External and Internal Anatomy Frog
BIO 22L
Experiment 10
EXTERNAL AND INTERNAL ANATOMY OF FROG
OBJECTIVES
At the end of the experiment, you should be able to:
1. describe the appearance of various organs found in the frog,
2. name the organs that make up various systems of the frog
3. learn appropriate terms used in the dissection of an animal
4. familiarize the method of pithing
MATERIALS
*Items that are in bold, italic, and underlined are materials that should be brought by the group on the day of
the experiment and not provided by the laboratory.
2 live medium-sized frog
dissecting kit
dissecting pan with wax
towels
thread
6-10 pcs. dissecting pins
plastic storage bag
scissors
compound and stereo/dissecting microscope
INTRODUCTION
Members of the phylum Chordata are called chordates. In order for an animal to be classified as a
chordate, it should have the four key characteristics, although these characteristics need not be present during
the entire life cycle. A chordate is an animal that has, for at least some stage of its life, a dorsal, hollow nerve
chord; a notochord; pharyngeal pouches; and a tail that extends beyond the anus. In the animal kingdom,
ninety-nine percent of all chordates are placed in the subphylum Vertebrata and are called as vertebrates. A
vertebrate is a chordate that has a strong supporting structure known as the vertebral column, or backbone.
In this experiment, the frog was chosen as an experimental animal to study the anatomy of vertebrates.
Frogs belong to the group called amphibians that live in water during their immature years and live primarily on
land during their adult years. The adult frog is a good example of the body organization of vertebrates that live
on land.
The major respiratory organ of adult frogs is their lungs which typically replaces their gills as frogs grow
old. The lungs appear as two spongy elongated bags located on both sides of the heart. Other amphibians
exchange gases with the environment through their skin.
In frogs, the circulatory system forms what is known as a double loop. The first loop carries
deoxygenated blood from the heart to the lungs and skin, and takes oxygen-rich blood from the lungs and skin
back to the heart. The second loop transports oxygen-rich blood from the heart to the rest of the body and
oxygen-poor blood from the body back to the heart. The frog’s heart has three major chambers. The two upper
chambers are the atria, which collect blood from the veins. Blood flows from the atria into the lower chamber,
the ventricle. The muscular ventricle pumps blood throughout the body through the arteries.
Frogs are also equipped with kidneys that filter wastes from the blood. The kidneys of frogs are long
dark organs embedded in the back wall. The excretory product travels through tubes called ureters into the
cloaca. From there, urine can be passed directly to the outside, or it may be temporarily stored in a small
urinary bladder just above the cloaca.
In the middle of the body cavity of the frog is the liver, the largest organ of the body. The reddishbrown liver consists of two lobes with a smaller lobe between them .The liver produces bile, which aids in the
digestion of fats. It also stores food in the form of glycogen and the liver also plays a role in detoxification. The
stomach of frogs, where food is partially digested is connected to the esophagus. The small intestine is the
narrow tube leading away from the stomach. Digestion is completed in the small intestine, as is most nutrient
absorption. The small intestine loops in tight coils down to the large intestine, a short wide tube. The large
intestine leads to the cloaca, a large sac that passes wastes out of the body.
External and Internal Anatomy Frog
BIO 22L
The laboratory exercise will give emphasis on the external structures of frog. It is found to be important
to have a strong background of the external structures of the animal to be used in dissection. In addition,
knowledge of anatomical terminologies would be of great help in the dissection process.
PROCEDURE
Before proceeding in the scrutiny of the external and internal structures of the frog, you should be familiar with
anatomic terms/terms of direction and movement used in the dissection process. Study the table on anatomical
terminologies and their definition for you to be guided and appreciate the dissection.
Table 6.1 Essential Terms of Direction and Movement and of Anatomy*
Terminology
abduction (abd.)
adduction (add.)
anterior (ant.)
caudal
cranial (or cephalic)
deep
depressor
distal (dist.)
dorsal (dors.)
erector
epiphysis
extension (ext.)
external (extern.)
fascia
foramen
fossa
flexion (flex.)
frontal (front.)
inferior (inf.)
insertion
internal (int.)
inverted (invert.)
joint
lateral (lat.)
