[FONT=Arial, Helvetica, sans-serif]The Comparative Anatomy of Eating[/FONT]
by Milton R. Mills, M.D.
What About Me?
The human gastrointestinal tract features the anatomical modifications consistent with an herbivorous diet. Humans have muscular lips and a small opening into the oral cavity. Many of the so-called “muscles of expression” are actually the muscles used in chewing. The muscular and agile tongue essential for eating, has adapted to use in speech and other things. The mandibular joint is flattened by a cartilaginous plate and is located well above the plane of the teeth. The temporalis muscle is reduced. The characteristic “square jaw” of adult males reflects the expanded angular process of the mandible and the enlarged masseter/pterygoid muscle group. The human mandible can move forward to engage the incisors, and side-to-side to crush and grind.
Human teeth are also similar to those found in other herbivores with the exception of the canines (the canines of some of the apes are elongated and are thought to be used for display and/or defense). Our teeth are rather large and usually abut against one another. The incisors are flat and spade-like, useful for peeling, snipping and biting relatively soft materials. The canines are neither serrated nor conical, but are flattened, blunt and small and function Like incisors. The premolars and molars are squarish, flattened and nodular, and used for crushing, grinding and pulping noncoarse foods.
Human saliva contains the carbohydrate-digesting enzyme, salivary amylase. This enzyme is responsible for the majority of starch digestion. The esophagus is narrow and suited to small, soft balls of thoroughly chewed food. Eating quickly, attempting to swallow a large amount of food or swallowing fibrous and/or poorly chewed food (meat is the most frequent culprit) often results in choking in humans.
Man's stomach is single-chambered, but only moderately acidic. (Clinically, a person presenting with a gastric pH less than 4-5 when there is food in the stomach is cause for concern.) The stomach volume represents about 21-27% of the total volume of the human GI tract. The stomach serves as a mixing and storage chamber, mixing and liquefying ingested foodstuffs and regulating their entry into the small intestine. The human small intestine is long, averaging from 10 to 11 times the body length. (Our small intestine averages 22 to 30 feet in length. Human body size is measured from the top of the head to end of the spine and averages between two to three feet in length in normal-sized individuals.)
The human colon demonstrates the pouched structure peculiar to herbivores. The distensible large intestine is larger in cross-section than the small intestine, and is relatively long. Man's colon is responsible for water and electrolyte absorption and vitamin production and absorption. There is also extensive bacterial fermentation of fibrous plant materials, with the production and absorption of significant amounts of food energy (volatile short-chain fatty acids) depending upon the fiber content of the diet. The extent to which the fermentation and absorption of metabolites takes place in the human colon has only recently begun to be investigated.
In conclusion, we see that human beings have the gastrointestinal tract structure of a “committed” herbivore. Humankind does not show the mixed structural features one expects and finds in anatomical omnivores such as bears and raccoons. Thus, from comparing the gastrointestinal tract of humans to that of carnivores, herbivores and omnivores we must conclude that humankind's GI tract is designed for a purely plant-food diet.
Summary
<table border="0" cellpadding="4" cellspacing="1" width="80%"><tbody><tr bgcolor="#000099" valign="top"> <td colspan="2">[FONT=geneva, verdana, arial, helvetica]
Facial Muscles[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Reduced to allow wide mouth gape</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Well-developed</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Reduced</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Well-developed</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Jaw Type[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Angle not expanded</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Expanded angle</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Angle not expanded</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Expanded angle</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Jaw Joint Location[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
On same plane as molar teeth</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Above the plane of the molars</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
On same plane as molar teeth</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Above the plane of the molars</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Jaw Motion[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Shearing; minimal side-to-side motion</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
No shear; good side-to-side, front-to-back</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Shearing; minimal side-to-side</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
No shear; good side-to-side, front-to-back</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Major Jaw Muscles[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Temporalis</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Masseter and pterygoids</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Temporalis</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Masseter and pterygoids</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Mouth Opening vs. Head Size[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Large</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Small</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Large</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Small</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Teeth (Incisors)[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Short and pointed</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Broad, flattened and spade shaped</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Short and pointed</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Broad, flattened and spade shaped</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Teeth (Canines)[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Long, sharp and curved</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Dull and short or long (for defense), or none</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Long, sharp and curved</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Short and blunted</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Teeth (Molars)[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Sharp, jagged and blade shaped</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Flattened with cusps vs complex surface</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Sharp blades and/or flattened</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Flattened with nodular cusps</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Chewing[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
None; swallows food whole</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Extensive chewing necessary</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Swallows food whole and/or simple crushing</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Extensive chewing necessary</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Saliva[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
No digestive enzymes</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Carbohydrate digesting enzymes</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
No digestive enzymes</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Carbohydrate digesting enzymes</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Stomach Type[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Simple</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Simple or multiple chambers</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Simple</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Simple</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Stomach Acidity[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Less than or equal to pH 1 with food in stomach</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
pH 4 to 5 with food in stomach</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Less than or equal to pH 1 with food in stomach</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
pH 4 to 5 with food in stomach</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Stomach Capacity[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
60% to 70% of total volume of digestive tract</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Less than 30% of total volume of digestive tract</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
60% to 70% of total volume of digestive tract</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
21% to 27% of total volume of digestive tract</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Length of Small Intestine[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
3 to 6 times body length</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
10 to more than 12 times body length</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
4 to 6 times body length</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
10 to 11 times body length</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Colon[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Simple, short and smooth</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Long, complex; may be sacculated</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Simple, short and smooth</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Long, sacculated</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Liver[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Can detoxify vitamin A</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Cannot detoxify vitamin A</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Can detoxify vitamin A</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Cannot detoxify vitamin A</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Kidney[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Extremely concentrated urine</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Moderately concentrated urine</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Extremely concentrated urine</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Moderately concentrated urine</td> </tr> <tr> <td height="10">
</td> </tr> <tr valign="top"> <td colspan="2" bgcolor="#000099">[FONT=geneva, verdana, arial, helvetica]
Nails[/FONT]</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Carnivore</td> <td bgcolor="#99ccff">
Sharp claws</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Herbivore</td> <td bgcolor="#99ccff">
Flattened nails or blunt hooves</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Omnivore</td> <td bgcolor="#99ccff">
Sharp claws</td> </tr> <tr valign="top"> <td bgcolor="#99ccff">
Human</td> <td bgcolor="#99ccff">
Flattened nails</td></tr></tbody></table>