The Digestive Apparatus and Abdomen

The Digestive Apparatus and Abdomen

The digestive system (apparatus digestorius) consists of the oral cavity, pharynx, alimentary canal, and accessory organs. The accessory organs include the teeth, tongue, salivary glands, liver, gallbladder, pancreas, and paranal sinus.

The wall of the digestive tube is richly supplied with secretory epithelium and intrinsic glands. It is lined throughout by a mucous membrane that is continuous with the surface integument at the mouth and anus.

Oral Cavity

The mouth (os. oris) in anatomic terms includes only the opening between the lips (rima oris) into the vestibule of the oral cavity. The oral cavity (cavum oris) is divided into the vestibule and the oral cavity proper and contains salivary glands, teeth, and the tongue.


The vestibule of the oral cavity (vestibulum oris) is the space external to the teeth and gums and internal to the lips and cheeks. It opens to the outside rostrally by means of the U-shaped slit, the mouth or oral fissure (rima oris), between the lips. This opening is in a dorsal plane. When the mouth is closed, the vestibule communicates with the oral cavity proper by means of the interdental spaces, which vary greatly in size. A space on either side, caudal to the last cheek tooth and nearly 1 cm long in large dogs, also establishes free communication between the two parts of the oral cavity.

The parotid and zygomatic salivary ducts open into the dorsocaudal part of the vestibule. The parotid duct opens through the cheek on the small parotid papilla (papilla parotidea), located opposite the caudal part of the superior fourth premolar tooth, approximately 5 mm from the fornix of the vestibule, which is formed by a reflection of the mucosa from the cheek to the gum. The main duct of the zygomatic gland opens lateral to the caudal part of the superior first molar tooth on a small papilla near the vestibular fornix (Fig. 7-1). A small mucosal ridge connects the main zygomatic and parotid duct openings. Usually one to four small accessory ducts from the zygomatic gland open caudal to the main duct. The submucosal labial and ventral buccal glands are few and are confined to the inferior lip and the adjacent part of the cheek. The secretion of these glands is discharged through approximately 10 openings located opposite the four inferior premolar teeth near the fornix of the vestibule.


The lips (labia oris) form the rostral and most of the lateral external boundaries of the vestibule. Superior and inferior lips (labium superius et labium inferius), formerly upper and lower lips, meet at the angles of the mouth (angulus oris). The lips bound the oral fissure, the external opening (the mouth) into the vestibule. Their margins are narrow and devoid of hair except the rostral two thirds of the superior lip on either side. Toward the angle on either side, the caudal third progressively increases in width to form a rounded border measuring as much as 1 cm. The margin of the inferior lip as far caudally as a level through the canine teeth is devoid of hair over a zone approximately 5 mm wide. Caudal to the canine teeth this smooth zone increases to 1 cm in width, and the narrow border becomes serrated by the formation of approximately 15 conical papillae several millimeters high.

No definite frenulum, or median mucosal fold, attaches the inferior lip to the gum, and the median mucosal fold of the superior lip is poorly developed, being thick but narrow. The mucosa of the inferior lip is firmly attached to the gum on either side in the space between the canine and the first premolar tooth (interdental space). A deep, straight, narrow cleft, the philtrum, marks the union of the two halves of the superior lip rostrally. The hair of both lips slopes caudoventrally. It is thinner and shorter rostrally, longer and thicker caudally. A few of these are tactile hairs (pili tactiles). On the superior lip and adjacent dorsal part of the muzzle the tactile hairs are imperfectly arranged in four rows. The wide orbicularis oris muscle and the insertions of several other facial muscles form the media of the lips.


The cheeks (buccae) form the caudal portion of the lateral walls of the vestibular cavity. The cheeks are small in the dog because of the large mouth opening. The cavity of the cheeks runs medial to the masseter muscles and extends as far caudally as the attachment of the buccinator muscles on the mandible and maxilla at their alveolar borders opposite the last two cheek teeth of the maxilla and mandible and the intervening rostral border of the basal half of the coronoid process. In rodents and most herbivores the cheeks are large and serve as storage space, especially during mastication and transportation. This function is of minor importance to the dog.

