FETAL PIG PICTORIAL REVIEW
DIGESTIVE SYSTEM
Internal Organs in situ
When you finish making the initial cuts, you will be able to see
many of the internal organs of the pig in their natural places.
The heart (1) lies in the middle of the thoracic cavity, surrounded
by a mambrane called the pericardium. If you have damaged the
pericardium beyond recognition, look at someone else's pig. By
the time you are done the dissection, all the pericardia will have
been removed. The top of the heart will be covered with the
thymus gland, which extends up into the throat. Much of the
thymus has been removed from the pig pictured here.
On either side of the heart are the lungs (2). Note that the left lung
is smaller. You will have cut the diaphragm (3), but it can still be
seen, as can its attachement to the body wall.
The large liver (4) may vary from reddish brown to blue. The
former is its natural colour, but blue latex often fills its numerous
blood spaces.
Just behind the liver on the left side, the stomach (5) can be seen,
and the spleen (6) raps around the left side of the liver. The spleen
is very delicate and may break when you try to separate it. It is not
surprising that ruptured spleens are one of the most common types
of internal injury.
The large umbilical vein (7) can be seen entering the liver. You
had to cut this vein during the initial incisions. Its other end can be
seen on the flap carrying the umbilical cord.
The intestines of a pig are arranged somewhat differently from
those of a human. The pinkish coloured jejunum (8) is seen on the
top right, the greener ileum (9) at the lower centre, and the coiled
up colon (10) at the top left.
The urinary bladder (11) folds back with the umbilical cord, and is
surrounded by the two large umbilical arteries (12). These arteries
take blood from the fetus to the placenta, where it picks up
nutrients and oxygen from the mother's blood, and loses waste
products of the fetus's metabolism.
Digestive Organs Beneath the Liver
By pushing the liver (1) up toward the head, you will be
able to view many of the organs of the digestive system.
The gall bladder (2) is usually quite evident as a sort of
blister on the third lobe of the liver. The next page looks at
it more closely.
The stomach (3) is easily identified as a large floppy
pouch under the left lobe of the liver. The duodenum (4)
arches away from the stomach to the pig's right. A deep
crease indicates the location of the pyloric sphincter (5). If
you probe this area, you will be able to detect the strong
muscular nature of this sphincter. The spleen (6) lies just
beow the stomach, and is attached to it by a thin
membrane. You may have to break this membrane to see
deeper structures.
The three major parts of the intestine, the jejunum (7), the
ileum (8) and the colon (9) are still visible .
Gall Bladder and Surrounding Organs
The gall bladder (1) is a soft sack embedded in the liver. In this view, with
the liver turned back, you can just make out the attached ducts, the cystic
duct (2) which drains the gall bladder, the hepatic duct (3) which carries
bile from the liver to the gall bladder, and the common bile duct (4) which
carries the bile from both organs to the duodenum (5). A person can live
without the gall bladder, because the livert can drain bile directly into the
duodenum, but a special restricted diet is required, since bile cannot be
stored up for a large fatty meal.
Now you see the same view with the membranes removed. This makes the
ducts much more obvious. It also clearly shows the hepatic portal vein (6),
which runs parallel to the common bile duct.
Viewing the Pancreas
The pancreas can be viewed by pulling the bottom of the
stomach up toward the head. It will be necessary to break
some of the membranes holding the stomach and spleen in
place. Use a blunt probe for this.
The pancreas (P) is a long, loosely organised gland, that
has often been described as looking like a "bunch of
grapes". If you look carefully, you may be able to find the
pancreatic duct, which goes behind the duodenum (D) and
joins the common bile duct before it enters the duodenum.
Intestine and Caecum
Role the intestines to the pig's right to see the view shown here.
Trace the path that food would take as it moves through the
intestine. If you like, you may try ot tease out the intestine toi see
how long it is. To do this, you will have to trim the mesentery
carefully. Make sure you are completely familiar with its
structure before you destroy it.
The jejunum (1) appears pinky in most fetal pigs. Remember that
the fetus is not swallowing anything except a bit of amniotic
fluid, so there is little in the intestine. The liver is secreting some
bile. By the time the intestinal juices get to the ileum (2) some of
the water has been absorbed and the bile is more concentrated.
This makes the ileum look greenish. The ileum ends where the
small intestine meets the colon (3) a a "T" junction. The other
end of the "T" (4) is the caecum. Note that the caecum is large
compared to the human appendix, but small compared to the pig's
horse-like relatives. The pig is descended from herbivores whose
caecum was undoubtedly vital for digeting cellulose, but, like
ourselves, it has become an omnivore and gets little if any
nutrition from cellulose.
