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psychoceramics: The Comparative Anatomy of Eating
- To: p--@z--.net
- Subject: psychoceramics: The Comparative Anatomy of Eating
- From: Ernie Karhu <ekarhu @ suneast.East.Sun.COM>
- Date: Tue, 26 Mar 1996 08:38:09 -0500
- Sender: owner-psychoceramics
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 bone.
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.
Colon
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 diet.
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.
Summary
Facial Muscles
CARNIVORE: Reduced to allow wide mouth gape
HERBIVORE: Well-developed
OMNIVORE: Reduced
HUMAN: Well-developed
Jaw Type
CARNIVORE: Angle not expanded
HERBIVORE: Expanded angle
OMNIVORE: Angle not expanded
HUMAN: Expanded angle
Jaw Joint Location
CARNIVORE: On same plane as molar teeth
HERBIVORE: Above the plane of the molars
OMNIVORE: On same plane as molar teeth
HUMAN: Above the plane of the molars
Jaw Motion
CARNIVORE: Shearing; minimal side-to-side motion
HERBIVORE: No shear; good side-to-side, front-to-back
OMNIVORE: Shearing; minimal side-to-side
HUMAN: No shear; good side-to-side, front-to-back
Major Jaw Muscles
CARNIVORE: Temporalis
HERBIVORE: Masseter and pterygoids
OMNIVORE: Temporalis
HUMAN: Masseter and pterygoids
Mouth Opening vs. Head Size
CARNIVORE: Large HERBIVORE: Small OMNIVORE: Large HUMAN:
Small
Teeth: Incisors
CARNIVORE: Short and pointed
HERBIVORE: Broad, flattened and spade shaped
OMNIVORE: Short and pointed
HUMAN: Broad, flattened and spade shaped
Teeth: Canines
CARNIVORE: Long, sharp and curved
HERBIVORE: Dull and short or long (for defense), or none
OMNIVORE: Long, sharp and curved
HUMAN: Short and blunted
Teeth: Molars
CARNIVORE: Sharp, jagged and blade shaped
HERBIVORE: Flattened with cusps vs complex surface
OMNIVORE: Sharp blades and/or flattened
HUMAN: Flattened with nodular cusps
Chewing
CARNIVORE: None; swallows food whole
HERBIVORE: Extensive chewing necessary
OMNIVORE: Swallows food whole and/or simple crushing
HUMAN: Extensive chewing necessary
Saliva
CARNIVORE: No digestive enzymes
HERBIVORE: Carbohydrate digesting enzymes
OMNIVORE: No digestive enzymes
HUMAN: Carbohydrate digesting enzymes
Stomach Type
CARNIVORE: Simple
HERBIVORE: Simple or multiple chambers
OMNIVORE: Simple
HUMAN: Simple
Stomach Acidity
CARNIVORE: Less than or equal to pH 1 with food in stomach
HERBIVORE: pH 4 to 5 with food in stomach
OMNIVORE: Less than or equal to pH 1 with food in stomach
HUMAN: pH 4 to 5 with food in stomach
Stomach Capacity
CARNIVORE: 60% to 70% of total volume of digestive tract
HERBIVORE: Less than 30% of total volume of digestive tract
OMNIVORE: 60% to 70% of total volume of digestive tract
HUMAN: 21% to 27% of total volume of digestive tract
Length of Small Intestine
CARNIVORE: 3 to 6 times body length
HERBIVORE: 10 to more than 12 times body length
OMNIVORE: 4 to 6 times body length
HUMAN: 10 to 11 times body length
Colon
CARNIVORE: Simple, short and smooth
HERBIVORE: Long, complex; may be sacculated
OMNIVORE: Simple, short and smooth
HUMAN: Long, sacculated
Liver
CARNIVORE: Can detoxify vitamin A
HERBIVORE: Cannot detoxify vitamin A
OMNIVORE: Can detoxify vitamin A
HUMAN: Cannot detoxify vitamin A
Kidney
CARNIVORE: Extremely concentrated urine
HERBIVORE: Moderately concentrated urine
OMNIVORE: Extremely concentrated urine
HUMAN: Moderately concentrated urine
Nails
CARNIVORE: Sharp claws
HERBIVORE: Flattened nails or blunt hooves
OMNIVORE: Sharp claws
HUMAN: Flattened nails
Ernest and politically incorrect
"Few of our ancestors were perfect ladies
and gentlemen; the majority, in fact,
weren't even mammals." -Robert Anton Wilson