The morphologic characteristics of the cheeks are closely related to that of the lips, with which they are continuous. The lips and the cheeks consist of three basic layers. The external layer is the hairy integument, the middle layer consists of muscle and fibroelastic tissue, and the inner layer consists of the mucosa. Projecting caudolaterally from the caudal part of the skin of the cheek are usually two coarse tactile hairs, 3 to 5 cm long. The middle layer of the cheek consists primarily of the buccinator muscle, although lateral to this are the m. zygomaticus and some fasciculi of the cutaneous fascia. The dorsal buccal glands in carnivores are consolidated to form the zygomatic gland, a large mixed salivary gland located in the rostroventral part of the orbit. The ventral buccal glands consist of a few small, solitary glands located in the submucosa, rostral to the masseter muscle and medial to the fibers of the ventral part of the buccinator. The mucosa of the lips and cheeks is thinly cornified, stratified squamous epithelium that, in some breeds, is partly or wholly pigmented. Although the lips are not used as prehensile organs, the various muscles in them provide for movement and expression of emotions, such as anger or fear.

Oral Cavity Proper

The oral cavity proper (cavum oris proprium) is bounded dorsally by the hard palate and a small part of the adjacent soft palate. Laterally and rostrally the dental arches and teeth form its boundary. The tongue and the reflected mucosa ventral and lateral to it form the floor of this cavity. When the mouth is closed the tongue nearly fills the oral cavity proper. The major sublingual and mandibular ducts open rostrally ventral to the body of the tongue, on the inconspicuous sublingual caruncle (caruncula sublingualis).

Extending caudally from the caruncle, lateral to the frenulum, to about a transverse plane through the inferior shearing teeth is a low ridge of mucosa approximately 2 mm wide and 1 mm high. This is the sublingual fold (plica sublingualis). It lies close to the body of the mandible and is formed by the underlying mandibular and major sublingual ducts and a variable number of lobules of the polystomatic portion of the sublingual gland.

Just caudal to the superior central incisor teeth is the incisive papilla (papilla incisiva), a rounded eminence that extends caudally to blend with the first transverse ridge formed of the mucosa covering the hard palate. On each side of this papilla an incisive duct (ductus incisivus) opens. This duct, formerly the nasopalatine duct, leaves the oral cavity by a slitlike opening and extends caudodorsally for 1 or 2 cm through the palatine fissure to open into the floor of the nasal cavity. Before opening into the nasal cavity, the duct communicates with the vomeronasal duct and organ.

The oral cavity is continuous caudally with the isthmus of the fauces and with the oral pharynx.


The palate (palatum) (Fig. 7-2) is a partly bony, partly membranous partition separating the respiratory and digestive passages of the head. The nasal cavities and nasal pharynx lie above it; the oral cavity and oral pharynx lie below it. The bony hard palate is rostral, and the membranous soft palate caudal. The hard palate (palatum durum) is formed by processes of the palatine, maxillary, and incisive bones on each side. The mucosa on the nasal side consists of pseudostratified ciliated columnar epithelium; that on the oral side consists of stratified squamous epithelium that is cornified. The hard palate is nearly flat. Laterally and rostrally it inclines slightly ventrally, and it is continuous with the portions of the incisive and maxillary bones that contain the alveoli of the superior teeth. Six to ten ridges and depressions cross it transversely on the oral side. Not all of these ridges are complete. In extremely brachycephalic heads they become nearly straight. Close inspection of the ridges reveals small blunt eminences.

