The present author, by the method of measuring human tympanic membrane by molding with alginate impression material, studied the detailed morphological changes of drum at normal pressure and-30mm Hg of the tympanum in 7 normal ears. 1. The present experiment was performed on 7 ears of 7 adults, 1655 years old, whose heasing acuity was normal and who had no abnormal findings in the tympanic membrane. 2. Average size of the tympanic membrane. Projecting on a flat surface the vertical diameter was 9.85mm, the diameter along the malleus 7.51mm and the horizontal diameter 7.55mm. In true lengths, the vertical diameter was 11.19mm, the diameter along the malleus 8.81mm and the horizontal diameter 8.77mm. The projection resulted in a shortening to about 14%. 3. The radius of the tympanic membrane from the umbo was shortest antero-inferiorly (3.63mm). The postero-inferior radius was next (4.39mm, and 4.42mm to the short process) and the postero-superior radius was longest (5.15mm). The average diameter of the pars flaccida was, along the vertical line, 2.37mm. 4. By negative pressure the tympanic memdrane was distended on the diameter along the malleus by 0.22mm (2.5%) and on the horizontal diameter by 0.17mm (1.97%). Centering the umbo the distention was 0.12mm (2.73%) on the postero-inferior radius, 0.09mm (2.48%) on the antero-inferior radius, 0.10mm (2.26%) along the malleus, and 0.08mm (1.55%) on the postero-superior radius. The distention was largest in the pars flaccida (0.22mm, 9.28%). 5. The average distance of the infundibulum to the imaginal surface of the tympanic membrane was 2.04mm, which was displaced 0.21mm medially by negative pressure. The height of the short process was 0.06mm. By negative pressure the process was displaced 0.18mm medially. The height of the basis of the pars flaccida was 0.49mm. The basis was retracted by negative pressure 0.35mm more. Namely, comparing to the degree of retraction of the umbo, that of the short process was smaller by 14.4% and that in the pars flaccida was larger by 67%. 6. The retraction in the pars tensa by negative pressure was most extreme in the posterosuperior part (0.26mm). This was larger than the retraction of the umbo by 24%. The retraction of the antero-inferior part was 0.18mm, that near the handle of the malleus 0.16mm, and that in the postero-inferior part was 0.13mm, which was slightest, and furthermore, smaller than the retraction of the umbo by 38%.
7. Otoscopically, the radius centering the umbo was 4.59mm in the antero-superior part, 3.11mm in the antero-inferior part, 3.16mm in the postero-inferior part, and 1.46mm in the postero-superior part. The radius was lengthened by negative pressure 0.32mm and 0.36mm in the antero-inferior and postero-inferior parts respectively, while it was shortened 0.01mm and 0.34mm in the antero-superior and postero-superior parts respectively. Namely, the umbo was displaced 0.35mm in the postero-superior direction and gave a criterion of measurement of the degree of retraction. 8. The average width of the posterior fold of the tympanic membrane, observed with an otoscope, was 0.75mm. By negative pressure it decreased by 0.36mm to a half. Thus it formed a sharp border by an extreme retraction of the posterior part of the tympanic membrane and accentuated the degre of the retraction.