10
DIGESTIVE
SYSTEM
The digestive system is concerned with (1)
ingestion of food, (2) physical and chemical digestion of
food, (3) absorption of nutrients, and (4) elimination of
undigested remains.
The system includes the digestive tract or
alimentary canal and the digestive glands associated with
it. The toad has a complete digestive tract, with two
openings- mouth and cloaca. The main parts of the tract
are: mouth, oral or buccal cavity, pharynx, esophagus,
stomach, small intestine, large intestine, rectum, and
cloaca. The digestive glands include the liver,
gallbladder, and the pancreas. These glands are not
found within the tract but they secrete digestive juices
that enter the alimentary canal through ducts.
Objectives At the end of this activity, the student should
be able to:
1. Describe the histology of liver and pancreas
2. Identify the organs of the digestive tract
3. Identify the other related organs and structures
necessary in the process of digestion
Materials
Compound light microscope
Prepared slides of:
Liver (x.s.)
Pancreas (x.s.)
Preserved toad
Dissecting pan
Dissecting set (must include
scissors, and scalpel)
forceps,
pins,
Procedure
A. Histology of the Liver and Pancreas
1. Liver
a. Obtain a prepared slide of the liver. Under LPO, note the presence of
numerous hepatic lobules separated from each other by connective tissue
partition or septa.
b. Examine one hepatic lobule. At the center of the lobule is the large central
vein. Radiating from the central vein are polygonal shaped hepatic cells or
hepatocytes arranged into cords or plates. In between hepatic plates are
spaces called hepatic sinusoids that lead toward the periphery of the hepatic
lobule. Around each lobule, one can observe portal canals or areas within the
septa. The portal area contains branches of the portal veins, hepatic arteries,
and bile ducts. Recall the structure of vein and artery. The bile duct is
different from the vein and artery in that it is lined by simple columnar
epithelium. Focus the epithelium under HPO.
c. Label the illustration in the Activity Sheet.
2. Pancreas
a. Obtain a prepared slide of the pancreas. The pancreas has both the
endocrine and exocrine functions. The exocrine pancreas, which perform the
digestive function, consist of closely packed acini arranged into small lobules.
The pancreactic acini consists of pyramidal-shaped cells surrounding a
lumen. Find these scattered in the pancreas.
b. Within the masses of acini cells are iolated pancreatic islets, or the Islets of
Langerhans. These are pale-staining structures composed of cells arranged
in cords. The islets have rich capillary network. The pancreatic islets perform
the endocrine function of the pancreas. Locate these structures.
c. Label the illustration in the Activity Sheet.
B. Gross Anatomy of the Digestive Tract
1. Get a preserved toad and a dissecting pan.
2. With a probe, pry open the mouth and cut the angle of the jaw. The buccal cavity
or the mouth will be studied first.
a. The skin folds in the margin of the upper jaw and lower jaw are called the
upper and lower lip folds, respectively.
b. At the tip of the upper jaw is a prominent depression called the median
subrostral fossa. This fits into a median elevation at the tip of the lower jaw,
the tuberculum prelinguale.
c. The sulcus marginalis is a pair of deep grooves found at the inner margin of
the upper jaw. Try closing the mouth and note that the sulcus marginalis
receives the elevation at the lower jaw.
d. Feel the margin at the upper jaw. The row of small teeth located along the
margin is the maxillary teeth. Another set of teeth is found on the roof of the
mouth called the vomerine teeth.
e. Insert the probe into the external nares and trace the canal into the roof of
the mouth cavity. These anterolateral openings are the internal nares.
f.
Note the bulging regions at the posterior half of the roof of the mouth. These
are the eyeball prominences. On the posterolateral side of the eyeballs,
medial to the angles of the jaw is a pair of openings, the openings of the
Eustachian tube. Probe these openings and note where they lead to.
g. The tongue is attached to the floor of the mouth cavity near the anterior end
by a short pedicle. Locate the two slit-like openings to the vocal sacs in the
posterolateral corner of the floor of the tongue. These are often lacking in the
females.
h. Behind the mouth is the pharynx that leads into the opening of a short
distensible tube, the esophagus. Ventral to the esophagus is the laryngeal
prominence with a vertical slit-like opening called the glottis. This will not be
traced here since it is part of the respiratory system.
i.
Label the illustration in the Activity Sheet.
3. With the forceps, gently pick up the skin at the mid-posterior region of the
abdomen. Holding the skin up, make a small opening using the scissors. Insert
the scissors into the opening and make a straight incision forward up to the tip of
the lower jaw. Make a transverse incision going to the armpits and base of the
trunk. Expose the muscles of the abdomen and chest by peeling the skin with a
blunt probe.
4. Using a pair of scissors, cut through the abdominal and chest muscles. Make an
incision at the posterior region of the abdomen. This time, the incision should be
made around 5mm to the right of the midventral line or the linea alba so as not to
cut the very large vein, the anterior or ventral abdominal vein. Continue the
incision through the abdominal muscles. Upon reaching the pectoral girdle area,
cut through the sternum. Make a transverse incision through the pectoral
muscles. Slowly pull the muscle flaps and expose the body cavities.