The soft palate (palatum molle or velum palatinum) continues caudally from the hard palate at an irregularly transverse level, passing just caudal to the last superior molar teeth in mesaticephalic heads. In extremely brachycephalic heads the junction of the hard and soft palates is more than 1 cm caudal to this transverse level. The soft palate is particularly long in the dog. In preserved heads the epiglottis is usually seen to lie above the thick caudal border of the soft palate. Occasionally it may lie ventral to the palate or even be recessed in the caudal margin of the soft palate. In brachycephalic breeds the soft palate may be so long as to interfere with the passage of air into the larynx. In the average dog’s head the soft palate is 6 cm long, 3 cm wide, and 5 mm thick where it is continuous with the hard palate. The soft palate gradually thickens, so that at the junction of its middle and caudal thirds it is approximately 1 cm thick, after which it becomes slightly thinner and ends caudally in a concave border. From its ventrolateral part a thin elliptical fold extends laterally to form the ventral wall of the tonsillar sinus.

On each side the caudal border of the soft palate is continued to the dorsolateral wall by the palatopharyngeal arch (arcus palatopharyngeus), or caudal pillar of the soft palate. This serves a part of the boundary between the nasal pharynx and the laryngopharynx. The palatopharyngeal muscle (m. palatopharyngeus) and the mucosa that covers it form this pillar. When the tongue is forcibly withdrawn from the mouth and moved to one side, a palatoglossal arch (arcus palatoglossus) is developed on the opposite side, running from the body of the tongue to the initial part of the soft palate. This serves as part of the boundary between the oral cavity and the oral pharynx. That portion of the soft palate caudal to a transverse plane through the caudal borders of the pterygoid bones is known as the palatine veil (velum palatinum). No uvula is present on the caudal border of the soft palate.

Structure of the Soft Palate

The soft palate, from the ventral to the dorsal surface, consists of the following layered structures.

Palatine Muscles

The muscles of the soft palate consist of the paired palatine muscles (mm. palatini) and the end ramifications of the paired tensor and levator veli palatini muscles (Fig. 7-3), which are nearly equal in size. These muscles are described with the muscles of the head. The right and left intrinsic palatine muscles lie close together on each side of the median plane, ventral to the palatine aponeurosis. The end ramifications of the paired extrinsic muscles, the levator and tensor veli palatini, blend with the palatine aponeurosis. The right and left pterygopharyngeal muscles pass lateral to the caudal part of the soft palate from origins on the pterygoid bones. Right and left palatopharyngeal muscles arise near the median plane from the palatine aponeurosis. They sweep laterally and dorsocaudally in the pharynx, forming the bases for the palatopharyngeal arches.

Palatine Nerves

The predominate source of sensory innervation to the palate is from branches of the maxillary nerve (n. maxillaris) from the trigeminal nerve (n. trigeminus). The major palatine nerve courses through the major palatine canal and supplies sensory fibers to the oral side of the hard palate. The nasal side is supplied by the caudal nasal nerve. The minor palatine nerve supplies the soft palate, which follows the minor palatine artery to enter the rostral end of the soft palate. Branches from the glossopharyngeal (n. glossopharyngeus) and the vagus nerve (n. vagus) also enter the soft palate and supply motor innervation to the palatinus, pterygopharyngeal, and palatopharyngeal muscles. The vagi contribute most to the supply of these muscles. The glossopharyngeal nerves supply sensory branches to the lateral walls of the oral pharynx and, to a lesser extent, the soft palate. They are sensory also to the caudal part of the tongue.

Palatine Aponeurosis

The palatine aponeurosis (raphe palati) consists essentially of the thin, fanned-out terminal tendons of the right and left tensor veli palatini muscles. It is located between the ventral margins of the right and left perpendicular parts of the palatine bones and the pterygoid bones, and attaches rostrally to the caudal margin of the hard palate.

Lying directly dorsal to the palatine aponeurosis are the small mixed dorsal palatine glands. The epithelium that covers them, and on which their numerous ducts open, is of the pseudostratified ciliated columnar type. It is continued rostrally on the dorsum of the hard palate to line the nasal pharynx. Caudally, before reaching the caudal border of the soft palate, it is continued by stratified squamous epithelium, which is the epithelium of the entire ventral surface of the palate.