In this diagram, you can also see the bottom of the rectum
(slightly out of focus below the caecum) as it passes between the
umbilical arteries on its way to the anus. make sure you trace the
full length of the rectum on your pig.
Intestinal Mesenteries
Close Up
The intestines are held in place by a pair of membranes called the mesenteries (1). The mesenteric arteries run between these parallel
membranes, and give rise to a vast number of tiny arteries (2) that take blood to the jejunum and ileum. These in turn break up into the
thousands of arterioles that supply the villi and help to absorb food.
Near the base of the mesenteries is a row of white, fatty-looking bumps. These are lymph nodes on the lymph ducts that drain the lacteals.
Since you take so much foreign material into your gut, it makes sense that you should have a strong line of defense against any invading
microorganisms that might escape the digestive processes.
RESPIRATORY SYSTEM
Organs of the Chest Cavity (Thorax) in situ
To open the thoracic (chest) cavity, you will have to cut
through the cartilage of the breast bone, and the collar
bones. Try not to damage the underlying blood vessels as
you do.
When you first open the chest cavity, your view will be
dominated by the heart (1) and lungs (2,3). Usually, the
right lung (2) will be larger than the left (3), though that is
not obvious in the view shown here. Each lung consists of
several lobes.
The heart may be covered (or partly covered) by a
transparent membrane, the pericardium, seen here almost
intact. The chances are, you will have destroyed much of it
while opening the chest.
Feel the inside of the rib cage and the surface of the lungs
with a gloved finger. They are very smooth and slippery,
due to the presence of thin, moist membranes, the pleura.
The pleura allow the lungs to slide across the walls of the
thoracic cavity with almost no friction during inhalation and
exhalation.
Note the diaphragm (5) between the lungs and the liver.
Remember that you cut the attachments of the diaphragm to
the body wall during the initial cuts. If you have a small pig,
the diaphragm may look like a translucent membrane with
muscle fibres just beginning to grow into it from the edges.
If your pig is older, it will be a definite layer of muscle, as
shown here.
Covering the top of the heart is part of the thymus (6), an
important component of the fetal immune system. thymus
tissue extends well up into the throat, and must be carefully
picked away to see other features of the cervical region. The
prominent thyroid, an endocrine gland, is visible as a brick
red or slightly purplish round mass.
Trachea and Lungs (Heart Removed)
The view shown here is one that you will not have, since it
required removal of the heart. You will need the heart in
place for the next part of the dissection. By moving it around
and looking behind it, however, you should be able to see
many of the features shown here, and perhaps to understand
them in context a bit better.
The cavity in which the heart sat is seen between the two
lungs (1). The muscles, glands and membranes have been
peeled away from the front of the trachea (2) which clearly
shows the rings of cartilage that keep it open. The trachea
branches into several main bronchi (3), of which three are
visible here.
The stumps of pulmonary arteries and veins are also visible
(4). Note that the colour of these vessels is not a reliable
guide to their identity; you would be better to note whether
they attach to the left atrium (veins) or the right ventricle
(arteries).
Close up of the trachea and bronchi
Detail of Bronchi and Bronchioles
This piece of lung has been scraped repeatedly with the
back of a blunt probe, wearing away the solid tissue. The
image was taken using a 20X dissecting microscope. The
network of tiny airways is clearly visible, as are a few of
the numerous blood vessels. A few bubbles of carbon
dioxide have formed where bronchioles are ruptured. Note
that the lungs of a fetal pig have not been inflated, and
would not contain much in the way of gases. All gas
exchange at this point is taking place thorugh the placenta.
Interior of Mouth
By cutting through the sides of the jaw, you can
open the mouth wide enough to view the structures
at the back. The ridged hard palate (1) is one of the
most obvious features, grading into the soft palate
(3) of the pharynx. Small "milk teeth" (2) are
already erupted in the mouth.
The large and muscular tongue (4) has been pulled
forward and down in this picture. Papillae, which
contain the taste buds, are visible, especially along
the edge.
At the back of the tongue is the epiglottis (5) which
fits into an opening from the nasopharynx (6),
alllowing direct passage of air from the nose to the
trachea. During swallowing, the epiglottis is pulled
down to cover the entrance to the trachea, allowing
food to pass over it to the oesophagus.
Depending on the position of your cut, you may be
able to see the maxillary salivary glands (7).