5. The cavities revealed are the: (1) anterior, small pericardial cavity containing the
heart, and (2) a large, posterior pleuroperitoneal cavity housing the lungs and
other internal organs or viscera. The two cavities are separated by a transverse
septum. Examine the inner side of the muscular body wall. A glistening
membrane, the parietal peritoneum, lines the body wall. The extension of the
parietal peritoneum forms the covering layer of the intestine and other viscera.
This extends to the visceral organs as the visceral peritoneum or the serosa.
6. The digestive tract is suspended by mesenteries which are extensions also of the
parietal peritoneum. The part that attaches the digestive tract for most of its
length to the median dorsal line is the dorsal mesentery. This is noted by gently
moving the organs away from the median line. The ventral mesentery, which
attaches the organs to the midventral line is usually lost except in the region of
the liver and the urinary bladder.
7. Examine the organs at the pleuroperitoneal cavity.
a. The liver is a large, trilobed, brownish organ occupying the anterior half of the
pleuroperitoneal cavity. Separate the left and middle lobes of the liver and
locate a greenish, spherical sac, the gallbladder. The gallbladder stores the
bile secreted by the liver. Hepatic ducts from the liver and cystic ducts from
the gallbladder may be traced to the point where it joins the common bile
duct. The common bile duct then enters the duodenum of the small intestine.
b. Raise the left side of the liver, the stomach can be seen dorsal to the liver.
The portion of the dorsal mesentery supporting the stomach is the
mesogaster. Examine the stomach. It is a sac-like organ with three regions.
The area nearest the esophagus is the cardiac region, which is guarded
internally by cardiac sphincter, a ring of smooth muscle. The next region that
bulges anteriorly is the fundus. Follow the stomach posteriorly. It terminates
into a narrow end, the pylorus, which is also guarded internally by the pyloric
sphincter. Gently lift the stomach and locate in the mesogaster a round,
dense body, the spleen. The spleen has no digestive function. Using a
scalpel, make a longitudinal slit into the walls of the fundus and pyloric
sphincter of the stomach. Empty its contents. Observe the longitudinal folds
in the inner wall of the stomach, called the rugae. The pyloric sphincter,
similar to the cardiac sphincter, consists of circular muscle that regulates the
flow of food into the small intestine.
c. From the pylorus, the small intestine begins and makes an abrupt angle to
the right. This short, straight, anterior-most segment of the small intestine is
the duodenum. In the mesentery between the pylorus and duodenum, look
for a yellowish gland, the pancreas, and this extends up to the dorsal surface
of the liver. The pancreas provide digestive enzymes and bicarbonates that
are secreted into the small intestine. Trace the ducts of the pancreas where
they join the common bile duct that enters the duodenum. The small
intestines proceed posteriorly as a coiled structure, the ileum. The ileum
terminates into a short straight tube, the large intestine. Part of the dorsal
mesentery supporting the small intestine and the large intestine is the
mesentery proper and the mesorectum, respectively.
d. The anterior region of the large intestine is the rectum where it leads into a
narrow region, the cloaca. The cloaca is a chamber that serves as a common
passageway to the outside for both digestive and urogenital systems. It
terminates at the cloacal aperture.
e. Label the photograph in the Activity Sheet.
References
Aceret, T., Lannu, A. 1988. A Laboratory Manual in General Zoology. 2nd ed.
Academe Publishing House. San Juan Metro Manila. 69-72.
Bailey, P.C., Hollman, D.C. Quarles, T.S. and Waits, E.D. 1970. Laboratory
Guide for an Introduction to Modern Biology. International Textbook Co.,
111-118.
Feldman, S. 1965. Experiments in Biological Design. Holt, Rinehart and Winston,
Inc., USA. 95-96.
Hymann, L. H. 1965. Comparative Vertebrate Anatomy. University of Chicago
Press, Chicago, 270-274.
11
THE
RESPIRATORY
SYSTEM
The exchange of gases is termed respiration.
Oxygen is obtained from the environment because it is
needed for metabolism. The resulting carbon dioxide
produced as a by-product of metabolism is in turn
released into the environment. Respiration process
consists of two stages: external respiration, which is
the exchange of gases between the environment and
the respiratory organs, and internal respiration, which
is the exchange of gases between body fluids and
tissue cells.
Frogs and toads utilize various respiratory organs
such as the lungs, skin, and mucous lining of the
mouth. These organs are characterized by semipermeable membrane through which gases diffuse
readily.
Objectives At the end of the activity, the student
should be able to:
1.
2.
Describe the histology of the lungs
Identify the organs involved in respiration
Materials
Compound light microscope
Prepared slide of frog or toad’s lungs
Preserved toad
Dissecting pan
Dissecting set (must include forceps,
scissors and scalpel)
pins,
Procedure
A. Microscopic Examination of the Lungs
1.
Obtain a prepared slide of toad’s lungs.
2.
Under LPO, note the numerous chambers of the
lungs that sometimes interconnect with one
another. These are the alveoli. Observe also the
very thin partition between alveoli. This is the
interalveolar septum.
3.
Observe that in the walls of the lungs are
numerous blood vessels, the capillaries.
4.
Label the photomicrograph in the Activity Sheet.
B. Gross Anatomy of the Respiratory System
1.
Use the same toad used in Activity 5.
2.
