, and nerves) that course through canals in the jaws.

Tooth eruption is a complicated and controversial process involving a number of factors: root growth, bone growth, pulpal proliferation, tissue pressure, and periodontal traction. Their relative importance is dis­puted, but the last factor is probably the most signifi­cant. The temporary teeth rise in the jaws after the crown is completed but before the root is formed; this process carries the tooth closer to the surface and provides the space necessary for the formation of the root. The move­ment of the crown is facilitated by a loosening of the connective tissue of the dental follicle (p. 142) and gum and by the presence of remnants of the epithelium of the dental lamina, which define the line of passage. However, if these remnants are large and cystic, as some­times happens, they may obstruct rather than facilitate the movement of the tooth, divert it from its true path, and give rise to troublesome anomalies of site and spacing. The retention of an epithelial covering over the unerupted crown ensures that no breach of continuity occurs when the tooth breaks through to the surface, as this remnant of the enamel organ fuses with the epithe­lium of the gums embracing the tooth (Figure 3-15).

The eruption of the permanent teeth is more com­plicated. These develop in bony crypts deep to the roots of the equivalent teeth of the temporary set. To erupt they must escape from this confinement and displace their predecessors. The erosion of the roof and the con­tinuous adjustment of the walls of the embedded alveo­lus involve the usual processes of bone remodeling, and it is hardly too fanciful to say that the permanent tooth and its alveolus migrate as a unit through the jaw to enter the alveolus of the temporary tooth. The replace­ment tooth then presses on the root of the temporary tooth, causing its resorption. The attachment of the temporary tooth is loosened, which allows it to shift and become increasingly mobile during mastication; it is soon shed, and the permanent tooth then rises in its place. Proper eruption of the permanent tooth depends on the temporary teeth holding places ready for them; if the latter are prematurely lost, the filling of the alveoli by bone may make it difficult for the permanent teeth to establish their proper occlusal relationships.

The dentition of the dog, although relatively simple, is well adapted to the feeding habits of the animal (Figure 3-16). The incisor teeth are small and peglike

Figure 3-15 Schematic drawings representing tooth eruption and replacement. A, Eruption of a deciduous tooth. The primor­dium of the permanent tooth is located on the lingual side of the deciduous tooth. B, The fully developed deciduous tooth within a bony alveolus. The crown of the permanent tooth has already formed. C, The permanent tooth is ready to break through. The root of the deciduous tooth has been resorbed; formation of the root of the permanent tooth is in progress.

Figure 3-16 Lateral view of the permanent dentition of the dog.

and are crowded together in the rostral part of each jaw. On eruption, each upper incisor presents a trilobed crown with a labial cutting edge. The lower incisors are bilobed. These features are lost as wear reduces the tooth to a simple prismatic peg. The name incisor sug­gests that these teeth are used for dividing food before it is taken into the mouth, but in this species a second and more efficient shear is provided by teeth farther back in the mouth. The incisors in the dog are employed mainly in nibbling and grooming.

The canine teeth are particularly well developed, so much so that the generic name (Canis) for doglike animals provides the term by which these teeth are known in all mammals. Canines are large, curved, and laterally compressed teeth of simple form and are capable of inflicting a deep wound; they are used for aggressive and holding purposes. A large part of each canine tooth is implanted in the jaw; the extent and position of the embedded part of the upper canine are revealed by a bony ridge over the alveolus.

The premolar and molar teeth together constitute the cheek teeth, a term more common and more useful in descriptions of the dentition of herbivorous species, in which the two groups have become assimilated to each other in form and function. In all mammals the first few (maximally four) cheek teeth are represented in both dentitions and are assigned to the premolar group; the remainder (maximally three) are represented only in the permanent dentition and are known as molar teeth. The premolars of the dog form an irregular but fairly closely spaced series of increasing size and complexity. The cusps or projections of the individual crowns are aligned one behind the other to form a discontinuous serrated cutting edge rather resembling that of the pinking shears of a dressmaker and effective for the same reason: the elongation of the blade makes possible a more rapid and cleaner division while the notches help hold the food in place. The more caudal molars also possess a cutting potential but are principally developed for crushing and are distinguished by their broader and more extensive masticatory surfaces. The cusps or eleva­tions that they carry are arranged in a pattern that is faithfully reproduced on the teeth of all members of the species; their homologues can be recognized, although sometimes only with great difficulty, in the teeth of other mammals.

Most of the cheek teeth, unlike the incisors and canines, have more than one root. Multiple roots, espe­cially if divergent, provide firmer anchorage but make extraction difficult, if not impossible, without previous division of the crown into portions corresponding to the individual roots.

The dentition of the cat is reduced to

3-1-3-1

3-1-2-1

in the permanent set (Figure 3-17). It is even more closely adapted to a fleshy diet, as the reduction of the molar series has largely eliminated the crushing poten­tial presented by the dog's dentition. The cutting action of the cat's cheek teeth earns them the description sec- odont; the dual-purpose structure of the dog's molars is better described as tuberculosectorial. The incisors of cats are remarkably small and the canine teeth relatively large.

In other domestic species, the diet is much more abrasive and requires considerably more crushing and grinding. The dentition is modified accordingly. The details are presented in the later chapters; here it is suf­ficient to note only the most conspicuous features.

In the dentition of the pig the broad crowns of the cheek teeth carry an elaborate formation of blunt cusps that make them very effective crushing instruments; teeth of this sort are said to be bunodont (Figure 3-18). The canine teeth of this species remain open at the embedded end (root) so that accretion of dental tissues continues throughout the animal's life. This persistent growth, coupled with their curved form, allows them to assume very striking forms in older individuals, particu­larly in boars.

The other species are more restricted to a herbivo­rous diet than the omnivorous pig, and the dentition of horses and ruminants must allow for continuous and considerable wear at the masticatory surfaces. This requirement is met by the enlargement of these surfaces, by the increase in height of the crowns, which are only gradually extruded (the delayed development of the roots allows growth to continue for some years after the teeth have come into wear), and, above all, by compli­cated folding of the enamel. This folding has two impor­tant consequences. It increases the amount of the hardest and most durable component of the tooth that is exposed and so reduces the rate of attrition. It pro­vides an alternation of harder and softer materials, which, wearing at different rates, produces an uneven­ness of the masticatory surface that gives it a rasplike quality (Figures 3-19 and 3-20).