The teeth (dentes) (Fig. 7-4) are highly specialized structures that serve for the procuring, cutting, and crushing of food as well as for social interaction. Each tooth is divided into three parts. The crown (corona dentis) is the exposed portion of tooth that extends beyond the gums (gingiva) and is covered by a thin layer of white enamel. With the exception of the canine teeth, all crowns in the dog end in tubercles (tubercula dentis). Below the enamel bulge, a circumferential widening at the base of every crown, lies the neck (cervix dentis) of the tooth. The neck includes the most coronal region of the root between the enamel bulge and the attached gingiva. The cementoenamel junction is located within the neck. The root (radix dentis) is the portion of the tooth located beyond the level of the attached gingiva, embedded in alveolar bone. The root tip is termed the apex of the root (apex radicis dentis). Many teeth have more than one root. A dog’s dentition is diphyodont, meaning there are two sets of teeth that develop sequentially. Dechambre (1912) reported total absence of teeth in a dog. Once teeth are fully erupted in the dog they cease growing. The first set is fully erupted and functional early in the second month after birth (Fig. 7-5). These teeth, known as deciduous teeth (dentis decidui), serve the animal during its most active puppyhood. Lawson et al. (1967) described the development and eruption of teeth and illustrated several stages by a series of radiographs. They found that eruption of deciduous teeth begins on approximately the twentieth day and is completed by the thirty-fifth day.

Esaka (1982) studied the development of premolar tooth germs and their rotation by means of radiographs, dissections, serial sections, and reconstructions. He found that the direction of rotation of the teeth varied along the arcade. His accompanying table showed the rotation and contact relationship of permanent tooth germs with deciduous teeth from birth to 6 months. Upon approaching maturity at 6 months of age, when the bones that contain the teeth have become larger, the small deciduous teeth are no longer adequate; they are shed (exfoliated) and replaced by the permanent teeth (dentes permanentes) (Fig. 7-6), which last throughout adult life. Permanent teeth are larger and stronger than deciduous teeth. As the maxilla and mandible continue to grow over the next two to three months permanent molar teeth erupt caudal to the premolar teeth. There are two molar teeth in each maxilla and three in each mandible. In an experimental study of diet and teeth, Mellanby (1929) began with a detailed review of dental structure in dogs that included the development of the teeth.

Observations on the development of the deciduous teeth in the fetal dog have been reported by Hörmandinger (1958), Satrapa-Binder (1959), Williams (1961), and Williams and Evans (1978). Postnatal calcification and eruption of the deciduous and permanent teeth have been studied by Meyer (1942), Höppner (1956), Arnall (1961), and Kremenak (1967). Cahill and Marks (1982) studied exfoliation of the third deciduous mandibular premolar in mongrels and purebred Beagles from the thirteenth to the twenty-seventh postnatal week. They used radiographs and histologic sections to correlate bone, tooth, and soft-tissue events. The timing and sequence were the same in Beagles and mongrels. Eruption of the permanent third premolars began during the sixteenth postnatal week and was completed in 7 weeks.

The central and middle deciduous incisors and the canine teeth of both superior and inferior arcades have usually erupted by the end of the first month. The lateral incisors erupt during the fifth or sixth week: the deciduous premolars between the fourth and eighth weeks. All permanent teeth, with the exception of the superior canine teeth, erupt lingual to their deciduous counterparts. The permanent superior canine teeth erupt mesially. Teeth erupt earliest in the large breeds. Table 7-1 gives the normal range of time for the eruption of each of the permanent teeth, which erupt at approximately the same time in each arcade. Eruption of the permanent canine teeth can slightly precede the shedding of the corresponding deciduous teeth.