Place the toad on the dissecting pan.
3.
Study again the oral cavity:
a. Observe a pair of openings at the anterolateral part of the snout. These are
the nostrils, also called external nares. As noted in the previous activity, these
openings lead into the internal nares.
b. At the roof of the oral cavity, near the angle of the jaw, observe the openings
of the Eustachian tube. The tube connects the mouth cavity with the cavities
of the middle ear. Probe these openings and find out if this will lead to the
tympanum.
4.
Find a slit-like opening at the posterior region of the pharynx. These are the
glottis and larynx. Notice that the glottis opens downward into the cavity of the
larynx or voice box. The larynx is supported or stiffened by cartilages. Locate
the anterior pair of arytenoid cartilages and a more posterior pair of cricoid
cartilages.
5.
Using the scissors, make an incision through the slit of the glottis. Find thin
elastic bands in each halves of the larynx. These are the vocal cords.
6.
The larynx is connected to the lungs through an extremely short tube, the
bronchus. This is quite difficult to locate.
7.
Examine the toad’s lungs. It is collapsed in the preserved toad and egg shaped
in a living toad. Feel the texture of the lungs. Notice that the lungs are
membranous sacs lined by small air pockets called alveoli (alveolus, singular).
Gas exchange takes place in the alveoli.
References
Aceret, T., Lannu, A. L. 1988. A Laboratory Manual in General Zoology. 2nd ed.
Academe Publishing House, San Juan Metro Manila. 69-72.
Bailey, P.C., Hollman, D.C., Quarles, T.S. and Waits, E.D. 1970. Laboratory Guide
for an Introduction to Modern Biology. International Textbolok Co., 111-118.
Feldman, S. 1965. Experiments in Biological Design. Holt, Rinehart and Winston, Inc.
USA. 95-96.
Hymann, L. H. 1965. Comparative Vertebrate Anatomy. University of Chicago Press.
Chicago 270-274.
12
CIRCULATORY
SYSTEM
The circulatory system primarily functions for:
(a) transport of respiratory gases, food materials,
waste products, hormones and minerals within the
body; (b) defense against foreign organisms; and (c)
maintenance of body temperature in warm-blooded or
homeothermic animals.
Vertebrates such as toads have a closed type of
circulatory system composed of heart, arteries,
arterioles, veins, venules, and capillaries. The heart
pumps deoxygenated blood to the lungs and
distributes oxygenated blood to all the organs. Vessels
called arteries and arterioles convey oxygenated blood
from the heart to the smallest vessels called capillaries
found in various tissues and organs of the body, with
the exception of the pulmonary artery that delivers
deoxygenated blood to the lungs for oxygenation.
Deoxygenated blood is then collected from the
capillaries to venules and to the veins before it returns
to the heart. The pulmonary vein, on the other hand,
collects oxygenated blood from the lungs and delivers
it back to the heart for distribution throughout the body.
Objectives At the end of the activity, the student
should be able to:
1. Identify the different parts of the amphibian
heart
2. Trace the arterial and venous pathways of the
toad
Materials
Dissecting set
Dissecting pan
Live toad
Procedure
1.
Obtain a toad that has been etherized.
2.
Place it ventral side up and pin its limbs
to the dissecting pan.
3.
Lift the skin of the abdomen with forceps. Using a pair of scissors, make a
slit from the posterior midventral portion of the abdomen to the tip of the
lower jaw. Separate the skin from the muscles to expose the linea alba.
Now, make a longitudinal slit through the muscles along both sides of the
linea alba (about a distance of 5mm from each side) up to the area of the
sternum.
4.
Using forceps, lift the cut muscle and observe a dark blood vessel
underneath the linea alba. This is the anterior abdominal vein that dips into
the liver. Cut the muscle beyond the dipping and carefully pull down the
muscle to separate the blood vessel.
5.
Cut the sternum longitudinally and carefully pull apart the two halves of the
pectoral girdle to expose the heart and other blood vessels. Note that a
shiny membrane, the pericardial membrane, envelops the heart. Severe
this membrane and study the parts of the heart.
A.
Heart
Locate this cone-shaped muscular organ lying in the thorax. Identify the
following associated structure and parts:
1.
2.
3.
4.
5.
Pericardium- a thin transparent membrane covering the heart
Pericardial cavity- coelom/cavity where the heart is situated
Atria (sing. atrium)- two globular structures atop the ventricle
Ventricle- most posterior conical structure of the heart
Conus arteriosus- cone-shaped tube arising from the ventricle on its
ventral side
6. Sinus venosus- this is best seen when the tip of the ventricle is lifted; a
thin-walled triangular sac on the dorsal side of the heart and is
continuous with the right atrium.
This will be done using the heart of preserved toad previously used. Make
a frontal incision to expose the internal structure of the heart. Identify the
following:
1. Interatrial septum- wall separating the right from the left atrium
2. Atrioventricular valves- thin flaps between the atria and ventricle; may
not be seen in the specimen
3. Semilunar valves- flaps that open into conus arteriosus; may not be
seen in the specimen
B.
The Venous System
The venous system is differentiated into three areas: systemic veins,
portal veins, and pulmonary veins.
