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Old 20-08-2005, 05:55 PM
Beach Runner
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Default Medical doctor comparative human anatomy

Im posting this for convenience of others

The Comparative Anatomy of Eating
by Milton R. Mills, M.D.

Humans are most often described as "omnivores". This classification
is based on the "observation" that humans generally eat a wide
variety of plant and animal foods. However, culture, custom and
training are confounding variables when looking at human dietary
practices. Thus, "observation" is not the best technique to use when
trying to identify the most "natural" diet for humans. While most
humans are clearly "behavioral" omnivores, the question still remains
as to whether humans are anatomically suited for a diet that includes
animal as well as plant foods.

A better and more objective technique is to look at human anatomy and
physiology. Mammals are anatomically and physiologically adapted to
procure and consume particular kinds of diets. (It is common practice
when examining fossils of extinct mammals to examine anatomical
features to deduce the animal's probable diet.) Therefore, we can
look at mammalian carnivores, herbivores (plant-eaters) and omnivores
to see which anatomical and physiological features are associated
with each kind of diet. Then we can look at human anatomy and
physiology to see in which group we belong.

Oral Cavity

Carnivores have a wide mouth opening in relation to their head size.
This confers obvious advantages in developing the forces used in
seizing, killing and dismembering prey. Facial musculature is reduced
since these muscles would hinder a wide gape, and play no part in the
animal's preparation of food for swallowing. In all mammalian
carnivores, the jaw joint is a simple hinge joint lying in the same
plane as the teeth. This type of joint is extremely stable and acts
as the pivot point for the "lever arms" formed by the upper and lower
jaws. The primary muscle used for operating the jaw in carnivores is
the temporalis muscle. This muscle is so massive in carnivores that
it accounts for most of the bulk of the sides of the head (when you
pet a dog, you are petting its temporalis muscles). The "angle" of
the mandible (lower jaw) in carnivores is small. This is because the
muscles (masseter and pterygoids) that attach there are of minor
importance in these animals. The lower jaw of carnivores cannot move
forward, and has very limited side-to-side motion. When the jaw of a
carnivore closes, the blade-shaped cheek molars slide past each other
to give a slicing motion that is very effective for shearing meat off

The teeth of a carnivore are discretely spaced so as not to trap
stringy debris. The incisors are short, pointed and prong-like and
are used for grasping and shredding. The canines are greatly
elongated and dagger-like for stabbing, tearing and killing prey. The
molars (carnassials) are flattened and triangular with jagged edges
such that they function like serrated-edged blades. Because of the
hinge-type joint, when a carnivore closes its jaw, the cheek teeth
come together in a back-to-front fashion giving a smooth cutting
motion like the blades on a pair of shears.

The saliva of carnivorous animals does not contain digestive enzymes.
When eating, a mammalian carnivore gorges itself rapidly and does not
chew its food. Since proteolytic (protein-digesting) enzymes cannot
be liberated in the mouth due to the danger of autodigestion
(damaging the oral cavity), carnivores do not need to mix their food
with saliva; they simply bite off huge chunks of meat and swallow
them whole.

According to evolutionary theory, the anatomical features consistent
with an herbivorous diet represent a more recently derived condition
than that of the carnivore. Herbivorous mammals have well-developed
facial musculature, fleshy lips, a relatively small opening into the
oral cavity and a thickened, muscular tongue. The lips aid in the
movement of food into the mouth and, along with the facial (cheek)
musculature and tongue, assist in the chewing of food. In herbivores,
the jaw joint has moved to position above the plane of the teeth.
Although this type of joint is less stable than the hinge-type joint
of the carnivore, it is much more mobile and allows the complex jaw
motions needed when chewing plant foods. Additionally, this type of
jaw joint allows the upper and lower cheek teeth to come together
along the length of the jaw more or less at once when the mouth is
closed in order to form grinding platforms. (This type of joint is so
important to a plant-eating animal, that it is believed to have
evolved at least 15 different times in various plant-eating mammalian
species.) The angle of the mandible has expanded to provide a broad
area of attachment for the well-developed masseter and pterygoid
muscles (these are the major muscles of chewing in plant-eating
animals). The temporalis muscle is small and of minor importance. The
masseter and pterygoid muscles hold the mandible in a sling-like
arrangement and swing the jaw from side-to-side. Accordingly, the
lower jaw of plant-eating mammals has a pronounced sideways motion
when eating. This lateral movement is necessary for the grinding
motion of chewing.

The dentition of herbivores is quite varied depending on the kind of
vegetation a particular species is adapted to eat. Although these
animals differ in the types and numbers of teeth they posses, the
various kinds of teeth when present, share common structural
features. The incisors are broad, flattened and spade-like. Canines
may be small as in horses, prominent as in hippos, pigs and some
primates (these are thought to be used for defense) or absent
altogether. The molars, in general, are squared and flattened on top
to provide a grinding surface. The molars cannot vertically slide
past one another in a shearing/slicing motion, but they do
horizontally slide across one another to crush and grind. The surface
features of the molars vary depending on the type of plant material
the animal eats. The teeth of herbivorous animals are closely grouped
so that the incisors form an efficient cropping/biting mechanism, and
the upper and lower molars form extended platforms for crushing and
grinding. The "walled-in" oral cavity has a lot of potential space
that is realized during eating.

