Fetal Pig Anatomy

INTRODUCTION

            In the following laboratory exercise, you will examine in some detail the external and internal anatomy of a fetal pig (Sus scrofa).  As the pig is a mammal, many aspects of its structural and functional organization are identical with those of other mammals, including humans.  Thus, a study of the fetal pig is in a very real sense, a study of humans.

            The fetuses you will see were salvaged from pregnant sows being slaughtered for food.  They are not raised specifically for dissection purposes.  The fetuses are removed from the sow and embalmed with a preservative, which is injected through the umbilicus.  Following this, the arterial and venous systems are injected under pressure with latex, a rubber-like compound.  Arteries (red) are injected through the umbilicus; veins (blue) are injected through one of the jugular veins at the base of the throat.

             During these exercises, keep several points in mind.  First, be aware that "to dissect" does not mean "to cut up," but rather primarily "to expose to view."  Actual cutting should be kept to a minimum.  Tissues are picked and teased apart with needle probes, forceps, and blunt probes in order to trace the pathways of blood vessels, nerves, muscles, and other structures. Never cut or move more than is necessary to expose a given part.  Second, pay particular attention to the spatial relationships of organs, glands, and other structures as you expose them.  Realize that their positions are not random. 

            Although the structures described below are identified on the accompanying figures, in some cases the figures contain more information than you need to know.  Don't panic – this extra information is provided to help you identify what you do need to know.  By the end of this exercise you should have a very good grasp of the connections between physiological processes and organ structure/function.

            At the end of each major section, there are a set of questions (Think about it). Additionally, there are boldface questions scattered through the text.  Make sure you figure out the answers to these questions before moving on.  All are fair game.

OBJECTIVES

1.        Perform a virtual whole-body dissection of a vertebrate.

2.        Identify the major anatomical features of the vertebrate body in a dissected specimen.

3.        Understand the relationship between structure and function in the vertebrate body and relate concepts to structures found in your pig.

4.         Apply knowledge and understanding acquired to problems in human physiology.

EXTERNAL FEATURES

1.      Determine the anatomical orientation of your specimen.

*The terms anterior and posterior are sometimes used synonymously with ventral and dorsal, respectively, for humans.

2.      Note the thin peeling layer of tissue covering the body of your pig.  This layer is the epitrichium, a layer of embryonic skin that peels off as hair develops beneath it.

3.        Identify the regions of the body (Fig. 1):

4.        Head:  Find the following:
5.        Trunk:  The terms sometimes used to describe the trunk vary whether one is discussing the dorsal or ventral surface.  The trunk can be described using the terms associated with the vertebral column:  thoracic (rib), lumbar (lower back), and sacral (pelvic) vertebrae.  Ventrally, the abdominal region dominates the area posterior to the thorax.  Note the umbilical cord; it connects the fetus to the placenta of the mother and later becomes the navel.  If you cut off the very tip (0.5 cm) of the umbilicus, you would more clearly see the following:
6.        Appendages:  Examine the legs of your pig.  Find the following:
7.        Determining the sex of your pig:
  1. Female:  Look for a single urogenital opening just ventral to the anus.  A prominent genital papilla projects from the urogenital opening. 
  2. Male:  Look for the scrotum, a sac-like swelling containing the testes and located ventral to the anus.  The male urogenital opening is faintly visible just posterior to the umbilicus.  Note that males as well as females have multiple nipples = teats = mammary papillae.
Think about it

1.      Notice how the number of toes is reduced in your pig.  The middle two digits form hooves.  Ungulates (hooved animals) like the pig walk with the weight of the body borne on the tips of the digits (unguligrade locomotion).  Cats and dogs use digitigrade locomotion (walking on the balls of their feet).  Humans typically use the entire foot for walking (plantigrade locomotion).  What form of locomotion do you use when you sprint?

2.      Although male mammals have nipples, as a general rule they do not lactate.  From an ultimate (why?) rather than a proximate (how?) standpoint, why is male lactation the exception rather than the rule (HINT: there are very few monogamous mammals)?