1. Systemic Veins
Using a blunt forceps, gently lift the apex of the heart and
look for a thin triangular sac. This is the sinus venosus which
receives deoxygenated blood from different parts of the body.
a. Precava
The precavae (anterior vena cava) are big veins originating
from the anterior right and left border of the sinus venosus.
Each of the precava divides into three branches: external
jugular vein, innominate vein, and subclavian vein.
The external jugular vein is the medial most branch of the
precava that eventually divides into a medial lingual vein, which
collects blood from the tongue, hyoid, and floor of the mouth
cavity, and an outer branch, maxillary or mandibular vein, which
collects blood from the jaws and thyroid glands.
The innominate vein is lateral to the first external jugular vein
that divides into internal jugular vein that collects blood from the
interior of the skull and subscapular vein that collects blood from
the shoulder and forelimbs.
The subclavian vein is the outermost branch that divides
intro brachial vein that collects blood from the forelimbs and
musculocutaneous vein that divides into a smaller cutaneous
vein that collects blood from the skin of the dorsal and
ventrolateral body wall and a bigger muscular vein that collects
blood from the muscles of the same region.
b. Postcava
The postcava (posterior vena cava) is a large unpaired vein
located on the posterior end of the sinus venosus. Follow this
posteriorly and notice that at the region of the liver, it branches
into the left and right hepatic veins, and at the region of the
kidneys, it branches into renal veins, spermatic veins from
testes of males or ovarian veins from ovaries of females. Both
spermatic and ovarian veins may be connected to the renal
veins or directly connected to the postcava. Using your forceps,
gently lift the postcava and look for lumbar veins which collect
blood from the dorsal region of the body wall.
2. Portal Veins
Unlike in mammals wherein there is only the hepatic portal
system, the frog has two portal systems- hepatic portal and renal
portal.
Hepatic Portal System
The hepatic portal vein is best seen by gently deflecting the
left lobe of the liver and lightly scratching the substance of the
pancreas to expose it. Connected to the hepatic portal vein are:
a.
b.
c.
d.
Gastric veins- coming from the stomach
Pancreatic vein- from the pancreas
Splenic vein from the spleen
Intestinal veins- from the small and large intestines.
Renal Portal System
The two renal portal veins are located on the lateral margin
of each kidney. Follow one of the renal portal veins posteriorly and
look for the dorsolumbar vein collecting blood from the dorsal body
wall and the oviducal vein from the oviduct. Downward, the renal
portal vein divides into a lateral femoral vein that collects blood
from the anterior and dorsal side of the thigh and a medial sciatic
vein that collects blood from the posterior side of the thigh.
Connected to the left and right femoral veins are the pelvic veins
which unites together and becomes connected to the anterior
abdominal vein which located beneath the linea alba. Before the
point of union of the pelvic veins, look for a small vesicular vein that
collects blood from the urinary bladder. Follow the anterior
abdominal vein upward and notice that it directly enters the
substance of the liver and becomes connected to the hepatic portal
vein.
3. Pulmonary Veins
This is best seen by carefully removing the membrane
connecting the pericardial sac with the upper portion of the lungs.
Gently deflect the heart on one side and notice that the pulmonary
veins enter the left atrium or auricle.
C.
The Arterial System
Arteries, except the pulmonary arteries, supply oxygenated blood to
different parts of the body.
On the ventral side of the heart is a cone-shaped or bulb-shaped structure
called conus arteriosus located between the auricles/atria. Anteriorly, it divides
into left and right branches called truncus arteriosus. Follow one truncus
arteriosus and observe that it divides into three branches: common carotid artery,
systemic arch, and pulmocutaneous artery.
1. Common Carotid Artery
The common carotid artery divides into two: a smaller
external carotid artery or lingual artery, which supplies blood to the
thyroid gland, muscles of the hyoid apparatus, and the tongue, and
a bigger branch, the internal carotid artery, which eventually divides
into three branches:
a. Palatine artery- supplying blood to the roof of the oral
cavity
b. Cerebral artery- supplies blood to the brain, and
c. Ophthalmic artery- supplies blood to the eyes
These blood vessels need not be followed. At the base of
the internal carotid artery is an oval structure, the carotid gland,
which regulates the blood flow going to the head.
2. Systemic Arch
The systemic arch (aortic arch) is the middle branch of the
truncus arteriosus. Follow this branch by tearing the parietal
peritoneum and gently deflecting the lungs and other internal
organs on one side. Four smaller branches arise from the systemic
arch:
a.
b.
c.
d.
Laryngeal artery- supplies blood to the larynx and jaw
Esophageal artery- supplies blood to the esophagus
Occipital artery- supplies blood to the orbit and nose
Posterior vertebral artery- supplies blood to the vertebral
column
The subclavian artery extends laterally and becomes the
brachial artery that supplies blood to the forelimbs.
Dorsal Aorta
The left and right systemic arches joins together to form the
dorsal aorta which can be seen best by tearing the remaining
portions of the parietal peritoneum.
Slightly below the point of junction of the systemic arches,
the dorsal aorta gives off a branch, the coeliacomesenteric artery,
which divides into two: an anterior coeliac artery and a posterior
anterior mesenteric artery.