Eruption of permanent teeth requires a coordinated resorption and formation of alveolar bone on opposite sides of the developing and erupting tooth. During eruption a pathway is formed by resorption of the overlying bone and deciduous tooth roots. As root formation of the permanent tooth begins there is considerable bone deposition. Although it is known that the presence of a dental follicle is necessary to trigger eruption, the full sequence of dental eruption has yet to be completely understood. Overall, eruption of permanent teeth can vary by a few weeks to a couple of months depending on the breed, nutrition, and systemic health.

Tetracycline antibiotics concentrate in growing bones and teeth and can inhibit bone growth or interfere with enamel formation. Because they fluoresce yellow or orange under ultraviolet light, they have been used to study incremental growth of bones and teeth. Owen (1963) studied the effects of tetracycline ingestion on the teeth of dogs and noted that dentine, cementum, and enamel can all incorporate the fluorescent component. Therefore the use of tetracycline or its derivatives is discouraged in animals younger than 6 months of age.

The teeth are arranged as superior (maxillary) and inferior (mandibular) dental arches (arcus dentalis superior et inferior). The inferior arch is anisognathic, narrower and shorter than the superior arch; therefore superior incisors are located slightly rostral to the inferior incisors. Incisors function mainly to nibble or nip during mastication, grooming, and social interactions. From the superior third incisor to the superior fourth premolar, superior and inferior teeth alternate in position along the dental arch resulting in a scissors bite. Canine teeth are used for puncturing and grasping. Stockard (1941) referred to the area between the canine and carnassial teeth as the “premolar carrying space,” in reference to hunting breeds. Overlap of the carnassial teeth, the superior fourth premolars, and the inferior first molars allows for optimal shearing of food. Direct occlusal contact of the superior and inferior molars provides for grinding of food (Fig. 7-7A). In dogs, food is mostly swallowed without mastication and complete occlusal contact is not necessary. Wood and Wood (1933) commented on the genetic and phylogenetic significance of a third superior molar in the modern dog.

Tooth Structure

The dense, pearly-white outer layer of the crown is enamel (enamelum). It is the hardest substance in the body, being 96% inorganic, composed of millions of crystals of hydroxyapatite. It cannot regenerate when damaged. It is thickest on the occlusal surfaces of the teeth, and its hardness gradually increases in the first year of life. Skobe et al. (1985) studied the ultrastructure of dog enamel and found three layers to be present: a rodless (aprismatic) surface layer, a middle layer of parallel rods that was not constant at all sites, and an inner layer with prominent Hunter-Schreger bands. Enamel rods of the dog tooth, as seen after acid etching of the surface, appear to be primarily hexagonal, although semicircular and spiral shapes are also present. The enamel rod is a column of mineral that extends from the dentinoenamel junction to the coronal surface of the tooth. They are perpendicular to the surface and each rod has two parts: a core of hydroxyapatite and a sheath of organic fibrous substance. A study of dental enamel in dogs was made by Glock et al (1942).

Dentin (dentinum), which is similar to bone in chemical composition, forms the bulk of the tooth and encloses the pulp cavity. It is capable of some regeneration. Elephant and walrus tusk is dentin that has lost its enamel covering and is known as “ivory.” Dentin consists of horizontal mineralized tubules that are stacked vertically. There is a flow of fluid through the dentinal tubules from the pulp cavity to the tooth surface, where it is not covered by enamel. Changes in the rate of flow of this fluid gives the tooth sensitivity to pressure. There are three forms of dentin: primary dentin is present when the tooth erupts, secondary dentin is laid down as the tooth ages, and tertiary or reparative dentin is created in response to wear or damage to the tooth. This latter form of dentin is relatively disorganized, and it stains faster than surrounding primary or secondary dentin. Gradual loss of enamel and dentin by wear that may occur in the aged is not accompanied by nociception, as the conducting axons recede or calcify in advance of the wearing surface.

The cementum in the dog is a thin covering found only on the roots. Grossly, the cementum cannot be differentiated from the dentin it covers. The cementum provides anchorage for the periodontal ligament on the root.

