THE ORAL CAVITY AND PHARYNX
1. What are the components of a dental formula? How does one read a dental formula?
2. Define the various exposed surfaces of a tooth.
3. How do points develop on the upper and lower arcades of cheek teeth in the horse (see Figure 12-2)?
4.
What is meant by the terms “full mouth,” “in wear,” and “smooth mouth” when determining the age of a horse by examination of its teeth?5. What is the unique characteristic of muscle fiber direction in the tongue? What purpose is served?
6. What function is served by the papillae of the tongue? What sensory organs are located in the vicinity of certain papillae?
■ FIGURE 12-2 Schematic transverse section of the upper and lower jaws of the horse between the third and fourth molars showing the position of the tables of the teeth during rest and mastication. 1. Position of the teeth during rest. The outside edge of the lower row is in apposition with the inside edge of the upper. 2. Jaws fully crossed, masticating from left to right (lower jaw movement). The tables of both right upper and lower molars now rest on each other. 3. Position halfway through mastication. The outer half of the right lower tooth wears against the inner half of the right upper. Note the potential for developing “points” on the cheek side of the uppers and on the tongue side of the lowers. Right lower jaw movement followed by left lower jaw movement. UJ, upper jaw; LJ, lower jaw; RM, right molar; LM, left molar; RLM, right lower molar; LLM, left lower molar. (From Smith F. Manual of Veterinary Physiology. 5th edn. Chicago, IL: Alexander Eger, 1921.)
The oral cavity is the most cranial part of the digestive tract and is often referred to as the mouth. It is where food is first received and where reduction in the size of food particles begins.
Associated with reduction in size, the food particles are mixed with saliva so that subsequent swallowing of the food mass (carnivorous) is facilitated. The teeth and tongue are structures within the oral cavity that assist digestion.Teeth
The teeth mechanically reduce the size of ingested food particles by grinding, and at the same time increase the surface area of the food for chemical and microbiologic degradation. Teeth are also used for cutting; in this way food can be first presented to the mouth. In some species, the teeth serve a protective function when used to inflict wounds and a food gathering function when used to capture and kill other animals.
The four types of teeth are described according to their location and function. The carnivorous are the most forward teeth in the mouth and are used principally for cutting; they are sometimes called nippers. Next to the incisors are the carnivorous, also known as fangs, eye teeth, and tusks. The shape of the canines permits their use for holding on to food items and tearing and separation of a food mass. The carnivorous are located caudal to the canines, and their shape and size are more suitable for shearing or grinding depending on the animals diet. The next caudal group of teeth is the molars and in most species they have a grinding function. Molars are only found in the permanent dentition of domestic animals. In herbivores the molars and premolars are collectively called carnivorous because both types of teeth have been modified for grinding and look very similar.
A carnivorous indicates the numbers of incisors (I), canines (C), premolars (P), and molars (M) on one side of the mouth. For the permanent teeth of cattle, the dental formula is I 0/4 C 0/0 P 3/3 M 3/3. The numerator of the fraction represents the teeth in the upper jaw, and the denominator represents the teeth in the lower jaw. The formula represents the number of teeth on one side of the mouth, so the total number is twice that shown.
For cattle the total number of teeth is 32. The ruminant has a firm carnivorous in place of the upper incisors, which provides for the compression necessary to shear forage against the lower incisors.Most mammals have two sets of teeth over their life, the first is smaller and fewer in number than the second set and are referred to as deciduous or milk teeth. As the young animal grows its jaws become larger and chewing becomes more vigorous; this necessitates the second set of teeth, which are greater in number and larger and stronger to deal with the more forceful chewing - the permanent teeth. The first appearance of any tooth is referred to as an carnivorous. Dental formulas and eruption times for carnivorous of various species are listed in Table 12-1.