* dextral
* sinistral
levator (lev.)
ligament
longitudinal (longit.)
medial (med.)
median
midline
midsagittal
oblique
palmar (palm.)
pectoral
pelvic
peripheral
plantar (plant.)
posterior (post.)
process
pronator (pronat.)
proximal (prox.)
rotator (rotat.)
sagittal (sagit.)
shaft
sheath
sphincter
superficial (superf.)
superior (sup.)
supinator (supinat.)
suture
symphysis
Definition
draws away from midline
draws toward the midline
situated near or toward the front end
referring to the tail
referring to the head
farther from the surface (in a solid form)
that which lowers
farther from the main mass of the body (or root)
toward the rear, back
that which draws upward
the extremity or head of a long bone
straightening
outside (refers to wall of cavity or hollow form)
fibrous envelopment of tissues
hole, perforation
shallow depression
bending or angulation
vertical; at right angles to the sagittal
lower, farther from crown of head
relatively movable part of a muscle attachment
inside (refers to wall of cavity or hollow form)
turned inward
connection between bones
farther from the midline (towards the sides)
the right side lateral direction
the left side lateral direction
that which raises
fibrous tissue binding bones together
refers to long axis (e.g. from head to tail)
nearer to midline (or center plane)
midway, being in the middle
divides body into a right and left side
vertical plane at midline dividing body into right and left halves
slanting
palm side of the hand
referring to the area related to the chest
referring to the area related to the hip region
near the surface of the body or organ
sole side of foot
near toward the hind end
projection (can be grasped with fingers)
that which turns palm hand downward
nearer to limb or point of reference
that which causes to revolve
vertical plane or section dividing body into right and left portions
body of a long bone
protective covering
that which regulates closing of aperture
nearer to surface (refers to solid form)
upper, nearer to crown head
that which turns palm of hand upward
interlocking of teeth-like ridge
union of right and left sides in the midline
External and Internal Anatomy Frog
A. Pithing
BIO 22L
tensor (tens.)
that which draws tight
transverse (trans.)
at right angle to long axis; body divided into upper and lower parts
ventral (vent.)
near or toward the belly
*Pansky, Ben at al. (1969). Review of Gross Anatomy. 2nd ed. USA: The McMillan Company.
Hold the frog using your left hand. Using your pointing finger, hold down the head of the frog via the
snout. Locate the depression at the posterior part of the head, the midline between the head and the body
of the frog. Carefully insert the pithing needle into the depression obliquely towards the frog’s brain. Rotate
the needle in circular motion (your aim here to smash up the frog’s brain). After, insert the needle in the
depression, this time towards the frog’s vertebral column. The pithing is successful only if the frog is
incapable of bodily movements. The aim of pithing is to paralyze the frog. ( Note: Your instructor will
demonstrate the proper frog pithing before you do it yourself!)
B. Identification of Sex of the Frogs under Study
To determine the frog’s sex, look at the hand digits, or fingers, on its forelegs. A male frog usually has
thick pads (swollen) on its "thumbs," which is one external difference between the sexes. Male frogs are
also usually smaller than female frogs. Male frog is noted with the presence of dark skin pigmentation
concentrated near the angles of the lower jaw. On the other hand, female frog is noted with lighter,
diffused pigmentation on the ventral side near the lower jaw. Female frogs usually do not have swollen
thumbs.
C. External Anatomy of the Frog
The pithed frog will be used for external structure examination. Place the pithed frog in the dissection
pan with the dorsal side up. Note the general form of the frog and identify its body regions and structures
as described below.
The body region of the frog is divided into two regions: the axial and appendicular region. The axial
region is composed of the head and the trunk.