DIGESTIVE SYSTEM

Objectives:

1.      Identify and describe the functions of the main organs of the digestive system.

2.      Gain an appreciation of the spatial relationships of the many organs and structures that contribute to the digestion of food and the nourishment of the body's cells.

            The digestive system of mammals consists of the alimentary canal (mouth, oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus) and other associated structures/organs/glands (salivary glands, gall bladder, liver, pancreas).

            The cavity behind the teeth and gums is the oral cavity.  Note the papillae on the tongue.  These provide friction for food handling and contain taste buds.  Like all young mammals, fetal pigs have milk teeth (baby teeth) that are later replaced by permanent teeth.

Figure 1.   External anatomy of the fetal pig. A.  Ventral view.  B.  Lateral view. 

C. Posterior view of female.  D.  Posterior view of male.

            There are 3 pairs of salivary glands (Fig. 2).  The saliva producing glands are:

            With scissors, you would carefully cut through the tissue and bone starting at the corners of the mouth and back toward the ears (keeping the roof of the mouth intact) until the lower jaw can be dropped and the oral (buccal) cavity exposed (Fig. 3).

Find the following structures:

Internal Anatomy of Digestive System

            Remember that most internal organs, including the digestive system, are located in the body cavity, or coelom.  A large muscular structure, the diaphragm, divides the mammalian body cavity into the thoracic cavity and the abdominal (peritoneal) cavity.  The thoracic cavity is further divided into a pericardial cavity (heart) and two pleural cavities (lungs).  Epithelial membranes line these cavities and cover the surface of all organs.  Names of the epithelial linings are determined by their location.  The word "parietal" refers to the wall of the body, and the word "visceral" in this case refers to organs within those cavities. 

Figures 2, 3, and 4.  Salivary glands and neck region (Figure 2), oral cavity (Figure 3),

and incision guide (Figure 4).

For example:

Coelomic fluid fills the space between membrane layers.  This moisture acts as a lubricant, allowing organs some degree of easy movement.  The organs are connected to each other and to the inner body wall by thin sheets of connective tissue called mesenteries, which suspend the organs and provide bridges for blood vessels, nerves, and ducts.

            Examine the neck, thoracic, and abdominal regions of your pig (Fig. 5).  First find the thymus gland, which partially covers the anterior portion of the heart and extends along the trachea to the larynx.  The thymus plays an important role in the development and maintenance of the immune system – this is where white blood cells mature into antibody-producing T-lymphocytes. 

            Immediately beneath the thymus in the neck is the thyroid gland, a small,solid, reddish, oval mass.  The thyroid secretes thyroxine, which in mammals influences the metabolic rate of cells, which in turn influences growth and development.  Because iodine is necessary for the production of thyroxine, our salt is often iodized.  If synthesis of thyroxine declines (e.g. due to a lack of iodine), the anterior pituitary increases the release of thyroid stimulating hormone (TSH).  This may stimulate the proliferation of thyroid cells, but if there is no iodine, thyroxine production will not increase, which causes additional TSH release.  The thyroid also produces calcitonin, a hormone that stimulates osteoblasts to lay down bone.  The consequence of this activity is a surprisingly rapid decline in blood calcium levels.  If blood Ca levels drop too low, or if extra Ca is needed, the parathyroid glands release parathormone.  The parathyroid is not a discrete organ in mammals – parathyroid tissue is embedded in the thyroid.  Parathormone raises blood Ca levels by activating osteoclasts, by stimulating Ca resorption in the kidney, and by activating vitamin D to enhance absorption of Ca from food.

             Note that the esophagus penetrates the diaphragm before entering the stomach.  Cut open the stomach lengthwise with your scissors.  The contents of a fetus's digestive tract is called meconium, composed of a variety of substances including bile stained mucus, amniotic fluid, sloughed epithelial cells, and hair.  Cleaned out, the stomach has folds (rugae).  What role might the rugae play?  Many glands that secrete pepsinogen and hydrochloric acid are embedded in the wall of the stomach.