The pulp (pulpa dentis), is soft tissue contained in a tooth (see Fig. 7-4). It is composed of sensory nerves, arteries, veins, lymphatic capillaries, and a connective tissue that holds those structures together. The pulp is contained in the pulp cavity (cavum dentis) of the crown and in the root canal of the root. An apical delta at the apex of each root consists of multiple small channels that allow free passage of the vessels and nerves in and out of the root canal (canalis radicis dentis).

Tooth Surfaces

The surface of the tooth that faces the lip or cheek is the vestibular surface (facies vestibularis), formerly called labial or buccal surface, and the surface that faces the tongue is the lingual surface (facies lingualis). The surface adjacent to the next tooth in the dental arch is the contact surface (facies contactus). For all teeth the contact surfaces are mesial and distal. The mesial surface is the contact surface adjacent to the next rostral or medial tooth, and the distal surface is the contact surface adjacent to the next caudal or lateral tooth. The surface that faces the ipsilateral opposite superior or inferior dental arch is the occlusal surface (facies occlusalis). In addition, direction along a tooth is indicated by the terms coronal, toward the crown, and apical, toward the apex of the root.

Tooth Groupings

The superior teeth are attached in the alveoli of the incisive and maxillary bones. Those with roots embedded in the incisive bones are the incisor teeth (dentes incisivi). The incisor tooth nearest the midplane on each side in each incisive bone is incisor 1, the central incisor. The second is incisor 2, the middle incisor, and the third is incisor 3, the lateral incisor. They are long slender teeth, arched slightly rostral and laterally compressed. Superior incisors increase in size from the central to the lateral incisor. The crowns of the superior lateral incisors are largest and slightly hooked caudally with its conformation more anatomically similar to a small canine tooth. The superior central and middle incisors have three tubercles each. Of the three incisal tubercles, the central one is largest and extends farthest coronally; the small mesial and distal tubercles are called mamelons, and a V-shaped ridge of crown with its apex nearest the gingiva, connects the side tubercles on the lingual surface. The lingual crown enlargement at the most apical aspect of the superior central and middle crowns is termed the cingulum, and when the mouth is closed the inferior incisor occlusal surfaces rest on the cingulum. Inferior incisors are similar in size and shape to the superior central and middle incisors. Depending on the occlusion and chewing habits of the dog, the tubercles and crown may slowly wear throughout life. The canine teeth (dentes canini) are separated from the lateral superior incisors by an interdental space matching the width of the inferior canine tooth (approximately 4-10 mm) and from the lateral inferior incisors by a space of roughly 1-3 mm. The canine teeth are by far the longest teeth in the dog having large roots that are nearly two times as long as their crowns. All four canine teeth are similar in length and width. They are transversely compressed with an oval cross-section. When the mouth is closed, the crown of the inferior canine tooth occupies the interdental space between the superior lateral incisor and the superior canine tooth (Fig. 7-7B). The superior roots produce an arciform alveolar juga and all four canine roots are located adjacent to the root apices of the first and second premolar teeth. Canine tooth roots are slightly wider in the midalveolar region and taper down to a rounded apex. Whereas the superior canine tooth root runs parallel to the maxillary bone, the inferior canine tooth root crosses the width of the rostral mandible making extraction challenging. The size, shape, and course of canine tooth roots necessitate a surgical approach for extraction unless significant alveolar bone loss has occurred. Lorber et al. (1979) found differences between male and female canine teeth in the dog. In the male, crowns were 23% longer, and roots were 40% longer than in females. Likewise, in the male, the width of the crown of the canine tooth was 26% wider, and the width of the root was 50% wider. The smaller crown-to-root ratio in males, together with greater root width, indicated to the authors a difference in the tooth anchorage mechanism. All teeth caudal to the canines are often referred to as the “cheek teeth” (St. Clair & Jones, 1957). They are divided into premolars and molars. In the permanent dentition, there are four premolar teeth (dentes premolares) in each of the four dental quadrants. There are no deciduous first premolar or molar teeth. The first premolar erupts between the fourth and fifth postnatal month and usually remains throughout life. The placement of the premolar teeth may be altered by changing the shape of the head through selective breeding, but regardless of head shape the teeth remain relatively constant in form and size (Stockard, 1941). This leads to tooth crowding in smaller breeds, which predisposes them to periodontal disease. The first premolar is the smallest, and has a single root. The fourth premolar is the largest. In the superior arcade, it has three roots—mesiovestibular, mesiolingual, and distal—and in the inferior arcade only two roots—mesial and distal. The second and the third premolar teeth are similar in all quadrants. Each premolar has two roots, mesial and distal. The crowns of the first three premolar teeth are similar in all quadrants. All three have a singular pyramidal shape. The first premolar tooth has one tubercle. The second and third premolars have similar crowns with an additional small tubercle on the distal aspect. The enamel bulge at the neck of the tooth is more prominent on the lingual surface (Fig. 7-7). On each tooth the mesial border slopes slightly distal. The distal border of the largest tubercle is steep. Adjacent to the base of the main tubercle on this border is a second smaller tubercle 1 or 2 mm high. The most distal surface of the crowns of the second and third premolars can be irregular with additional small tubercles located in this region.