In the triangular head region, notice the most anterior portion, this called the snout. Pair of slit-like
opening immediately postero-dorsal to the snout is the external nares or nostrils. These are continuous with
the internal nares or choenae within the mouth. The large posterior opening that extends posterolaterally
up to the base of the head is the mouth. This is bordered dorsally and ventrally by immovable dorsal and
vental folds, respectively. Located posterior to the nostrils and protrude on the dorso-lateral sides of the
head is the eyes, composed the immovable upper eyelid, which borders the dorsal side of the eyes and is
usually thicker than the lower eyelid (borders the ventral side of the eyes and is thinner and more movable
than the upper eyelid). Notice the thin and transparent structure continuous with the inner fold of the lower
eyelid that moistened the eye is nictitating membrane. This allows the frog to see underwater. Located
anterior to the eye along the median dorsal line is the brow spot (usually small light-colored circular spot
that may or may not be prominent due to numerous pigmentation of the frog’s skin. Behind each eye is an
oval-shaped membrane known as the tympanum or tympanic membrane, which serves as a covering of the
eardrum and is continuous to the buccal cavity.
In the trunk region, noticeable is the mid-dorsal line, which is a demarcation line at the dorsal side—a
reference point that divides the body symmetrically. Located about the middle of the trunk is the hump—
dorsally elevated region that corresponds to the articulation of the ilium pf the pelvic girdle and the
transverse process of the sacral vertebrae. A common opening of the digestive tract and the urogenital
system is the cloaca, situated at the median and posterior end of the trunk.
The frog’s appendicaular region is composed of the forelimbs and the hind limbs. Fore limbs refers to
the appendages located at the anterior side of the body—shorted in size compared to the hind limbs. Each
fore limb is made up of 4 digits and a rudimentary, undeveloped fifth digit. The fore limb is used to raise or
support the body when the frog is at rest. The fore limbs are subdivided into parts (from proximal to distal):
External and Internal Anatomy Frog
BIO 22L
the upper arm, the forearm (subdivided into: manus (hand), carpus (wrist), palm), and the phalanges or
digits (fingers).
Longer appendages of the frog located at the posterior side of the body are the hind limbs—adapted for
jumping and swimming. Each fore limb is composed of five digits with a rudimentary sixth prehallux found
at the inner side of the foot. These distinct toes are connected together by a membranous extension of the
skin, the web. Foot is well-developed with an elongated ankle. The hind limbs are subdivided into parts
(from proximal to distal): the thigh, the shank, the tarsus (ankle), the pes (foot), the sole and the
phalanges or digits (toes).
D. Internal Anatomy of the Frog
D.1 Overview of the Internal Organs
Place the frog in the pan with the ventral side facing up. Pin the limbs to the wax in the pan. Pick up
the loose skin just above the anal opening using your forceps. Make an incision through the raised skin.
Cut the skin along the center of the body to the base of the head. Cut laterally from the central cut to each
of the limbs. Pins the skin flaps back from the body wall. Make the same cuts through the muscle of the
body wall as you did through the skin. Raise the body wall with the scissors as you cut to avoid damaging
the structure below. When you reach the forelimbs, cut through the sternum. Pin back the muscle flaps to
expose the internal organs. In order to fully examine the internal structures, the eggs (present in females)
and fingerlike projections are removed (fat bodies).
Carefully lift the reddish-brown liver, with two large lobes and a smaller lobe in between them. On the
other side of the liver is the green sac called gall bladder. Locate the glottis—a small slit in the opening of
the buccal cavity. Insert a probe into it and follow the probe where it will meet a dead end, the stomach
which appears as the oval whitish sac. Run your finger over the pyloric valve until you reach the valve.
Locate the reddish triangular organ in the middle of the upper body. This internal organ is the heart.
Identify the small pea-shaped organ in the connective tissue near the small intestine. This organ is the
spleen.
Locate the lungs by looking for the two spongy elongated bags found on both sides of the heart.
Observe the long, dark organs known as the kidneys embedded on the back walls of the specimen. If your
frog has eggs, it is a female ready for breeding. The egg-producing ovaries appear as thin-walled gray fold
tissue. The coiled white tube on each side of the kidneys is the oviduct. This is a passage leading the eggs
from the ovaries to the cloacae. For a male frog, the two yellow bean-shaped testes are located next to the
kidneys. Sperm reaches the cloacae through the Wolffian duct. Now, carefully examine the parts that
belong to the following organ systems: respiratory, digestive, and urogenital system.