These animals carefully and methodically chew their food, pushing the
food back and forth into the grinding teeth with the tongue and cheek
muscles. This thorough process is necessary to mechanically disrupt
plant cell walls in order to release the digestible intracellular
contents and ensure thorough mixing of this material with their
saliva. This is important because the saliva of plant-eating mammals
often contains carbohydrate-digesting enzymes which begin breaking
down food molecules while the food is still in the mouth.

Stomach and Small Intestine

Striking differences between carnivores and herbivores are seen in
these organs. Carnivores have a capacious simple (single-chambered)
stomach. The stomach volume of a carnivore represents 60-70% of the
total capacity of the digestive system. Because meat is relatively
easily digested, their small intestines (where absorption of food
molecules takes place) are short -- about three to five or six times
the body length. Since these animals average a kill only about once a
week, a large stomach volume is advantageous because it allows the
animals to quickly gorge themselves when eating, taking in as much
meat as possible at one time which can then be digested later while
resting. Additionally, the ability of the carnivore stomach to
secrete hydrochloric acid is exceptional. Carnivores are able to keep
their gastric pH down around 1-2 even with food present. This is
necessary to facilitate protein breakdown and to kill the abundant
dangerous bacteria often found in decaying flesh foods.

Because of the relative difficulty with which various kinds of plant
foods are broken down (due to large amounts of indigestible fibers),
herbivores have significantly longer and in some cases, far more
elaborate guts than carnivores. Herbivorous animals that consume
plants containing a high proportion of cellulose must "ferment"
(digest by bacterial enzyme action) their food to obtain the nutrient
value. They are classified as either "ruminants" (foregut fermenters)
or hindgut fermenters. The ruminants are the plant-eating animals
with the celebrated multiple-chambered stomachs. Herbivorous animals
that eat a diet of relatively soft vegetation do not need a
multiple-chambered stomach. They typically have a simple stomach, and
a long small intestine. These animals ferment the difficult-to-digest
fibrous portions of their diets in their hindguts (colons). Many of
these herbivores increase the sophistication and efficiency of their
GI tracts by including carbohydrate-digesting enzymes in their
saliva. A multiple-stomach fermentation process in an animal which
consumed a diet of soft, pulpy vegetation would be energetically
wasteful. Nutrients and calories would be consumed by the fermenting
bacteria and protozoa before reaching the small intestine for
absorption. The small intestine of plant-eating animals tends to be
very long (greater than 10 times body length) to allow adequate time
and space for absorption of the nutrients.


The large intestine (colon) of carnivores is simple and very short,
as its only purposes are to absorb salt and water. It is
approximately the same diameter as the small intestine and,
consequently, has a limited capacity to function as a reservoir. The
colon is short and non-pouched. The muscle is distributed throughout
the wall, giving the colon a smooth cylindrical appearance. Although
a bacterial population is present in the colon of carnivores, its
activities are essentially putrefactive.

In herbivorous animals, the large intestine tends to be a highly
specialized organ involved in water and electrolyte absorption,
vitamin production and absorption, and/or fermentation of fibrous
plant materials. The colons of herbivores are usually wider than
their small intestine and are relatively long. In some plant-eating
mammals, the colon has a pouched appearance due to the arrangement of
the muscle fibers in the intestinal wall. Additionally, in some
herbivores the cecum (the first section of the colon) is quite large
and serves as the primary or accessory fermentation site.

What About Omnivores?

One would expect an omnivore to show anatomical features which equip
it to eat both animal and plant foods. According to evolutionary
theory, carnivore gut structure is more primitive than herbivorous
adaptations. Thus, an omnivore might be expected to be a carnivore
which shows some gastrointestinal tract adaptations to an herbivorous

This is exactly the situation we find in the Bear, Raccoon and
certain members of the Canine families. (This discussion will be
limited to bears because they are, in general, representative of the
anatomical omnivores.) Bears are classified as carnivores but are
classic anatomical omnivores. Although they eat some animal foods,
bears are primarily herbivorous with 70-80% of their diet comprised
of plant foods. (The one exception is the Polar bear which lives in
the frozen, vegetation poor arctic and feeds primarily on seal
blubber.) Bears cannot digest fibrous vegetation well, and therefore,
are highly selective feeders. Their diet is dominated by primarily
succulent lent herbage, tubers and berries. Many scientists believe
the reason bears hibernate is because their chief food (succulent
vegetation) not available in the cold northern winters.
(Interestingly, Polar bears hibernate during the summer months when
seals are unavailable.)