Vocal Cords and Epiglottis
In this view, the tongue has been pulled far forward and down, causing
the epiglottis (1) to pull out of the nasophrynx (4). The opening to the
larynx and trachea is clearly visible, as is one of the cartilaginous vocal
cords (2) that stretch across it. Vibrating vocal cords and the resonance of
the larynx beneath them create sounds. Just behind the opening of the
larynx is the opening of the oesophagus (3)
CIRCULATORY SYSTEM
Circulation in the Fetal Pig
The following pages will show you the major blood vessels of the
fetal pig, as they appear in real specimens. The pages are sequenced
in the order in which you would find the various vessels as you
dissected. You may access them in a different order by clicking
below. Use this page to improve your understanding, and to study.
Note that the blood vessels in these pigs have been injected with
red (arteries) and blue (veins) latex. The larger arteries have very
thick walls and may appear white. Some vessels may fail to receive
latex. The left carotid artery and the left jugular vein have been
opened to allow injection of the latex, and may be broken or
distorted.
Veins Anterior to the Heart
The veins draining the head and arm regions are quite
symmetrical. Internal(1) and external(2) jugular veins
drain each side of the head and join to form the left and
right common jugulars(3). These are joined by left and
right subclavian veins(4) respectively to form the short left
and right innominate veins(5) the two innominates join to
form the anterior vena cava(6) which drains into the right
atrium(7) of the heart.
These veins serve to bring deoxygenated, nutrient poor
blood back from the front end of the pig. In the right
atrium it is mixed with the deoxygenated nutrient-rich
blood from the posterior vena cava(8). The blood then
enters the right ventricle and is pumped to the lungs for
oxygenation. Remember, however, that the lungs in a fetal
pig are not yet functional, so oxygen must be acquired in
the placenta. You will see the mechanism for this in a later
view.
In the next step, these veins must be removed to see the
underlying arteries.
Arteries above the Heart
Blood is pumped from the left ventricle through the
aorta(1), which is visible behind the pulmonary artery. As
the aorta arches to the left side of the pig (your right,
remember) it gives rise to two major arteries (in humans it
would be three). The larger brachiocephalic artery(2) gives
rise first to the right subclavian artery(3) and then to the
right carotid(4) and left carotid arteries(5). In humans the
left carotid has its own branch directly from the aortic
arch.
The second branch from the aorta is the left subclavian,
which feeds the left arm and shoulder.
All of these arteries carry oxygenated blood under high
pressure to anterior systems of the pig.
Arteries of the Heart
The heart is a very complex organ, especially in a mammalian
embryo.
In the fetal pig, the right(1) and left(2) atria are clearly visible as thinwalled sacs on the top of the heart. The right(3) and left(4) ventricles
are fused into one muscle mass, but their positions can be inferred
from the shape of the heart.
This view of the heart clearly shows the coronary arteries(5) radiating
over the wall of the heart muscle. The arteries arise from the very base
of the aorta, and coronary veins (no latex) return blood to the right
atrium.
The pulmonary artery(6) rises from the right ventricle on the ventral
side of the heart, and arches back over the heart, where it branches to
take blood to each lung. Because the lungs are not yet functional in a
fetus, it would be wasteful to send huge amounts of blood to an organ
that is not fulfilling any purpose. A short broad shunt vessel, the
ductus arteriosus(7), takes blood from the pulmonary artery and
diverts it into the aorta(8).
The heart rolled to the right to show the ductus:
This shunt must close at birth, to separate the systemic and pulmonary
circulation. It then atrophies to become a small ligament, the ligamentum
arteriosum, that holds the pulmonar artery and aorta together in the adult.
Failure of the ductus to close is called a patent ductus, and requires
surgery in some children to tie off the vessel.
The aorta itself arises from the left ventricle, dorsal to the pulmonary
artery. It arches over to the left and back, travelling down behind the left
side of the heart along the backbone.
Veins of the Heart Area
The venous system of the heart is quite simple. The
numerous pulmonary veins leaving each lung merge to
become the four main pulmonary veins(1), which enter the
left atrium. Sometimes the heart must be twisted around to
see these. Some of you will have destroyed some of the
pulmonary veins when removing a lung, but the points of
entry should still be visible on the heart itself.
The much larger venae cavae(2) enter the right atrium. The
anterior and posterior vena cava merge right at the point
where they enter the heart. The posterior vena cava which
travels along the backbone, parallel to the aorta, passes
right through the liver and diaphragm before entering the
heart.
The coronary veins(3) are particularly clear on this
specimen. They can be seen conecting to vena cava.