D.2 The Respiratory System
The following organs are included: the lungs, the lining of the mouth and skin. These structures are
moist with small blood vessels embedded to meet most of the frog’s oxygen demand. These organs help
the frog to stay underwater in a longer period. During hibernation, a significant decrease in the frog’s
metabolism is observed along with skin respiration to meet the oxygen requirement.
The following parts of the frog’s respiratory system are to be identified: the external and
internal nares, the olfactory canal (nasal), the buccal cavity, the glottis, the larynx or voice box—located
posterior to the glottis.
D.3 The Disgestive System
The system is divided into two main groups: the gastrointestinal tract or the alimentary tract and the
accessory organs. The former is a continuous tube running from the mouth to anus, in which the following
structures are included: the mouth, the pharynx, esophagus, stomach, small intestine and large intestine.
The later is composed of teeth, tongue, liver, gall bladder and pancreas.
External and Internal Anatomy Frog
BIO 22L
Open the mouth of the frog and study the covering and the floor of the mouth. Cut through the joint
between the upper and the lower jaw to expose the buccal cavity. The following parts of the frog’s digestive
system are to be identified: the maxillary teeth; the sulcus marginalis—a pair of groove on the inner side of
the maxillary teeth that receives the lower jaw when the mouth is closed; the median subrostral fossa—a
prominent depression of the sulcus marginalis at the anterior tip of the upper jaw; the lateral subrostal
fossa—a pair of depression of the sulcus marginalis lateral to the pulvinars; the pulvinar rostrale—a pair of
low elevations on each side of the median subrostral fossa.; the orbital prominence—distinct two large
rounded bulges of the eyeball; the vomerine teeth—the fine teeth projecting from the vomers in between
the internal nares; the Eustachian tube—a pair of slit-like openings found medial to the angles of the jaw—it
leads to the cavity of the middle ear; the tuberculum prelinguale—a prominent median elevation at the tip
of the lower jaw—this fits into the median subrostral fossa when the mouth is closed; the prelingual fossa—
distinct two shallow depressions on each side of tuberculum prelingual; the tongue; the vocal sac—a pair of
slit-like opening on the floor of the mouth near to the angle of the jaw—found only in male frogs; the
opening of esophagus—a large transverse slit, posterior to the laryngeal prominence; the laryngeal
prominence—circular elevation anterior to the esophageal opening; and the glottis; the pharynx—posterior
portion of the buccal cavity which opens into the esophagus; the esophagus—a short tube that connects the
pharynx to the stomach; the stomach; the pylorus—a constriction at the posterior end of the stomach; the
spleen; the gall bladder; the small intestine—irregularly coiled—the more anterior portion is the duodenum
(wider and shorter), the more posterior, narrower and longer coiled division of the small intestine is the
ileum—the small intestine is suspended form the dorsal body wall by the mesenterium; the large intestine;
the cloaca; the anus—a small opening at the posterior end of the cloaca; the liver; and the pancreas—a
small elongated and irregularly shaped gland located between the stomach and duodenum.
D.4 The Urogenital System
This system consists of the excretory and the reproductive systems. The elimination of waste products
of metabolism is for the excretory system, while the production of gametes and secretion of sex hormones
are for the reproductive system. The following parts of the frog’s urogenital system are to be identified—for
the female reproductive system: the ovaries—paired, lobular, saccular organ on the ventral wall of the
kidney, suspended from the dorsal wall by mesovarium; the oviducts or Mullerian ducts; and the copora
adiposa or fat bodies—for the male reproductive system: the testes—a pair of elongated, yellowish
structures on the ventral surface of the kidney and is attached to the kidney by a mesochoirum; the vas
efferentia—small, slender tubules lying on the mesochorium; the vasa deferentia or vas deferens—the term
given to the mesonephric duct for the passage of sperm; the vestigial oviducts—a non-functional pair of
slender white wavy tubes, one along each side of mesonephric duct which join posteriorly; and the fat
bodies.