            Two muscular rings (smooth muscle), the cardiac (closer to the heart) and the pyloric sphincter (adjoining the small intestine), control the movement of food through the stomach.

            The majority of digestion and absorption takes place in the small intestine.  It is composed of the duodenum, the jejunum, and the ileum, the latter two being difficult to distinguish.  The duodenum, into which bile and enzymes from the gall bladder and pancreas enter, passes posteriorly and then curves to the left.  The coils of the small intestine are held together by mesenteries.  A rule of thumb is that the small intestine in both pigs and humans (omnivores) is about five times the length of the body.  Note the lymph nodes embedded in the mesenteries.  These nodes filter pathogens from the lymph.

            The caecum is a small blind-ended sac found at the juncture of the ilium and the colon (large intestine).  This juncture is also the site of the ileocecal valve.  In the pig, the caecum houses bacterial symbionts that help break down cellulose (a major component of plants) – much in the same way that gut protozoans in termites allow the termites to eat wood.  Many herbivorous mammals (pigs, horses, rodents, rabbits) use "hindgut fermentation" in the caecum to digest cellulose. One clade of ungulates, the "ruminants" (camels, giraffes, deer, sheep, cattle) use "foregut fermentation".  Ruminants have a multi-chambered stomach in which cellulose breakdown takes place.  This breakdown is aided by their ability to regurgitate the contents of their fermentation chamber back into their mouth for further mechanical breakdown (i.e., chewing cud).  In humans the caecum is known as the appendix and is not used in digestion.  Although the human appendix contains some lymphatic tissue, its function is poorly understood and it can be removed without any harmful effects.  So why haven't we lost our appendix completely?  Recent evidence suggests that the smaller it gets, the more likely it is to get obstructed, inflamed, and infected (appendicitis).  Too large an appendix is wasteful, too small is dangerous.  Barring a mutation that eliminates it completely, we are stuck with a slightly wasteful, occasionally dangerous tradeoff.  Evolution is not about perfection.

Figure 5.  Views of the internal organs of the fetal pig.  A.  Neck, thorax, and abdomen, as they appear after just opening, with no disturbance. B.  Close-up of neck region.  C. Close-up view of thorax.  D. Abdomen, view of intestines.

            The colon (large intestine) can be divided into three major regions:  ascending, coiled, and descending.  The colon runs from the caecum to the rectum.   The colon functions to absorb water for compaction of the feces.  Just past the rectum is the anus, the site of the final muscles of the alimentary canal, the anal sphincter.

Other Associated Organs

            The liver, the largest organ in the abdominal cavity, has a multitude of functions, most of which are underappreciated.  For example, in the fetus, blood cell production takes place in the liver as well as the bone marrow.  In the adult, the liver:

            The gall bladder, a small, usually greenish sac which lies on the underside of the right central lobe of the liver, stores bile secreted by the liver.  Bile from the liver enters the common bile duct via the hepatic duct; bile from the gall bladder enters via the cystic duct.   Bile is composed of bile salts (which emulsify fats (breaks them into small droplets) in the duodenum) and bilirubin, which is a bile pigment.  Bilirubin is a byproduct of the breakdown of hemoglobin from old red blood cells, which takes place in the liver and spleen. 

The pancreas is an elongate granular mass between the stomach and the small intestine.  Actually the pancreas consists of two lobes:  one that runs transversely and another than runs longitudinally along the duodenum.  The pancreas secretes digestive enzymes and other substances into the small intestine via the pancreatic duct (which you will not be able to see).  Remember, the pancreas is an endocrine as well as an exocrine organ.  Endocrine glands have no ducts; they secrete their products (hormones) directly into capillaries.  Hormones act as chemical signals to mediate other physiological processes.  Scattered throughout the exocrine tissue of the pancreas are small islands of endocrine tissue (Islets of Langerhans).  Although these islets are too small to see with the naked eye, they are extremely important.  They secrete insulin, glucagon, and somatostatin directly into the tiny blood vessels that run through the pancreas.  Insulin and glucagon lower and raise blood glucose levels, respectively, and somatostatin regulates levels of both insulin and glucagon.

