How to Draw Mouths

Figure Drawing and Composition, Richard G. Hatton

The form of the upper lip, shown in A is caused by the shadow line of the turn of the lips ahown at B. There is a hollow depression at C which folds into the mouth. At D the lower lip end. The mounds below the lip at E are reflected in the two sides of the chin at F. The chin's prominence at G is reflected by the tip of the nose.
Ebook of Figure Drawing By Richard G. Hatton here.

Figure drawing and composition;: Being a number of hints for the student and designer upon the treatment of the human figure,Buy a copy at Amazon.com.

Figure drawing and composition: Being a number of hints for the student and designer upon the treatment of the human figure by Richard G. Hatton Another copy at Amazon.com.

Figure Drawing (A Companion to Figure Composition) Similar book by the same author at Amazon.com.

As with the head is is useful to have a knowledge of the skeletal structure.

how to draw mouths
Art Drawing Made Easy, Campana's Popular Library

The Nose and Mouth from below, in three-quarter view and from the side
Art drawing made easy (Campana's popular art library) at Amazon.com
Art Drawing Made Easy Another copy at Amazon.com

Buchanan's Initiatory Drawing Lessons
For the Use of Writing Academies and Private Families
Edinburgh, 1828