The wide-open mouth of a yawn is sometimes accompanied by an involuntary spray of saliva. This phenomenon is a normal consequence of several biological systems intersecting at a single moment. Understanding why saliva escapes during a yawn requires looking closely at the mechanics of the yawn, the continuous process of saliva production, and how the surrounding muscles interact. The unintentional expulsion of saliva results from these forces temporarily disrupting the body’s usual management of oral fluids.
The Physical Act of Yawning
Yawning is a complex, stereotyped motor behavior involving more than just opening the mouth. It begins with a slow, deep inhalation of air, followed by a brief period of holding the breath, and concludes with a relatively rapid exhalation. This action is coordinated by a network of nerves and muscles originating in the brainstem and hypothalamus.
The characteristic wide mouth opening, or gaping, is achieved by the depression of the mandible, or lower jaw. This movement stretches the muscles of the face, jaw, and throat, including those involved in the swallowing process. The deep breath also involves the diaphragm and intercostal muscles, coordinating the event across the upper body.
The entire sequence typically lasts only a few seconds and powerfully stretches the airway and surrounding tissues. This maximal stretching temporarily changes the shape and tension of the oral cavity and pharynx. This physical transformation sets the stage for the subsequent expulsion of saliva.
Saliva Production and Management
The mouth maintains constant moisture thanks to the continuous work of three major pairs of salivary glands: the parotid, submandibular, and sublingual glands. These glands collectively produce between 0.5 to 1.5 liters of saliva daily. Saliva lubricates the mouth, aids in digestion, and protects teeth.
The submandibular glands contribute a significant amount of the total saliva volume, and their ducts open under the tongue. Under normal conditions, saliva is constantly swallowed in a reflex known as deglutition, which prevents excessive pooling. This routine process manages the steady flow of fluid into the oral cavity.
This normal management relies on a relatively stable muscle environment around the glands and ducts. The constant production and swallowing work together to maintain oral hygiene and lubrication without any unintentional spillage. The act of yawning, however, dramatically alters this stable environment.
The Mechanism of Involuntary Saliva Expulsion
The involuntary expulsion of saliva during a yawn is primarily a mechanical event resulting from the sudden, powerful muscle movements. The wide-open jaw and the intense stretching of the facial and mandibular muscles physically compress the major salivary glands, particularly the submandibular glands. This sudden pressure acts like squeezing a sponge, forcing a brief surge of saliva out of the ducts and into the mouth.
The submandibular glands’ ducts, known as Wharton’s ducts, are located just behind the lower front teeth, making their output particularly susceptible to being propelled forward. This forced output is a temporary, involuntary surge that overwhelms the body’s normal ability to contain and swallow the fluid.
The wide gape of the mouth drastically changes the shape of the oral reservoir. The accompanying deep inhalation and exhalation create a rapid rush of air. This combination of compressed glands, a rapid surge of fluid, and a powerful airstream overcomes the typical swallowing reflex. The result is a small, involuntary stream of saliva propelled out of the mouth, an event sometimes informally termed “gleeking.”