LAB 9: VISCERA/DIGESTIVE SYSTEM
EEB 3273/Schwenk
Preparation: Homberger & Walker, Ch. 10, small print.
Background:
The evolution of an active-feeding lifestyle (as opposed to a filter-feeding one) required several changes in what
was ancestrally an uncomplicated digestive system. Active feeders generally take relatively large food items at
infrequent intervals. This food is consumed faster than it can be digested and so must be stored. In most
vertebrates, the stomach is the major storage organ. In birds, the crop also serves to store food. A large food
bolus must also be highly distensible to allow passage of the bolus and to increases storage volume as needed.
The main role of the digestive system, of course, is to reduce food items to molecular nutrients that can be
absorbed through the gut wall into the bloodstream. Mechanical reduction of food occurs in the mouth in
species that chew their food. Birds do not chew, but often soften food in the crop (an expanded part of the
esophagus) and then mechanically process it in the first part of the stomach (the gizzard). Most chemical
digestion occurs within the gut tube, but in some tetrapods, particularly mammals, chemical digestion begins in
the mouth with the introduction of salivary enzymes (e.g., salivary amylase, which begins the conversion of
complex carbohydrates—starches—into simpler sugars). In addition to its storage function, the stomach
secretes hydrochloric acid to break down large molecules and enzymes to begin protein digestion. However,
most chemical digestion occurs within the small intestine. This is also where nutrient absorption takes place
Taken all together, it is often possible to get a good idea of an animal's diet from the shape and complexity of
the gut. In general, carnivores tend to have relatively short guts, although stomach capacity is large (they
typically feed infrequently, but consume large quantities when food is available after a kill). In contrast,
herbivores typically have long, convoluted guts with vastly increased internal surface area that slows down
passage time of plant food to provide additional time for digestion. In herbivores, the gut also usually contains
a specialized area for cellulose digestion in which symbiotic bacteria and other microbes live in large numbers.
No vertebrate produces the enzymes necessary to break down cellulose directly, so the microbes do it for them,
releasing nutrients into the gut for absorption (as well as their own bodies as they perish). Microbial digestion
within the gut occurs in the absence of oxygen using an anaerobic metabolic process called fermentation. In a
group of mammals known as ruminants, fermentation occurs in a multi-chambered, complex stomach
(artiodactyls, e.g., cows, goats, sheep, antelope—also, one bird, the hoatzin), hence these animals are called
foregut fermenters (the term ‘ruminant’ comes from their habit of ‘ruminating’ or ‘chewing the cud’). Other
herbivores use a blind sac or diverticulum off the large intestine called the caecum for the same function.
These are called hindgut fermenters because the fermentation process occurs in the posterior part of the gut
(rodents, rabbits, horses, tapirs, rhinos). The human vermiform appendix is a vestigial caecum (suggesting that
our ancestors were much more herbivorous than we are) that has taken on a new and important immune
function).
Two basic sections of the gut tube can be recognized developmentally—the foregut and hindgut. The foregut
differentiates into the pharynx, esophagus, and stomach, while the hindgut becomes the small and large
intestines, and cloaca (Latin for ‘sewer’ because it receives the openings of the gut tube, urinary and
reproductive ducts). Additional folds and outpocketings (diverticula) of the embryonic gut tube become
respiratory structures (lungs and/or swim bladder) and digestive organs associated with the gut tube (liver, gall
bladder and pancreas). The bulk of the tissue of the digestive system is muscle and connective tissue derived
from embryonic splanchnic mesoderm (remember, it is only the lining of the gut that is endodermal in origin).
The basic structure and pattern of the digestive system is phylogenetically quite conservative, with differences
among species representing ‘variations on a theme.’
Many of the terms for which you are responsible will be familiar. In addition, very little actual dissection is
required. Apart from opening up the body cavity with ventral incisions, your will primarily be occupied with
manipulating the organs and identifying them.
EEB 3273—Lab 9—Page 1
Today's Lab:
Cut open your shark and your cat to examine the digestive system. We will not focus on the mouth and
pharynx, but rather on the posterior parts—those organs termed the viscera (Latin for "guts"). When doing
your dissections, be sure to preserve the material of the urogenital system (kidneys, bladder, reproductive
organs and various connecting tubes) for a future lab. Also be sure to preserve the blood vessels, most of which
are injected with colored latex. For today, focus specifically on comparison of the shark and cat digestive
tracts. Know the function of each organ.
I. SHARK DIGESTIVE SYSTEM
Material: preserved sharks
Identify:
esophagus
valvular intestine (‘spiral valve’)
pancreas
pylorus
spleen
liver (stores energy as fat)
dorsal mesentery
—pyloric region
bile duct
puboischiadic bar
—cardiac region
gall bladder
gill raker
—esophageal papillae
stomach
—internal rugae (folds)
II. CAT DIGESTIVE SYSTEM
Material: preserved cats
Identify:
PLEURAL CAVITY
parietal pleura (covers body wall)
visceral pleura (covers lungs)
esophagus
PERITONEAL CAVITY
diaphragm
liver
parietal peritoneum (covers body wall)
gall bladder
—transverse
visceral peritoneum (covers visceral organs)
bile duct
—colon
greater omentum (dorsal mesentery)
spleen
—rectum
esophagus
pancreas
—anus
stomach
small intestine
—body
—duodenum
—fundus
—jejunum
—cardiac region
—ileum
—pyloric region/pylorus
—intestinal folds (internal)
—internal rugae
—intestinal villi (internal)
EEB 3273—Lab 9—Page 2
large intestine
caecum (cecum)
appendix
Transverse section of human thorax through pleural cavity. The lungs lie within the cavity, which represents a space between two
layers of pleura—a thin layer of tissue derived from the somatic layer of the lateral plate mesoderm (parietal pleura) and the
splanchnic layer of the lateral plate mesoderm (visceral pleura). The space between the two is exaggerated in this picture—in life, the
lungs virtually fill the space and the two layers of pleura are pushed against one another. Fortunately, the pleural membranes (and the
peritoneal membranes in the main (abdominal) body cavity (coelom) secrete a watery, ‘serous’ fluid that lubricates all surfaces,
preventing adhesions.
"2313 The Lung Pleurea" by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013..
Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons. For color version, click on link:
http://commons.wikimedia.org/wiki/File:2313_The_Lung_Pleurea.jpg - mediaviewer/File:2313_The_Lung_Pleurea.jpg
EEB 3273—Lab 9—Page 3