The coeliac artery divides into three:
a. Left gastric artery- supplies blood to the left side of
the stomach and left side of the pancreas
b. Hepatic artery- supplying blood to the liver and
portions of the pancreas
c. Pancreatic arteries- go to the rest of the pancreas
The anterior mesenteric artery also divides into three:
a. Splenic artery- supplying blood to the spleen
b. Intestinal arteries- supplying blood to the small
intestine
c. Anterior hemorrhoidal artery- supplying blood to
the large intestine and cloaca
At the level of the kidneys, the dorsal aorta gives off
branches going to the kidneys as renal arteries, spermatic arteries
supplying blood to the testes for males or ovarian arteries to the
ovaries of females, and adiposal arteries, supplying blood to the
adipose tissues.
Gently lift the dorsal aorta and take note of the unpaired
lumbar arteries supplying blood to the dorsal wall. On the ventral
side of the dorsal aorta at the region of the cloaca and in female
frogs, a branch goes to the oviduct as oviducal arteries.
Following the dorsal aorta posteriorly, it eventually divides
into left and right common iliac arteries. Follow one of the iliac
arteries and look for the epigastricovesical artery, which further
divides into:
a. Epigastric artery- located anteriorly; supplies blood
to the abdominal wall
b. Rectovesical artery- located posteriorly; supplies
blood to the cloaca and urinary bladder
At the region of the thigh is the femoral artery that supplies
blood to the muscles and anterior part of the thigh. The common
iliac continues to the entire length of the hindlimb as the sciatic
artery.
3. Pulmocutaneous Artery
This is the posterior branch of the truncus arteriosus that
eventually divides into pulmonary artery, which carries blood to the
lungs, and cutaneous artery, which goes anteriorly and dorsally behind
the tympanum and divides into:
a. Auricularis artery- supplies blood to the lower jaw
b. Dorsalis artery- supplies blood to the dorsal part of the skin
c. Lateralis artery- supplies blood to the lateral part of the skin
These three small branches of arteries may no longer be
followed.
References
Kent, G. 1992. Comparative Vertebrate Anatomy. Missouri : Mosby Year Book.
Rubite, R. and Yap, Audwin. 1987. Laboratory Manual in Zoology. Unpublished.
Weichart. 1976. Comparative Vertebrate Anatomy.
13
UROGENITAL
SYSTEM
The term urogenital is a combination of the
Greek word “ouro” meaning urine and “genitales" refers
to reproduction. Functionally, these are two separate
systems, namely the urinary or excretory system and
reproductive system. These systems develop from a
common germ layer origin and utilize common ducts to
release their products. Hence, they are usually
considered together.
The main organs of the excretory system are
the kidneys. These lie dorsal to all other organs in the
abdominal cavity and to the peritoneum, thus
retroperitoneal in position. The kidneys are made up of
nephrons that remove waste products of metabolism
from the body as well as regulate the amount of ions
inside the body of the animal. Urine from the kidneys is
collected by mesonephric ducts/ Wolffian ducts which
in turn convey urine to the urinary bladder. The urinary
bladder lies ventral to the cloaca.
The paired gonads namely the testes and the
ovaries are the principal organs of the reproductive
system of sexually reproducing animals. These organs
are sites for the production of gametes. Moreover,
these are also sources of important hormones needed
for the development and sexual maturation of the
animal. The sperm is conveyed to the outside through
the kidneys and then into the mesonephric duct. The
eggs, on the other hand pass through the oviduct
which empties into the cloaca.
Objectives At the end of this activity, the student
should be able to
1. Identify histologically the parts of frog's kidneys,
testes, and ovaries
2. Identify the organs of the urogenital system of both
male and female toad
3. Trace the pathway of urine, egg cells, and sperm
cells in the toad
Materials
Preserved frog/toad
Dissecting set
Dissecting pan
Prepared slides
Frog's kidney (x.s.)
Frog's testes (x.s.)
Frog’s ovary (x.s.)
Compound light microscope
Procedure
A. Histologic Examination of the Urogenital System
1.
Frog’s Kidney
a. Obtain a prepared cross section slide of frog’s kidney. Focus
under LPO.
b. Find scattered, within the field of view, tufts of capillaries each
surrounded by clear area. These are the renal corpuscles. Each
renal corpuscle is made up of the glomerulus (glomeruli, pl.),
which are the tufts of capillaries, and the Bowman’s capsule,
which is represented by the clear spherical area surrounding the
glomerulus.
c. Interspersed among the renal corpuscles are the renal tubules.
These are represented by rounded structures lined with
epithelial cells. Identify the type of epithelium lining the renal
tubules.
d. The renal corpuscles and renal tubules make up a nephron,
which is the functional unit of a kidney. It is in this structure
where the process of urine formation occurs.
e. Label the illustration in the Activity Sheet.
2.
Frog’s Testes
a. Secure a prepared slide of cross section of testes. Focus under
LPO.
b. Notice the several compartmentalized ovoid structures within
the field of view. These compartments represent portions of the
seminiferous tubules. The cells lining these compartments are
germinal epithelia, which function for the production of sperm
cells.
c. Focus on one seminiferous tubule under HPO. Notice the
varying stages of development of sperm cells. There is a
progression of maturity of sperm cells as from the basal layer of
the seminiferous tubule to the central cavity or lumen.
d. Identify the immature and mature sperm cells. The immature
sperm cells appear ovoid in shape with well-defined nucleus.