In general, bears exhibit anatomical features consistent with a
carnivorous diet. The jaw joint of bears is in the same plane as the
molar teeth. The temporalis muscle is massive, and the angle of the
mandible is small corresponding to the limited role the pterygoid and
masseter muscles play in operating the jaw. The small intestine is
short ( less than five times body length) like that of the pure
carnivores, and the colon is simple, smooth and short. The most
prominent adaptation to an herbivorous diet in bears (and other
"anatomical" omnivores) is the modification of their dentition. Bears
retain the peg-like incisors, large canines and shearing premolars of
a carnivore; but the molars have become squared with rounded cusps
for crushing and grinding. Bears have not, however, adopted the
flattened, blunt nails seen in most herbivores and retain the
elongated, pointed claws of a carnivore.

An animal which captures, kills and eats prey must have the physical
equipment which makes predation practical and efficient. Since bears
include significant amounts of meat in their diet, they must retain
the anatomical features that permit them to capture and kill prey
animals. Hence, bears have a jaw structure, musculature and dentition
which enable them to develop and apply the forces necessary to kill
and dismember prey even though the majority of their diet is
comprised of plant foods. Although an herbivore-style jaw joint
(above the plane of the teeth) is a far more efficient joint for
crushing and grinding vegetation and would potentially allow bears to
exploit a wider range of plant foods in their diet, it is a much
weaker joint than the hinge-style carnivore joint. The
herbivore-style jaw joint is relatively easily dislocated and would
not hold up well under the stresses of subduing struggling prey
and/or crushing bones (nor would it allow the wide gape carnivores
need). In the wild, an animal with a dislocated jaw would either soon
starve to death or be eaten by something else and would, therefore,
be selected against. A given species cannot adopt the weaker but more
mobile and efficient herbivore-style joint until it has committed to
an essentially plant-food diet test it risk jaw dislocation, death
and ultimately, extinction.

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.


Facial Muscles

CARNIVO Reduced to allow wide mouth gape
HERBIVO Well-developed
OMNIVO Reduced
HUMAN: Well-developed

Jaw Type

CARNIVO Angle not expanded
HERBIVO Expanded angle
OMNIVO Angle not expanded
HUMAN: Expanded angle

Jaw Joint Location

CARNIVO On same plane as molar teeth
HERBIVO Above the plane of the molars
OMNIVO On same plane as molar teeth
HUMAN: Above the plane of the molars

Jaw Motion

CARNIVO Shearing; minimal side-to-side motion
HERBIVO No shear; good side-to-side, front-to-back
OMNIVO Shearing; minimal side-to-side
HUMAN: No shear; good side-to-side, front-to-back

Major Jaw Muscles

CARNIVO Temporalis
HERBIVO Masseter and pterygoids
OMNIVO Temporalis
HUMAN: Masseter and pterygoids

Mouth Opening vs. Head Size


Teeth: Incisors

CARNIVO Short and pointed
HERBIVO Broad, flattened and spade shaped
OMNIVO Short and pointed
HUMAN: Broad, flattened and spade shaped

Teeth: Canines

CARNIVO Long, sharp and curved
HERBIVO Dull and short or long (for defense), or none
OMNIVO Long, sharp and curved
HUMAN: Short and blunted

Teeth: Molars

CARNIVO Sharp, jagged and blade shaped
HERBIVO Flattened with cusps vs complex surface
OMNIVO Sharp blades and/or flattened
HUMAN: Flattened with nodular cusps


CARNIVO None; swallows food whole
HERBIVO Extensive chewing necessary
OMNIVO Swallows food whole and/or simple crushing
HUMAN: Extensive chewing necessary


CARNIVO No digestive enzymes
HERBIVO Carbohydrate digesting enzymes
OMNIVO No digestive enzymes
HUMAN: Carbohydrate digesting enzymes

Stomach Type

HERBIVO Simple or multiple chambers
HUMAN: Simple

Stomach Acidity

CARNIVO Less than or equal to pH 1 with food in stomach
HERBIVO pH 4 to 5 with food in stomach
OMNIVO Less than or equal to pH 1 with food in stomach
HUMAN: pH 4 to 5 with food in stomach

Stomach Capacity

CARNIVO 60% to 70% of total volume of digestive tract
HERBIVO Less than 30% of total volume of digestive tract
OMNIVO 60% to 70% of total volume of digestive tract
HUMAN: 21% to 27% of total volume of digestive tract

Length of Small Intestine

CARNIVO 3 to 6 times body length
HERBIVO 10 to more than 12 times body length
OMNIVO 4 to 6 times body length
HUMAN: 10 to 11 times body length


CARNIVO Simple, short and smooth
HERBIVO Long, complex; may be sacculated
OMNIVO Simple, short and smooth
HUMAN: Long, sacculated


CARNIVO Can detoxify vitamin A
HERBIVO Cannot detoxify vitamin A
OMNIVO Can detoxify vitamin A
HUMAN: Cannot detoxify vitamin A


CARNIVO Extremely concentrated urine
HERBIVO Moderately concentrated urine
OMNIVO Extremely concentrated urine
HUMAN: Moderately concentrated urine


CARNIVO Sharp claws
HERBIVO Flattened nails or blunt hooves
OMNIVO Sharp claws
HUMAN: Flattened nails