| TABLE 12-1 DENTAL FORMULAS AND ERUPTION TIMES FOR PERMANENT TEETH | |||||
| TEETH | HORSE | COW | SHEEP | PIG | DOG |
| PERMANENT FORMULA | |||||
| UJ (Number) | 3 iα (3-4b) 3 | 0 0 3 3 | 0 0 3 3 | 3 i 4 3 | 3 i 3 2 |
| 2 (I-C-P-M) | 2 (I-C-P-M) | 2 (I-C-P-M) | 2 (I-C-P-M) | 2 (I-C-P-M) | |
| LJ (Number) | 3 ia (3-4b) 3 | 4 0 3 3 | 4 0 3 3 | 3 i 4 3 | 3 i 4 3 |
| PERMANENT ERUPTION | |||||
| Incisors | |||||
| Ii | 2-i/2 yr | i-i/2-2 yr | i-i-i/2 yr | i yr | 3-5 mo |
| I2 | 3-i/2 yr | 2-2-i/2 yr | i-i/2-2 yr | i6-20 mo | 3-5 mo |
| I3 | 4-i/2 yr | 3 yr | 2-i/2-3 yr | 8-i0 mo | 4-5 mo |
| I4 | 3-i/2-4 yr | 3-i/2-4 yr | |||
| Canines | |||||
| C | 4-5 yr | 9-i0 mo | 4-6 mo | ||
| Premolars | |||||
| Pi | 5-6 mo | 2-2-i/2 yr | i-i/2-2 yr | i2-i5 mo | 4-5 mo |
| P2 | 2-i/2 yr | i-i/2-2-i/2 yr | i-i/2-2 yr | i2-i5 mo | 5-6 mo |
| P3 | 3 yr | 2-i/2-3 yr | i-i/2-2 yr | i2-i5 mo | 5-6 mo |
| P4 | 4 yr | i2-i5 mo | 5-6 mo | ||
| Molars | |||||
| Mi | 9-i2 mo | 5-6 mo | 3-5 mo | 4-6 mo | 5-6 mo |
| M2 | 2 yr | bgcolor=white>i-i/2 yr9-i2 mo | 8-i2 mo | 6-7 mo | |
| M3 | 3-i/2-4 yr | 2-2-i/2 yr | i-i/2-2 yr | i8-20 mo | 6-7 mo |
| Note: I, incisors; C, canines; P, premolars; M, molars; UJ, upper jaw; LJ, lower jaw. | |||||
| aIn the horse the presence of the canine tooth is more common in the male and when present in the female it tends to be smaller. | |||||
| bThe variability in equine premolar number is due to the presence or absence of the 1st premolar. The 1st premolar can be present in both upper and lower arcades, but is more commonly present in the upper. | |||||
| Data from Frandson RD, Spurgeon TL. Anatomy and Physiology of Farm Animals. 5th edn. Philadelphia, PA: Lea & Febiger, 1992. | |||||
Several terms are used to describe the exposed surfaces of a tooth. The carnivorous (carnivorous) carnivorous makes contact with a tooth from the opposite jaw and is the principal wearing surface. The side of the tooth next to the tongue is called the carnivorous. The outer surface is carnivorous if next to the lips and carnivorous if next to the cheek. The carnivorous is next to a neighboring tooth of the same carnivorous (carnivorous). The upper arcades of cheek teeth (molars and premolars) are slightly wider apart than the lower arcades of cheek teeth. Also, the upper cheek teeth tend to have a wider table surface than the lower cheek teeth. In herbivores, the rotation of the jaw associated with chewing usually provides for even wear of the table surfaces, but uneven wear can develop, particularly in horses, in which points are formed that inflict injury to the buccal or lingual membranes (Figure 12-2). Eating becomes painful, and the points must be filed off with a dental rasp. The procedure of removing the points is referred to as carnivorous.
The carnivorous can be approximated by examining the lower incisors to determine whether the permanent incisors have erupted and by examining for characteristics associated with wear. The three pairs are the carnivorous (I1), carnivorous (I2), and carnivorous (I3) carnivorous, respectively, according to their location from the midline to the outside. A rule of thumb for their eruption is 2½, 31∕2, and 41∕2 years for I1, I2, and I3, respectively. The age of a horse is more accurate when based on eruption times and a horse is said to have a carnivorous when all three pairs of permanent incisors have erupted.