The sizes of immature sperm cells vary depending on the
degree of maturity. Mature sperm cells, on the other hand,
appear thread-like. Locate a group of spermatozoa.
3.
Frog’s Ovaries
a. Obtain a prepared slide of the frog’s ovaries. Focus under LPO.
b. Within the ovary, the eggs are arranged into individual follicles
composed of oocytes and follicle cells. Notice that the entire
ovary is surrounded by a covering called theca externa.
c. Find the egg cells, specifically the primary oocytes, at various
stages of maturity. The degree of maturation of the egg cells
can be determined by the amount of the yolky material
accumulated in the cytoplasm. Mature oocytes will have more
accumulated yolk and pigments.
d. Observe one mature oocyte. Take note of the nucleus of the
oocyte. It is sometimes referred to as the germinal vesicle. Its
nuclear membrane is serrated. Within the nucleus are several
nucleoli. Some of these nucleoli function in organizing the
distribution of accumulating yolk in the growing oocyte.
e. Focus on the cell membrane of the oocyte under HPO. Observe
the ovoid cells lying immediately outside the cell membrane.
These cells are the follicle cells, which function in nourishing the
growing oocyte.
f. Aside from the growing oocytes, also observe masses of
pigments located within the ovarian tissues. These serve as
sources of pigment for maturing oocyte.
g. Surrounding the follicle cells is a thin layer of connective tissue,
the theca interna. The theca interna, or cyst wall, however, does
not cover the whole egg cell. It is absent at the region of future
rupture during ovulation release of eggs. The cyst wall forces
the egg to separate out from the surrounding follicle cells.
B. Gross Anatomy of the Urogenital System of Toad
1.
Secure a preserved toad used in the previous activity. Rinse off excess
formalin with tap water.
2.
Use the blunt end of a probe to search for the kidneys. Gently push the
internal organs on one side and make a slit in the parietal peritoneum to
fully expose the kidneys. Locate these brownish elongated organs situated
at the retroperitoneal region of the animal. These lie close to the dorsal
body wall, which are separated from the main body cavity by a very thin
membranous peritoneum.
3.
On the medial part of the kidney, find a yellowish strip of tissue embedded
in it. This is the chromaffin tissue which is equivalent to the adrenal glands.
This structure may be difficult to see in some specimen.
4.
From the outer margins of the kidneys, locate the threadlike mesonephric
duct (Wolffian duct) which serves as passageway for both urine as well as
reproductive products into the cloaca.
5.
Locate the urinary bladder, which appears as a soft mass of thin tissue just
ventral to the large intestine. It is bilobed and empties into the cloaca.
Since the mesonephric duct opens into the cloaca, urine backs up into the
urinary bladder from the cloaca unless the aperture or vent is open.
6.
Expose the cloaca by cutting through the ischiopubic symphysis with
scalpel and push the pelvic bones aside. Cut open the cloaca just left of
the midventral line, separate the cut edges. Find the urinary bladder
opening at the ventral wall of the cloaca and the opening of the
mesonephric ducts on the dorsal wall.
7.
For male specimen, locate the paired testes, which are a pair of yellowish,
elongated structures found at the ventral surface of each kidney. A
mesentery called mesorchium keeps the testes in place. Find the vas
efferentia, which are minute, slender tubules lying on the mesorchium that
function on conducting the sperm from the testes to the kidney. The
mesonephric duct found at the lateral margins of the kidney is also termed
as the vas deferens when it functions for the passage of spermatozoa.
Anterior to the testes, observe the yellowish finger-like structures called
corpora adiposa (fat bodies). Also anterior to the testes is the Bidder’s
organ, which is a grayish mass of tissues and represents a rudimentary
ovary. If the testes of a toad are removed experimentally and the Bidder’s
organ is allowed to remain with blood supply, the latter becomes a
functional ovary.
8.
For female specimen, locate paired, lobulated, saccular organs on the
ventral wall of the kidneys. These are the ovaries. They are suspended
from the dorsal body wall by mesenteries called mesovarium. The ovary
becomes very much enlarged when the small, globular, pigmented eggs
are produced. A young ovary appears as white and very much folded
structure. Locate at the anterior part of the ovaries the yellowish finger-like
structures, the corpora adiposa (fat bodies), which function in the storage
of food during breeding season. Find also the highly convoluted, white,
long tubes on each side of the ovary called the oviducts (Muellerian ducts).
Locate the anterior end of the oviduct, which lies near the bases of
esophagus and lungs. Look for the ostium, the most anterior, funnelshaped opening of the oviduct. This serves as the entrance of eggs, which
are released from the ovary into the body cavity. Follow the oviduct
posteriorly where it enlarges into a distensible sac, called the uterine
enlargement, which stores eggs temporarily. Beyond the uterine
enlargement, the oviduct empties into the cloaca, anterior to the entry of
the mesonephric duct. The eggs are coated and released into the water
where external fertilization takes place.
9.
Switch specimen with a classmate whose toad is of opposite sex. Study
the reproductive system of both sexes.
References
Duran, A.P. 1987. Laboratory Manual in General Zoology. A.P. Duran Enterprises.
