Purpose:
To learn to identify the internal
organs of the frog.
To relate the structures of the internal anatomy
to their functions. To compare the internal anatomy of the frog
with the human.
Procedure: Lay the frog on its dorsal side. Insert your scissors through the abdominal
muscles just above the hind legs.
Cut a straight line up to the tip of the jaw.
You will have to cut through the sternum. Make four cuts laterally from the center to the sides of the body.
This will expose the internal organ systems.
Data:
Internal Anatomy Overview
Before continuing with
a study of the digestive tract, identify the organs
in the pleuroperitoneal cavity (Fig. 3-5).
The large, dense, trilobed organ occupying much
of the anteroventral part of the cavity is the liver. Lift it up and find the gall bladder, a small dark sac attached
to the dorsal surface of the liver.
The stomach lies dorsal tot he liver on the
left side of the body. One lung
lies beside the anterior part of the stomach; the other,
in a comparable position on the other side of the body.
Life up the posterior part of the stomach. The small, dense, round body on the left side of the mesentery supporting
the intestine is the spleen,
an organ in which blood cells are manufactured and stored. A small
intestine leads from the posterior end of the stomach. The lobate whitish tissue lying in the loop
between the first part of the small
intestine and the stomach is the pancreas. Follow the coils of the small intestine posteriorly and they will
lead to the wider, large
intestine. The finger-like lobes of fat which must be
pushed aside are the fat
bodies. One mass is attached tot he anterior end of
each testis or ovary.
The testes of a male are a pair of small,
oval bodies, one on each side of the mesentery
that supports the intestine.
Ovaries occupy the same position, but vary greatly in size according
to the reproductive state of the female. Shortly before ovulation, they are filled with ripe eggs and occupy
all available space in the pleuroperitoneal cavity. It may be necessary to remove one in order
to see the other organs clearly.
An elongated dark kidney
lies against the back, dorsal to each gonad (testis
or ovary). In females, a long coiled white tube, the oviduct, will be found lateral to each
kidney. Males may have a smaller vas
deferens.
The urinary bladder is a bi-lobed sac just
anterior to the hind legs and ventral to the large intestine.
Data:
The Digestive
Tract
Pass a probe or a pair
of blunt forceps through the pharynx and down the esophagus. By feeling with
your fingers, you can tell when the probe enters the
stomach. Since a frog lacks a neck, the esophagus is
quite short. There
is no line of demarcation between it and the stomach
on the outer surface of these organs, but internally
there is a change in the type of cell lining their cavities.
The stomach is
a large, saccular organ in which food is stored and
digestion initiated.
It curves toward the right side of the body and
usually has a J-shape. Cut it open. If it is not
completely filled with the remains of food organisms,
its lining forms conspicuous longitudinal folds.
Its posterior termination is a thick, muscular
pyloric sphincter,
whose contraction keeps food in the stomach until
it is partly digested by the gastric juice and broken
up mechanically by a churning action. Glands in the stomach wall secrete pepsin, which initiates the splitting of protein, and hydrochloric acid, which is essential
for the activity of pepsin.
Digestion is completed
and food is absorbed in the small
intestine. Its
first part, the duodenum,
curves up toward the liver; the rest of it is comparable
to the mammalian jejunoileum. Slit open a part of the small intestine and observe the irregular
folds which increase the internal surface area available
for the absorption of digested food.
There are many mucus-producing
glands in the lining of the small intestine, but most
of the secretions that act here come from the liver
and the pancreas. The pancreas
produces enzymes that act on all categories of food
(carbohydrases, lipase, proteinases, and nucleases.)
The liver produces no enzymes, but the bile
salts in the bile, which originates in the liver, emulsify fats, thereby
facilitating the action of lipase, and also aid in fat
absorption.
Bile leaves the liver
through inconspicuous hepatic
ducts which coalesce with a cystic
duct. As
the common bile duct passes posteriorly through the
pancreas, it receives several minute pancreatic
ducts. It finally emerges from the pancreas
to enter the duodenum.
The pancreas releases its secretions only when
food is in the duodenum, but bile, which is secreted
continuously, backs up into the gall
bladder where it is stored until food enters
the intestine.
The large
intestine, or colon,
is a short segment from which some water and ions are
absorbed, and where fecal materials is temporarily stored.
It has a large anterior diameter, but it tapers
posteriorly as it passes through the pelvic canal to
enter the cloaca. In order to see this region, carefully cut
through the pelvic girdle and spread the hind legs apart. Cut only on the midventral line, and do not
injure the urinary bladder.
The cloaca is the
terminal part of the digestive tract which also receives
the urinary and genital ducts.
Notice that the urinary bladder enters it ventrally.
You may see other urogenital ducts that enter
the cloaca dorsally. The cloaca opens on the body surface at the
cloacal aperture.
Use the Dissection
Works computer program to identify each part in
the diagram below.
Click
here for copy to label
Data
Table – Digestive System
|
Organ
|
Function
|
|
Esophagus
|
|
| Stomach |
|
|
Pyloric Valve
|
|
|
Duodenum - Small Intestine
|
|
|
Jejunoileum - Small Intestine
|
|
|
Pancreas
|
|
|
Colon - Large Intestine
|
|
|
Cloaca
|
|
|
Liver
|
|
| Gall Bladder |
|
| Messentary |
|
|
Fat Bodies
|
|
Frog Digestive System: Click
here for copy to label.
