ブタ (Sus scrofa domesticus) の頬歯の形態に関する研究

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The swine has the permanent dentition formula of I 3/3 C 1/1 P 4/4 M 3/3=44, and it showes the fundamental tooth formula of the mammals. Its cheek teeth (P, M) are called the bunodont molars and have the projections such as spines and tubercles. The first anterior cheek tooth (P_1) is the monophyodonty as the deciduous tooth to perform the function of the permanent tooth. As the swine is considered to be the domesticated wild boar, a fairly large variety is seen in its body size, nature and configuration. The author made the observations with 31 skulls (male 5 and female 26) having the permanent dentition and 21 skulls having the deciduous dentition (with the first posterior cheek tooth), studied the cheek tooth morphology and obtained the following results : (1) The cheek teeth consist of 7 teeth, 4 anterior (P_1, P_2, P_3 and P_4) and 3 posterior (M_1, M_2 and M_3) cheek teeth. Where P_1 tends to be absent, its presence in both upper and lower jaws were seen in 73% of all cases observed. Its absence was overwhelmingly greater in the lower jaw with 12% rate of absence in both left and right sides though it was present both sides in the upper jaw. (2) The size of tooth increased successively from P_1 backward to M_3. While <P_1>___- to P__-_3 were of the triconodont tooth form and the occlusal surface of P__-_4 formed a triangle and three-cusp, M__-_1 and M__-_2 were four-cusped and M__-_3 had the excessive talon. From P^^-_1 to P^^-_4 were the triconodont tooth form, M^^-_1 and M^^-_2 were four-cusped, and M^^-_3 were seven to eight-cusped, having projecting tubercles in between the cusps. Mesiodistal development of talonid was remarkable in M^^-_3. By comparison of the coefficient of variation, P_1 showed the greatest value, followed by the distal side of M_3. (3) The mesiodistal diameter of talon (-id) of M__-_2 was only 1mm greater than that of M__-_1 but in M__-_3 it was as much as 9mm longer than in M__-_2. The difference between M^^-_1 and M^^-_2 was 1.2mm but leaped to 12mm between M^^-_2 and M^^-_3 whose mesiodistal diameter was the greatest of all teeth. The buccolingual diameter of talon(-id) in M__-_2 was 3mm greater than that of M__-_1, slightly larger by 0.6mm in M__-_3 than in M__-_2. The difference between M^^-_1 and M^^-_2 was 2.8mm, and 2mm between M^^-_2 and M^^-_3. In both upper and lower jaws, the difference of buccolingual diameter of talon(-id) was more remarkable between M_1 and M_2. (4) The developmental degree of median buccal accessory cusp that belongs to the posterior cheek tooth was not distinct in M__-_1 but it developed well in M__-_2. That of the lower jaw indicated similar degree, fairly development. (5) In the study of the relations between developmental degree of median buccal accessory cusp and the size of the dental crown, it was observed that the greater the dental crown buccolingual and mesiodistal diameter, the better the accessory cusp growth, especially in M__-_1 and M__-_3. Whereas it was indicated the greater the mesiodistal and buccolingual diameter of the talonid of M^^-_1 and the better the accessory cusp growth, no relation was seen between the growth of accessory cusp and tooth size in the other lower posterior cheek teeth. (6) These observations are summarized to conclude that the tooth reduction begins to occur from the first anterior cheek tooth, particularly so in the lower jaw of the cheek teeth of swine. Accordingly, the tubercles and spines are better developed and complicated toward posterior end. From the fact that the mesiodistal diameter of talonid is particularly large in M^^-_3, it is considered that the occlusal surface area is made larger to make mastication more effective in the distal teeth rather than mesial teeth.
九州歯科学会の論文

著者

池田廣重
九州歯科大学第1口腔解剖学教室

ブタ (Sus scrofa domesticus) の頬歯の形態に関する研究

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