ALTHORS’ ABSTRACT OF THIS PAPER ISSI:ED BY THE BIBLIOGRAPEIC SERVICE, OCTOBER 1 THE GEOMETRICAL SYMMETRY O F GROWTH O F THE UPPER INCISORS O F THE ALBINO R A T FREDERICK S. HAMMETT AND ELIZABETH S. JUSTICE The Wistar I n s t i t u t e of A n a t o m y and Biology, Philadelphia THREE FIGURES The growth of the incisor teeth of the normal albino rat is apparently continuous throughout life. That these teeth do not reach prodigious length is due largely to the fact that a n adequate apposition of the upper and lower incisors ensures the grinding away of their ends as they are pushed out during growth. When such apposition is imperfect or lacking, the teeth become abnormally long and it is in such cases that the mechanical direction of growth is observable to better advantage than by measurements of the radius of curvature of normally ground teeth. According to Owen ( ’40-’45), the upper incisors a r e large segments of a small circle. Such a conception, while true, does not afford a picture of the unopposed growth of these teeth. There recently came into our possession a n adult male albino rat in which the growth of the upper incisors had been unopposed. The animal was 287 days old. I t was apparently healthy. It had a body weight of 120 grams and a body length of 200 mm. According to Donaldson’s tables ( ’15), a rat of this age should have a body weight of 273.7 grams and a body length of about 220 mm. I t is thus seen that our rat was some 56 per cent below standard weight and 10 per cent below the standard length. This low body weight can be attributed to a difficulty in getting adequate food for normal growth in body weight, obviously due to the lack of effective incisor teeth. That a certain amount of comminution of food occurred, 141 142 F. S. HAMMETT A N D E. S. JUSTICE however, was shown by tlie fact that the molars were somewhat worn. The very much greater retardation of growth i n body weight, due to the quantitative lack of food, as compared with the relatively slight retardation of growth in body length, falls in line Kith the theory expressed in a n earlier paper ('23) that growth by increase in cell number is more stable than is growth by increase in cell size towards conditions which tend to disturb growth in general. Although similar abnormalities have been reported by Tiedersheim ('03), Beretta ( '13), Addison and Appleton ('15)' and Hamrnett ('B), certain features in our specimen seemed to make a brief report worth while. First, as to the apparent cause of the condition. Bcretta ('13) has classified the causal nexi of such abnormalities under three headings, viz.: 1) Overgrowth due to lack of opposing teeth; 2) overgrowth due to deviation of the jaws, and, 3) overgrowth of lower incisors due to prognathism. Our specimen falls into the first class. I t is peculiar in that there was apparently a congenital defect in development of the lower incisors. To all outward appearances the loci of the anlagen were normal, but the effectiveness of the anlagen in directing and producing normal incisors was lacking. The conditions found a r e shown in figures 2 and 3. The anlage in the left inferior maxilla gave rise to a meager, abortive product which emerged from the jaw caudad to its normal point, and protruded laterally, upward and slightly forward. The anlage in the right maxillary bone failed to give rise to an externally extruding tooth. On the inner surface of the jaw, however, as shown in figure 3, there was a portion of a tooth, the growth of which was directed caudad and which took its origin from the right lower incisal anlage. These abnormalities were evidently due to congenital defects. As a result of tliis lack of development of the lower incisors, the growth of the upper incisors was unopposed. The mec1:anical direction of this growth first claimed our atten- INCISOR T O O T H GROWTH 143 tion bj- reason of its remarkable symmetry. On first inspection the impression was given that the course of the growth of these teeth had followed the line of a complete and practically perfect circle. I n figure 1 is a slietch of the condition observed in the left upper incisor. The right incisor, instead of growing around outside, penetrated the skull directly beneath its own anlage. Careful measurements across these teeth showed that everywhere the same diameter of 14 mm. obtained from outer edge to outer edge. Thus the preliminary Fig. 1 Showing the symnietrieally circular character of the growth of the unopposed upper left incisor. Fig. 2 Shouring the abortive l e f t lower incisor. Fig. 3 Showing the place of emergence of the c:iuclally directed lower incisor. Inner surface of jaw. impression was confirmed and a demonstration made that the mechanical direction of the growth of the upper incisors of the albino r a t is t o v a r d s the formation of a complete and practically perfect circle. It seems noteworthy to us that the processes concerned in the growth of these teeth should be so nicely adjusted that the amount of material added to the labial portion bears that certain relation to the amount of material added to the lingual portion so that the growth assumes the geometrical symmetry of the circle. 144 F. S. HAMMETT A N D E. S. JUSTICE L1TERATUR.E C I T E D ADDISON,W. H. F., AND APPLETON,JR., J. L. 1915 The structure and growth of the incisor teeth of the albino rat. Jour. Morph., vol. 26, pp. 43-46. BERETTA, A. 1913 La normala dentatura dei roditori in rapport0 alle anomalie dentali in questi osservate. La Stomatologie, T. 10. Abstract in Deutsche Monatsschrift f u r Zahnheilkunde, April, S. 287. DONALDSON, HENRYH. 1915 The Rat. Memoirs of The Wistar Institute of Anatomy and Biology, Philadelphia, no. 6. HAMMETT, F. S. 1922 Studies of the thyroid apparatus. VII. A differential effect of thyro-parathyroidectomy and parathyroidectomy on the incisor teeth of the albino rat. The American Jour. of Physiol., vol. 62, pp. 197-201. 1923 Studies of the thyroid apparatus. IX. The effects of the loss of the thyroid and parathyroid glands at 100 days of age on the growth in body length, body weight and tail length of inale and female albino rats. The American Jour. of Physiol., vol. 63, no. 2, pp. 218-244. OWEN, R. 1840-1845 Odontography. London. WIEDERSHEIN,R. 1902-1903 E i n abnormes R.attengebiss. Anat. Anz., Bd. 22, S. 569-573.