Hickman, C.P., F.M. Hickman and L. Kats. 1997. Laboratory Studies in Integrated
Principles of Zoology. 9th edition. The McGraw-Hill Companies, Inc. pp. 251-265.
Pahl, George, FSC. 1987. Principles of Zoology Investigations. De La Salle University
Press, Manila.
14
NERVOUS
SYSTEM
The organ system that integrates all body
functions is the nervous system. Integration is done
through transmission and processing of information from
the external environment or within the organism's body.
The major divisions of the vertebrate nervous
system include the central nervous system (CNS) and
the peripheral nervous system (PNS). The CNS serves
as the main processing center of information. It includes
the brain and the spinal cord. The PNS, on the other
hand, include all other nervous tissues outside the CNS.
The PNS is subdivided into afferent or sensory and
efferent or motor divisions. The afferent division sends
information to the CNS, while the efferent division carries
information away from the CNS. The efferent division
either supplies the skeletal muscles (somatic nervous
system), or the visceral organs (autonomic nervous
system). The autonomic nervous system, in turn is made
up of two divisions that act antagonistic with each other,
the sympathetic and parasympathetic nervous system.
The diagram below summarizes the organization of the
vertebrate nervous system.
The information being transmitted within this
system is in the form of nerve impulses. The pathways
where these impulses are conducted are through groups
of neuronal axons bundled together called nerves, if they
are situated in the PNS, or tracts, if they are within the
CNS. The cell bodies where these axons extend from
usually cluster together to form ganglia, if they are
outside the CNS, or nuclei, if they are within the CNS.
Vertebrate
Nervous System
Central Nervous
System
Brain, Spinal
Cord
Peripheral
Nervous System
Efferent
(Motor)
Division
Somatic
Nervous System
Afferent
(Sensory)
Division
Autonomic
Nervous System
Sympathetic
Nervous System
Parasympathetic
Nervous System
Objectives At the end of this activity, the student should be able to:
1.
2.
3.
Identify the major parts of the brain of the toad
Identify and locate the sympathetic trunk and 10 pairs of spinal nerves in
toad
Identify the major parts observed in a cross section of spinal cord
Materials
Compound light microscope
Prepared slide of spinal cord
Preserved frog/toad
Dissecting pan
Dissecting set (must include forceps, pins, scissors and scalpel)
Paper towel
Procedure
A.
Examination of the Cross Section of the Spinal Cord
1.
Recall the structure of the spinal cord. Obtain a prepared slide of the cross
section of the spinal cord. Focus under scanner or LPO.
2.
Locate the cavity at the center of the spinal cord, the central canal. In live
specimen, this is filled with cerebrospinal fluid.
3.
At the outline of the spinal cord, find the pia mater, which is the membrane
inseparably fused with the main mass of the spinal cord. In some slides
(depending on its preparation), locate the dura mater, which is a loose
membrane covering the spinal cord. Recall that the pia mater and dura
mater make up the meninges of the CNS.
4.
Determine the dorsal and ventral aspect of the spinal cord. A shallow
fissure called the dorsal fissure marks the median dorsal side of the cord.
A deeper, ventral fissure marks the ventral aspect of the cord. Notice that
the ventral fissure extends deep to the central canal.
5.
Take note that there are two distinct areas that can be observed
surrounding the central canal. These areas are:
a.
b.
B.
Gray matter- an inner darkly stained, butterfly-shaped or H-shaped
structure
White matter- an outer, lightly stained area
6.
Shift to HPO and focus on the central canal. Note that the canal is lined
with ependymal cells, darkly stained columnar cells.
7.
Focus the inner gray matter. Observe that the butterfly-shaped structure
has dorsolateral extension, called the dorsal horn (dorsal cornua) and the
ventrolateral extension called the ventral horn (ventral cornua). Locate
within the ventral horn the cell bodies of motor neurons, which are darkly
stained, star-shaped cells.
Peripheral Nervous System (Sympathetic Trunk and Spinal Nerves)
1.
Obtain a preserved frog/toad and place it on a dissecting pan ventral side
up.
2.
Carefully lift the viscera. Locate where the systemic arteries join to form
the dorsal aorta. Notice the whitish nerve fibers associated with it. This is
the sympathetic trunk, which is part of the autonomic nervous system. Its
major function is to regulate many involuntary functions.
3.
Remove all the viscera to expose the underlying vertebral column.
4.
Notice the spinal nerves emerging from each vertebra. Locate the brachial
plexus. A plexus is a network of communicating nerves. This includes the
first three spinal nerves that innervate the arm, neck, and shoulder.
5.
Find the 4th, 5th, and 6th spinal nerves that innervate the dorsal body wall.
These spinal nerves do not form plexus.
6.
Locate the succeeding 7th to 10th spinal nerves, which form the sciatic or
lumbosacral plexus. The nerves are directed posteriorly and innervate the
hindleg.
In addition to the spinal nerves, an additional component of the peripheral
nervous system is the cranial nerves. These are nerves that emerged from the brain. In
the frog/toad, there are 10 pairs of cranial nerves. The location of these nerves will be
described later. These will not be identified since most of them are quite small to be
traced.
C.
Central Nervous System (Brain and Spinal Cord of Frog/Toad)
1.
