|SECTION X||INTERESTING VARIATIONS IN ANIMAL TEETH COMPARED TO HUMAN TEETH USING DENTAL FORMULAE|
A dental formula for the human primary dentition can be represented by placing the abbreviation for incisors (I) followed by an upper number representing the number of incisors in an upper quadrant over a bottom number representing the number of incisors in a lower quadrant (I 2/2), then the number of canines (C) in an upper and lower quadrant (C1/1), and then the number of molars (M) in an upper and lower quadrant (M2/2). The formula used to represent teeth in the human primary dentition is as follows:
The dental formula for the human permanent dentition, adding the new abbreviation for premolars (PM), is as follows:
It is interesting to note that the dentition of animals can be represented by the same type of formula as described above. Look at the formulas for animals in Table 1-6, and note that cows have no upper incisors or upper canines. They have three upper and three lower premolars on each side. Did you know that dogs have twice as many premolars as humans if you include uppers and lowers, as well as the right and left sides? Did you know that the tusks on an elephant are maxillary central incisors? Elephants have the largest diastema in the world, large enough for the massive trunk between their central incisors.
|TABLE 1-6||Some Dental Formulae (Order of Teeth per Quadrant) and Interesting Facts about Teeth in Animals2–4|
*Pigs and hippopotami have the same formula, except that they have two or three upper and two or three lower incisors.
†Elephants have deciduous molars (Dm) but no premolars. An elephant’s skull is not larger than necessary to house its brain. The size is needed to provide mechanical support for the tusks (one third of their length is embedded in the skull) and the enormous molars. Each molar weighs about 9 pounds and is nearly a foot long mesiodistally on the occlusal surface. Tusks (the central incisors) can be as long as 1½ feet and weigh 440 pounds.5
‡Guinea pigs have the same formula, except that they have only one maxillary incisor.
The beaver has four strong curved incisors. They have very hard, bright orange enamel on the labial surface and much softer exposed dentin on the lingual surface. As the dentin wears off, this leaves very sharp cutting edges of enamel. The incisors continue to grow throughout life. The posterior teeth have flat, rough edges on the occlusal surface, and they stop growing at 2 years of age. There is a large diastema immediately posterior to the incisors, and flaps of skin fold inward and meet behind the incisors to seal off the back part of the mouth during gnawing.
Therefore, splinters are kept out. The flaps of skin relax for eating and drinking.
The shrew has two hooked cusps on the upper first incisor. Its primary dentition is shed in utero. The shrew’s 1- to 1½-year life span is limited by the wear on their molars. Death occurs by starvation once the molars wear out. Also, their small body can store only enough food for 1 to 2 h, so they must feed almost continually. Their diet consists of small invertebrates, woodlice, and fruit.
The vampire bat has large canines, but its highly specialized upper incisors, which are V shaped and razor edged, are what remove a piece of the victim’s skin. The bat’s saliva contains an anticoagulant, and its tongue rolls up in a tube to suck or lap the exuding blood.
Some vertebrates do not have any teeth (complete anodontia) but have descended from ancestors that possessed teeth. Birds have beaks but depend on a gizzard to do the grinding that molars would usually perform. Turtles have heavy jaw coverings, which are thin edged in the incisor region and wide posteriorly for crushing. The duck-billed platypus has its early-life teeth replaced by keratinous plates, which it uses to crush aquatic insects, crustaceans, and mollusks. The whalebone whale and anteaters also have no teeth, but their diets do not require chewing.
LEARNING EXERCISE 1
Sketch a tooth and adjacent gingiva in cross section, and label the following structures: enamel, dentin, cementum, root canal, pulp chamber, apical foramen location, dentinoenamel junction, cementoenamel junction, dentinocemental junction, periodontal ligament space, alveolar bone, gingiva, gingival sulcus, anatomic crown, and anatomic root. Use Figures 1-10 and 1-12 as a guide.
LEARNING EXERCISE 2
Identify the teeth visible in Figure 1-64 using the Universal Numbering System. Remember that as you are viewing this mouth, the left side of the photograph is the right side of the mouth. Begin with the second molar in the maxillary arch and continue to the central incisor. Then drop to the mandibular central incisor and continue numbering back to the mandibular second molar. Compare your responses to the answers that follow. Then identify the same teeth using the International System and finally the Palmer System.
Universal tooth numbers for teeth in order: 2, 3, 4, 5, 6, 7, 8; 25 for central incisor, 26, 27, 28, 29, 30, 31. The correct numbers using the International System are 17, 16, 15, 14, 13, 12, 11; 41 for central incisor, 42, 43, 44, 45, 46, 47. Then, use Table 1-1 to confirm the correct method for identifying each of these teeth using the Palmer System.