The superior fourth premolars and the inferior first molars are the largest shearing teeth in the mouth. They are the carnassial or sectorial teeth (dens sectorius) (Fig. 7-7C). The superior fourth premolar teeth have three stout diverging roots. The mesiovestibular and mesiolingual roots are more slender than the wide distal root. The space between all roots is referred to as the furcation, which is normally filled with interradicular bone. The mesiovestibular and distal roots form prominent alveolar juga that end just ventral to the ventral border of the infraorbital canals. The distal root of each superior fourth premolar tooth is triangular and somewhat transversely flattened, becoming wide at the neck. The mesiolingual root is flattened in an oblique plane; a small tubercle extends lingually off the mesial developmental ridge of the crown indicating the position of this root. According to Annis (1974), the superior fourth premolar is the tooth most commonly involved with root abscesses. An abscess of one of the roots can manifest by either the formation of a suborbital swelling on the face rostroventral to the eye with or without a fistula or by fistula formation within the mouth at the mucogingival junction. A permanent cure involves extraction or endodontic therapy of the affected tooth. The fourth premolar tooth in the superior arcade is similar in form to the two teeth mesial to it but it is larger. A developmental groove separates the largest mesial tubercle from a more prominent distal tubercle.

The molar teeth (dentes molares) have no deciduous predecessors. There are two in each superior quadrant and three in each inferior quadrant. In each quadrant the first are the largest and the last are the smallest. The masticatory surfaces of the superior molars are multituberculate and are at two levels. The vestibular tubercules are higher than the low lingual tubercle. The lingual surfaces of the superior molar teeth are irregularly flattened and make occlusal contact with the last two inferior molars and the distal third of the first inferior molar. The inferior molars are also multituberculate.

Each superior molar tooth has three slightly diverging roots. The lingual root of each of these teeth is more massive than either of the two vestibular roots, although it is shorter. It is slightly compressed rostrocaudally and is so shaped that the greatest compression force is transmitted through it to the compact bone that lies adjacent to the neck of the tooth.

The inferior first molar tooth is greater than twice the size of the second and third molar teeth. The distal third of the first molar inferior teeth is adapted for crushing and grinding; the mesial portion is sharp and pointed, and is well-suited for shearing. The shearing portion of each inferior first molar possesses the largest tubercle of any inferior cheek teeth and it is roughly quadrilateral in form. A small fossa formed in the hard palate mucosa slightly lingual to the midportion of the superior fourth premolar tooth receives the coronal tip of this large tubercle when the mouth is closed. For a discussion of the terminology used for tooth cusps and crests, see Szalay (1969) and Every (1972, 1974). Szalay uses a modified Cope-Osborn terminology for mammalian tritubercular cusps and crests, whereas Every proposes a new term for each.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on The Digestive Apparatus and Abdomen
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