Place the specimen dorsal side up. With the use of forceps, cut away the
skin and muscles of the head from the external nares to the base of the
skull.
2.
Using the scalpel, scrape the dorsal part of the cranium until the bone is
thin and flexible. BE SURE TO SCRAPE AWAY FROM YOU.
3.
Using the tip of the scalpel or forceps, chip away small pieces of the
cranium until the brain is exposed. Use scissors to cut away the heavier
bone along the sides of the brain.
4.
Beginning with the first vertebra (atlas), snip through each side of vertebra
between the neural spine and the articular processes (zygapophyses).
Remove the dorsal piece to expose the spinal cord. Continue this process
until the while spinal cord is exposed.
5.
Carefully lift the exposed brain and spinal cord to a clean paper towel. The
procedure for the removal of the CNS of the frog/toad requires careful
execution and patience.
6.
Notice that the most part of the brain and spinal cord is covered by grayish
membrane. This represents the meninges. In the frog/toad, the meninges
are made up of two layers, the tough dura mater that adheres to the walls
of the cranium and neural canal, and the inner pia mater that closely
adheres to the brain and spinal cord. Carefully lift off the membranous
meninges, in order to study the different parts of the brain.
7.
The brain of the frog/toad, just like in any vertebrates, can be divided into
three main divisions, namely the forebrain (prosencephalon), midbrain
(mesencephalon), and hindbrain (rhombencephalon). Place the brain on a
clean petri dish dorsal side up. Locate the forebrain, which is represented
by the cerebrum and diencephalon.
8.
Find the most anterior lobes of the brain, the cerebrum. This is considered
to be the frog’s thinking center. The lobe constricts anteriorly to form the
olfactory bulb. The olfactory bulb extends further anteriorly to form the
olfactory nerves, the first pair of the cranial nerves.
9.
Posterior to the cerebrum, observe the diencephalon or thalamus. This
can be seen as a depressed region and functions in processing sensory
information. Projecting from the roof of the diencephalon is a club-shaped
endocrine gland, the pineal gland. Anterior to the pineal gland covering a
portion of the diencephalon, locate a network of fine blood vessels known
as the choroid plexus. If the brain is turned on its ventral side, at this level,
find the optic nerves, the second pair of cranial nerves. The optic nerves
cross to form the optic chiasma. Hence, the diencephalon is sometimes
referred to as twist brain. Locate the infundibulum at the floor of the
diencephalon. This is seen as a funnel-shaped down growth. Posterior to
the infundibulum is the pituitary gland, an endocrine gland.
10.
Immediately posterior to the diencephalon, locate the very prominent optic
lobes. This represents the midbrain, which functions in coordinating
responses to visual inputs. The cranial nerves III (oculomotor nerve) and
IV (trochlear nerve) emerge from the midbrain.
11.
Posterior to the optic lobes, find the cerebellum and medulla oblongata,
which are parts of the hindbrain. Locate a depressed ridge immediately
posterior to the optic lobes. This is the cerebellum, which functions in
balance. The cerebellum in frog/toad is relatively small compared to birds
and mammals. Why is this so?
12.
Locate the medulla oblongata posterior to the cerebellum. This is seen as
an inverted triangle that tapers posteriorly. The medulla oblongata
regulates most visceral functions. The last six pairs of cranial nerves- CN
V (trigeminal), CN VI (abducens), CN VII (facial), CN VIII (auditory), IX
(glossopharyngeal), and X (vagus) emerge from the medulla oblongata.
13.
Posterior to the medulla oblongata, locate the spinal cord. The spinal cord
functions in linking the brain and most parts of the body. It also functions in
spinal reflex actions. A reflex is defined as an involuntary, predictable
response to a stimulus.
14.
Notice that the spinal cord has two enlargements, namely the brachial
enlargement at the level of the arms and shoulder, and the lumbar
enlargement found at the posterior aspect of the trunk. These
enlargements represent aggregates of nerve supplying the forelimbs and
hindlimbs, respectively.
15.
Observe that the spinal cord is an elongated cylindrical structure and
tapers posteriorly. The pointed end of the spinal cord is referred to as the
filum terminale, which is housed within the neural canal of the urostyle.
16.
Locate at each side of the spinal cord the roots of the spinal nerves. Recall
that there are 10 pairs of spinal nerves in the frog/toad. Towards the
posterior end of the spinal cord, the roots tend to crowd themselves and
together with the filum terminale, they appear like horse’s tail, thus referred
to as cauda equina.
The brain and spinal cord of vertebrates are hollow structure with cavities. In live
specimen, these cavities are filled with fluid called cerebrospinal fluid. The cavities of
the brain are called ventricles, while the central canal refers to the cavity of the spinal
cord.
References
Duran, A.P. 1987. Laboratory Manual in General Zoology. A.P. Duran Enterprises.
Hickman, C.P. Jr., L.S. Roberts and A. Larson, 2001. Integrated Principles of Zoology,
11th edition. McGraw Hill Higher Education
Mader, S.S. 1998. Biology Laboratory Manual. 6th edition. WCB McGraw Hill.
Miller, S. A. and J.P. Harley. 2002. Zoology, 5th edition. McGraw Hill Higher Education.