Posterior Arteries of the Systemic Circulation
By removing the left lung and rolling the heart to
the right, you can see the aorta as it descends
through the diaphragm. It gives rise to small
intercostal arteries that feed the muscles of the
chest.
After it has passed through the diaphragm, it
gives rise to two moderate sized arteries. These
can bge seen by rolling the abdominal organs to
the right side of the pig, or removing some of the
liver and intestines. The first, the coeliac(1)
quickly branches into gastric, hepatic, splenic
and pancreatic arteries that feed the upper
alimentary organs. The second, the anterior
mesenteric artery(2) gives rise to the hundreds of
branches that fan out through the mesentery,
taking blood to all the villi of the small intestine.
Two large branches from the aorta, the renal
arteries(3 - hard to see here), take blood to the
kidneys for cleansing, and two small, thin
gonadal arteries(4) branch off very close to them.
A small artery then branches off the aorta and
runs anteriorly along the rectum(5). this is the
posterior mesenteric artery, which supplies the
large intestine and picks up water in the colon.
At the end of the aorta, it branches into four main
trunks. Two go to the legs, the iliac arteries(6),
while the other two go ventrally along the sides
of the bladder and into the umbilical cord. These
two umbilical arteries(7) are responsible for
transporting metabolic wastes to the placenta and
carrying nutrient poor and deoxygenated blood to
the placenta where it may obtain required
resources by diffusion from the mother's blood.
Closer view
Detail of the Base of the Aorta
Veins of the Trunk Area
Blood from the legs is drained by the large iliac veins(1),
which merge to form the posterior vena cava(2). Small
gonadal veins (usually not visible) and renal veins(3) join
this large vessel, which travels forward through the liver
and diaphragm to the heart. At the top of the liver, it is
joined by the hepatic vein(4), which is usually buried right
in the liver tissue. The posterior vena cava joins the
anterior vena cava just as it enters the right atrium(5).
Hepatic circulation
The liver receives its oxygen from a small hepatic artery
that branches off the coelic artery, but most of the blood
flowing through the liver comes from the gut. All the
blood delivered by the coeliac artery, the anterior
mesenteric artery and the posterior mesenteric artery is
collected into the large hepatic portal, which runs parallel
to the common bile duct. Usually, the hepatic portal does
not get injected with latex, so it can be difficult to see.
After it enters the liver, the hepatic portal distributes blood
to a vast network of sinuses in the liver, where it can be
screened and its solutes adjusted. These sinuses are then
drained by small veins which merge to form the hepatic
vein. The hepatic vein, shown in the photo, was buried
right in the tissue of the liver, and had to be dissected out.
The hepatic vein then takes blood to the posterior vena
cava.
Interior of the Heart
The two views of the dissected heart on the left show the organ before and after the latex was removed from the inner chambers. Try to identify
the valves and chambers, and the entrances of the various blood vessels. Note that this is difficult to do in a two dimensional view. Take what
you see here and try to apply it when you look at a real heart in the classroom.
URITOGENITAL SYSTEM
External views of male and female fetal pig
Externally, the differences between male and female fetal
pigs are small. There is no external penis in the male, and
the scrotum is not well developed until sexual maturity. In
females, the entrance to the vagina is hidden beneath the
tail.
The two specimens in this view show the small abdominal
differences. The male (bottom) clearly shows the genital
pore just behind the umbilical cord. This is the opening
through which the erect penis will be everted for mating.
Running back from the genital pore along the midline of
the abdomen, a faint white line is visible. This is the
internal penis.
Slight development of the scrotum is visible between the
legs. This is much clearer in the rear view, accessible by
clicking at the end of this text.
In the female (top) there is a lack of the above features.
The main external feature is the genital papilla, a
triangular flap of tissue that covers the opening of the
urogenital tract. It is found just ventral to the anus, beneath
the tail.
Female Reproductive Organs of the Abdomen
This close up view shows the primary reproductive organs
of the female. The ovaries (1) are small kidney-shaped
organs. They usually cover the fallopian tubes (2) which
just show below the right ovary (left side!) in this view.
In this view, showing the top end of the ovary (1), one can
see the fallopian tube (2) terminating in the funnel-shaped
funiculum (3). Note that the funiculum lies right next to
the ovary, but is not attached. It must capture eggs released
directly into the body cavity. Both the ovary and the
fallopian tube are attached to the back wall of the abdomen
by a strong ligament (4).