Push aside the visceral organs to expose the kidney. Slit the parietal peritoneum near the vertebral
column and identify the following organs included in the excretory system of the frog: the kidney—a pair of
reddish, elongated, and flattened organ which is line ventrally by the parietal peritoneum; the adrenal
glands—a pair of yellowish, irregularly-shaped gland located on the ventral surface of the kidney; the
mesonephric duct—a pair of slender, straight white tubes on the postero-lateral edge of the kidney, which
conducts waste products from the kidney to the cloaca; the urinary bladder—a bilobed sac on the ventral
surface of the cloaca that serves as temporary storage of urine; and the cloaca.
External and Internal Anatomy Frog
BIO 22L
NAME (SN, GN, MI):_____________________________ DATE PERFORMED: ___________________________
SUBJECT/SECTION: _____________________________ DATE SUBMITTED: ____________________________
INSTRUCTOR: _________________________________ RATING: ____________________________________
Report for Experiment 6
External and Internal Anatomy of Frog
A. Pithing
Table 6.1 List all the major organs affected by pithing the frog
Organ System
Affected Function
B. Identification of Sex of the Frogs under Study
Table 6.2 Put a check mark corresponding to the characteristics of the frog under study and identify the sex of
the frogs. (Show the frog to your instructor for checking.)
Characteristics
Pigmentation at the Lower Jaw
Thumb Pads
Body Size
Body Structure
Identified Sex
Frog “X”
 Yes
Frog “Y”
 No
 darkly pigmented
 lightly pigmented
 swollen
 flat
 big
 small
 bloated
 not bloated
 male
 female
 Yes
 No
 darkly pigmented
 lightly pigmented
 swollen
 big
 flat
 small
 bloated
 not bloated
 male
 female
External and Internal Anatomy Frog
C. External Anatomy of the Frog
Figure 6.1 Draw the external anatomy
Ventral View of the Frog
Dorsal View of the Frog
BIO 22L
External and Internal Anatomy Frog
BIO 22L
Note of the following labels for
each structure identified
23. pes
24. prehallux
D. Internal Anatomy of the Frog
Figure 6.2 The Buccal Cavity and the Respiratory System of the Frog
1. axial region
2. appendicular region
3. snout
4. external nares
5. mouth
6. eyes
7. upper eyelid
8. lower eyelid
9. nictitating membrane
10. brow spot
11. tympanic membrane
12. trunk
13. hump
14. cloaca
15. forelimb
16. hindlimb
17. manus
18. carpus
19. phalanges
20. thigh
21. shank
22. tarsus
External and Internal Anatomy Frog
Frog’s Buccal Cavity
BIO 22L
Identify the labeled structures
in the buccal cavity of the frog:
1.
2.
3.
4.
5.
6.
7.
8.
9.
a
b
The Digestive System of the Frog*
* labels should be based on the mentioned structures in the procedure D.3
Figure 6.3 The Urogenital System of the Frog
External and Internal Anatomy Frog
BIO 22L
*labels should be based on the mentioned structures in the procedure D.4
Male Frog’s Urogenital System
Female Frog’s Urogenital System
Table 6.3 Major Internal Organs and their Specific Functions
Organs
Buccal Cavity
Cloaca
Esophagus
Fat Bodies
Gallbladder
Glottis
Kidney
Large intestine
Liver
Lungs
Nares
Ovaries
Pancreas
Pharynx
Skin
Small Intestine
Spleen
Stomach
Testes
Urinary Bladder
QUESTIONS
Specific Function(s)
External and Internal Anatomy Frog
BIO 22L
1. Where is the tongue attached to the jaw? How would this place of attachment and the tongue’s
stickiness be useful to the frog?
____________________________________________________________________________________
____________________________________________________________________________________
2. What is the function of the webbing between the toes of the frog?
____________________________________________________________________________________
3. How does the length of the small intestine relate to its function in the absorption of nutrients?
____________________________________________________________________________________
4. During the cold winter, the frog’s body temperature cools and the frog becomes inactive. Where does
the frog get food when it cannot catch prey?
____________________________________________________________________________________
5. In what situation would the location of the frog’s nares be an advantage in breathing?
____________________________________________________________________________________
6. Give the evolutionary significance of the prehallux and the brow spot.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
CONCLUSION
___________________________________________________________________________________________
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REFERENCES (In standard bibliographic format)