The text explores the structure and proportions of the mouth from various perspectives, particularly focusing on the differences between seeing a mouth from the front, three-quarters, and profile. It discusses the lines of operation and how they change based on the mouth's orientation, with detailed examinations of the upper and lower lips. The latter part describes various expressions and their impact on the shape and orientation of the mouth.
The extended scales reach to the bottom of the board. In this way, it can be seen that the mouth viewed at three-quarters (fig. 3) occupies exactly three-quarters of the width of the mouth viewed from the front; it can also be seen that the small side or the side seen in foreshortening a b of fig. 3 occupies exactly one quarter of the width AB (fig.1) or one-third of the width a d, fig. 3. It can also be seen that the main line of operation passes through the middle of the mouth at point b, the base of the upper lip, as in the mouth seen from the front (fig. 1) where it passes through point a. There is, however, a difference, and here it is: in fig. 3, the main line of operation does not pass through the middle of the curve forming the top of the upper lip, because this upper lip is not perpendicular, but instead leans forward.
If one supposes a line that divides the upper lip into two equal parts, with the mouth seen from the front, this line will appear perpendicular like the line a b, fig. 1. This line seems to blend with the main line of operation, whereas in fig. 3 this same line, represented by the line b c, seen from the side, appears oblique, and point c no longer seems perpendicular to point b, and the main line of operation, which is almost always perpendicular, can no longer, in this position, pass through point c. The smaller the three-quarters become, the more this line appears slanted, and consequently, the more point c, the middle of the upper part of this lip, seems to move away from the main line of operation, while this main line of operation always goes through point a of fig. 1, b of fig. 3, and d and a of figs. 4 and 5, centers of the base of this lip.
In the space (fig. 3), there must be as many details as in space A (fig. 1). Here (fig. 1), the curve is a single piece. In fig. 3, this curvature seen in foreshortening becomes a series of undulations. Similarly, the curvature A a of fig. 1 appears, in fig. 3, deeper inward; finally, in figs. 4 and 5 the space d must contain everything that is in space A of fig. 1. The mouth represented by fig. 6 is placed concerning the observer to be viewed in profile. Then only half is seen, and the line of fig. 6 is the same as the line a b (fig. 1) as c b (fig. 3) and as c d (fig. 4 and 5).
The width of the mouth seen in profile loses its status and is equal to a quarter of the width AB of the mouth viewed from the front.
The lower lip is spherical: in the middle of this lip, from top to bottom, there is a slight groove which, taking the shape of this lip, presents a curved line. The mouth seen from the front, this groove seems a straight line there, glued to the main line of operation. In the mouth viewed at regular three-quarters (fig. 3), this groove becomes a curved line p b. The smaller the three-quarters become, the more the line becomes undulating. See the lines of (fig. 4), p d (fig. 5) and r (fig. 6), becoming the contour of the lower lip.
In all positions, front, three-quarters and profile, provided that the head is upright, the height of the lips is always the same, as seen by fig. 8. Drawing horizontal lines from the upper and lower ends of the upper lip of the mouth seen in profile, one has, between these lines, the thickness of the upper lip of the mouth viewed at three-quarters; the lower lip of the mouth seen in profile will still give the thickness of the lower lip of the mouth seen at three-quarters.
Fig. 9 represents a mouth that smiles: fig. 10 represents a mouth that laughs out loud. Fig. 11 represents a laughing mouth: but this mouth leans forward, so that the lower teeth are visible.
Fig. 12 represents a mouth that inhales, and fig. 13 represents a mouth that blows. Fig. 14 represents a mouth that sings. Fig. 15 represents the mouth of a person suffering from sharp pain and gritting their teeth.
Fig. 16 represents an open and inverted mouth. In this position, the mouth being seen from below, the lines appear curved.
Fig. 17 represents a pouting mouth. Here, the corners of the mouth drop significantly, while in a laughing mouth, the corners of the mouth are raised. The more laughter increases, the more the corners of the mouth rise. The more bad mood increases, the more the corners of the mouth rise.