The fallopian tubes bring ova to the uterine horns (3)
which join in hte midline to form the uterine body (4). The
uterus is thus "Y-shaped". This allows for the attachment
of placentas for a whole litter of young pigs along the
uterine horns. The ovary and uterus are held in place by
the broad ligament (5), which later in life will have to bear
most of the weight during pregnancy .
The uterine body passes through the pelvic opening
between the urethra (5), which drains the bladder (6), and
the rectum (7).
Female Urogenital Organs of the Pelvic Area
The bladder (1), lies between the two umbilical arteries
(2). In this view it is pulled over to the pig's right side. The
left ureter (3), which drains urine from the kidney, can be
seen entering the bladder
The ovary (4) lies just posterior to the kidney (not visible
in this view). The uterine horns (5) pass over the umbilical
arteries and the ureters, and join to form the uterine body
(or common uterus)(6)
In this view, the pelvic canal has been dissected. The birth
canal consists of the uterine body and the vagina (7). Urine
leaves the body via the urethra (8). Note that, unlike in
humans, the vagina and urethra join to form a short
urogenital sinus (9) before reaching the outside.
Abdominal Organs of the Male Urogenital System
This view of the male abdomen, with the intestines held
out of the way, shows some features common to both
sexes. A tough membrane, still intact on the pig's left side,
separates the urinary organs from the digestive ones. This
membrane has been removed on the pig's right side (your
left) to show the kidney (1), which extracts nitrogen and
other waste from the blood. The kidneys are drained by the
ureters (2) which carry urine to the urinary bladder (3) for
storage.
In this view, the bladder has been pulled back. In the intact
animal, it would lie in front of the rectum. Note the
umbilical arteries on either side of the bladder. After birth,
these will atrophy and become ligaments connecting the
aorta and the front wall of the abdomen.
Little of the reproductive system can be seen in the
abdomen. The testes, which originated in the abdomen,
have been pulled down into the scrotum through the
inguinal canal (5). All that can be seen are the vasa
deferentia (sing. vas deferns), which emerge from the
inguinal canal, pass over the ureters and umbilical arteries,
and enter the urethra (not clearly visble) just after it leaves
the bladder.
Organs of the Male Reproductive System
In this view the scrotum and inguinal canal on the pig's left side have been
dissected. The testis (1) is clearly visible as a smooth oval body inside the scrotum.
Over the past few weeks it has been pulled from its point of development in the
abdomen through the inguinal canal and into the scrotum. It will continue to
descend even further into the scrotum. It is pulled by a contraction of the
gubernaculum, better seen in some of the alternative views.
The top end of the epididymis (2) is visible. The epididymis consists of thousands
of tiny tubes that collect mature sperm from the tiny cavities in the testis and store
them until ejacualtion.
The left vas deferens (3) can be seen ascending through the inguinal canal and
passing over the umbilical artery. it will carry sperm to the urethra. The gonadal
artery and vein run paralllel to it, as do very sensitive nerves (not visible). Any
blocakge of the inguinal canal can cause sterility, pain, and varicose veins in the
scrotum (varicocoele). The most common blockage is due to an inguinal hernia, in
which a small bulge of the intestine is forced into the canal.
Also visible in this view is the penis (4) which has been dissected out. Notice that
it runs along the ouside of the pelvis to the anal area, where it meets the urethra
that has descended through the pelvic canal. (not visible here)
Organs of the Male Reproductive System - Deep Dissection
By dissecting through the pelvic bones, one can see the
deeper structures of the male tract. The urethra (2) drains
the urinary bladder (1) and enters the penis (3), allowing
urine to be moved out of the body. The urethra appears
very thick at "2" because it is surrounded by strong
muscles which control urinary flow and also contract
during ejaculation.
The left testis and epididymis are dissected out of the
scrotum (6). The gubernaculum (7) has contracted to pull
the testis into the scrotum.
Sperm travel from the testis to the epididymis, which loops
beneath the testis an meets the vas deferens (8). The vas
runs through the inguinal canal, over the umbilical artery
and ureter, and joins the urethra just below the bladder.
Two sets of seminal galnds can be seen attached to the
urethra, the seminal vesicles (9) and the bulbourethral
gland (10). These add nutritive and protective fluids to the
semen as it passes on its way to the penis
The urethra (1) is now a narrow tube. Most of the volume of the penis is taken up
by the corpus cavernosa (2), a spongy tissue that helps to erect the penis when it is
engorged with blood. Two other erectile bodies, the corpora spongiosa run on
either side of the urethra. They are not as clear in htis image. One of the arteries
that provides blood to erect the penis is visible (3).