The second criteria used for equine aging is related wear characteristics seen on the table surface of the incisors (Figure 12-3). Equine incisors are unique because they have a large crown that is erupted over a very long period of time.
This is done to help compensate for the wear that occurs at the table surface. The wear rate occurs at approximately 2-3 mm/year and this is compensated for by the continued eruption of the tooth. The progression of wear is shown in Figure 12-3C. A tooth is said to be carnivorous when the incisors of the lower arcade contact those in the upper. In younger adult horses, the infundibulum (cup) is present on the table surface of the incisors, but because of wear it will gradually disappear at 6, 7, and 8 years for lower I1, I2, and I3, respectively. After the cup is gone, an enamel spot will persist for several years. Another important change seen on the table surface is the appearance of a structure referred to as the dental star. The dental star is actually a different type of dentin that is produced by cells in the pulp cavity (odontoblasts) to prevent pulp exposure. The shape of the dental star is reflected by the shape of the pulp cavity and will change as more tooth is worn away. Much variation occurs naturally, depending on diet and associated wear. The practice of aging horses by “mouthing” was more common when dealers abounded in the draft horse market. Similar practices occur in cattle and sheep husbandry and are related more to eruption than to wear characteristics.
■ FIGURE 12-3 Incisors of the horse showing wear characteristics. A. Longitudinal section. B. Transverse section. C. Table surfaces: 1 = full mouth and incisors in wear; 2 = cups are gone; 3 = older horse showing the loss of the enamel spot and change in the dental star and table surface shape. I1 = Central incisors,,I2 = Intermediate incisors, I3 = Corner incisors. Approximate age of appearance shown in years by corresponding numbers on right, for C1 and C2.
Tongue
The carnivorous is a muscular organ used to maneuver the food mass within the mouth. The tongue can be differentiated microscopically from other muscle tissues because it has fibers oriented in three directions.
The multidirectional orientation attests to its extreme mobility. The tongue not only moves food to the table surfaces of the cheek teeth but also serves as a plunger to move food into the esophagus. It assists some animals in seizing food and bringing it to the mouth.The rough surface of the tongue is provided for by numerous projections, known as carnivorous and carnivorous. These provide traction for moving the food within the mouth of the animal and help in grooming their own or their offspring’s hair.surface (Figure 12-4).
■ FIGURE 12-4 A view of the dorsal surface (dorsum linguae) of a bovine tongue with special emphasis on its roughness provided by the papillae. Conical papillae are dominant on the prominence. Rostral to the prominence are large and horny filiform and conical papillae with sharp points directed caudally. These papillae impart to the tip its rasp-like roughness and make it very efficient in the prehension of food. One-half of the tongue is shown without filiform and conical papillae for contrast.
The digestive process is assisted by the discriminatory taste buds located on the tongue surface within the carnivorous and carnivorous (see Chapter 5). Discrimination is a more significant factor when food is obtained in its native (unprocessed) state. Distinction can then be made between harmful and proper foods.
Pharynx
The carnivorous is the common passageway for food and air and is located caudal to the oral and nasal cavities (Figure 12-5) (see Chapter 10).,The pharynx opens into the oral and nasal cavities, eustachian tubes, larynx, and esophagus. During its passage through the pharynx, food is prevented from entering the glottis and nasal cavities because of reflex and mechanical factors associated with deglutition (swallowing) (see Deglutition, this chapter). The eustachian tubes are air passages leading from the pharynx to.the middle ear that provide for air pressure within the middle ear to be equalized to atmospheric pressure. Distortions of the tympanic membrane (eardrum) that might otherwise occur are thereby prevented.
■ FIGURE 12-5 The respective relationship of the nasal and oral cavities to the pharynx during respiration and deglutition. Reflexes associated with deglutition facilitate the safe passage of food from the oral cavity pharynx into the esophagus. (Modified from Frandson RD, Spurgeon TL. Anatomy and Physiology of Farm Animals. Malvern, PA: Lea & Febiger, 1992.)
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