Jaw and Lips

What is it about?

Oral structures like lips and jaw have always drawn a strong attention in people’s mind, likely because they are so clearly and visibly engaged in producing speech as well as expressing our emotional states. This is obvious from the many idioms and proverbs that include a reference to these structures, as for example, “The lips of the righteous feed many: but fools die for want of wisdom” or “he snatched vic­tory from the jaws of defeat.” These structures have also fascinated people in a more academic way, from surgeons who need to repair cleft lips in children born with such an affliction to speech scientists, who wish to understand the mecha­nisms by which the lips and jaw move and coordinate in producing speech and in other oral motor functions. In this chapter, I will review the various aspects by which lips and jaw can be characterized, starting with their anatomical and physiological features, and then highlighting their specific roles in different oral motor functions such as chewing, swallowing, and perhaps most importantly in the context of this volume, speech production. I will take this broad perspective since the use of lips and jaw in speech production from an evolutionary point of view is a rather recent oral motor behavior in comparison to the far older existing functions of chewing, swallowing, and facial expressions that humans as a species share with other primates. Speech is unique to humans and has specific character­istics that will differentiate it from other oral motor functions, as will be discussed in this chapter, but understanding its origins and relationship to these other oral motor functions is crucial for those who wish to study it.

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