Prehension, Mastication, Swallowing

Prehension is the process of siezing or grasping or otherwise getting food into the mouth. Different species use different techniques to prehend food - for example, horses and goats rely considerably on their lips, whereas cattle, dogs and cats don't use their lips to any extent, but rather, gather many foods with their tongues. Studying comparative prehension can be entertaining, but is of minimal value for understanding digestion.

As with prehension, there are considerable differences among species in techniques used for drinking, that basically boil down to being either a "sucker" or a "lapper". Drinking is usually an efficient process, although beards and moustaches can sometimes interfere.

Mastication, or chewing, is the first step in the breakdown of complex foodstuffs and serves several functions, including:

• breaking large pieces into small pieces, resulting in a massive increase in surface area, which is where digestive enzymes work
• softening of food and transformation into a size conducive to swallowing
• lubrication of food by impregnating it with saliva

Chewing is, to a large extent, a reflex, although you can voluntarily masticate as well. To study this phenomenon, watch a cow ruminating or look around and watch someone chewing gum. The presence of food (or gum) in the mouth causes a reflex inhibition of the muscles of the lower jaw. Those muscles relax and the lower jaw drops, causing a stretch reflex which causes muscle contraction and closure of the mouth. During mastication, the tongue and, to a lesser extent, the lips and cheeks acts to keep food between the grinding surfaces of the teeth. This can be demonstrated by trying to chew your next meal while holding your tongue still. Incidentally, chewing is hard work and expends a lot of energy.

Deficits in the ability to effectively masticate are a very common cause of digestive disease in animals. Many of these problems are associated with poor teeth, and most are easily diagnosed by simple inspection. A particularly common problem in horses is the occurence of "points" on the molar teeth.

The final step in pregastric digestion is swallowing, also known as deglutition. This is really a very complex process that can be thought of as occurring in three steps:

• First, a bolus of food is pressed backward into the pharynx by the tongue. This is the only step that is voluntary - the remaining steps occur by reflex.
• Once the bolus reaches the pharynx several actions are initiated, which basically involve shunting the bolus into the esophagus while at the same time closing alternative routes of escape. The lumen of the larynx is squeezed shut and the epiglottis swings backward to cover the larynx. The larynx is also pulled forward and down making the opening to the esophagus larger.
• Finally, the tongue presses backward and a peristaltic contraction in the pharynx propels the bolus into the esophagus, where the actual act of swallowing takes place.

During swallowing, boluses of food are propelled through the esophagus by strong peristaltic contractions. In dogs and humans, it takes 4-5 seconds for the bolus to traverse the esophagus. If the bolus is not delivered in "one pass", secondary waves of peristalsis are initiated at the point of distention, which almost always result in delivery of the bolus to the stomach. Congenital and acquired disorders in esophageal motility that interfere with this usually reliable delivery of food are rather common in both animals and man.

Advanced and Supplemental Topics

• Dental Anatomy (General and for specific species)
• "Points" in Horses

Send comments to Richard.Bowen@colostate.edu