The spleen is a long, flat, red-brown organ which lies across the stomach.  It is not part of the digestive system and is actually the largest organ of the lymphatic system.  It stores and releases red bloods cells into the bloodstream, recycles old red blood cells from circulation, and aids in the development of white blood cells.  Despite all of these important functions, your spleen can be removed with few ill effects.  

Think about it

1.      Saliva contains water (to moisten food), mucus (to lubricate food), salivary amylase (to break down starch), bicarbonate (to buffer acids in food), and antibacterial agents.  Why might these last three components be necessary when the stomach is the next destination anyway?

2.      Everyone knows different parts of the tongue are especially sensitive to different tastes.  But why should we devote tongue space to bitterness?

3.      In humans, the uvula hangs as a pendant from the posterior end of the soft palate.  During swallowing, it lifts upward and closes off the nasopharynx.  Why is this important?

4.      Is diarrhea a defense strategy to rid your body of pathogens or a way for intestinal pathogens to spread to others (still occurs in less developed countries with no sewage treatment)?

5.      In olden days, coal miners often suffered from rickets, a disease characterized by brittle bones.  Miners rarely see the sun, a "source" of vitamin D.  What's going on here?

6.      Would you expect carnivores to have longer or shorter intestines than herbivores?

7.      What happens if the contents of the colon pass too rapidly through the colon?  too slowly?

8.      What are some possible advantages and disadvantages of foregut and hindgut fermentation?

RESPIRATORY SYSTEM

The respiratory system is responsible for bringing a fresh supply of oxygen to the blood stream and carrying off excess carbon dioxide.  In mammals, air enters the body through the external nares and enters the nasal cavities dorsal to the hard palate. As air passes through these convoluted cavities, it is humidified and warmed to body temperature and dust is caught in the mucus of the membranes that line the cavities.  Air moves from here into the nasopharynx, where it passes through the glottis into the larynx. 

The larynx is a hard-walled chamber composed of cartilaginous tissue.  In the course of hominid evolution, the larynx has moved downward (caudally).  As a result, human vocalizations tend to come out of the mouth, where the tongue can manipulate them.  In chimps, the larynx is higher in the throat, with the result that vocalizations are very nasal (and thus less controllable and understandable).  Our descended larynx comes with a price – it makes choking on food far more likely.  Interestingly, human babies retain an elevated larynx.  It makes baby talk difficult, but it also allows babies to nurse and breathe at the same time.

           The vocal cords are elastic ridges that stretch across the space within the larynx.  When air passes over the vocal cords during exhalation, the cords vibrate and produce sound. In adult humans, laryngitis results from viral infection of the vocal cords.  They swell and regular speech is difficult to impossible. 

            The trachea, distinguished by its cartilaginous rings (incomplete on the dorsal side), divides into the two bronchi (singular bronchus), which enter the lungs and divide into bronchioles.  Bronchioles terminate in alveoli, where gas exchange takes place.

            The right lung typically consists of four lobes and the left of two or three.  The lungs in your fetal pig are small and fairly solid because they have never been inflated.  Inflation causes lungs to have a spongy appearance.  Note the position of the diaphragm in relation to the lungs.  Contraction of the diaphragm enlarges the thoracic cavity and pulls air into the lungs.  Remember that only mammals have a true muscular diaphragm; other terrestrial vertebrates use a variety of methods to inflate their lungs.

            Examine the lungs and note the pleural membranes (one lining the inner surface of the pleural cavity and the other covering the outer surface of the lung).  As mentioned earlier, the intrapleural space is filled with fluid.  This fluid allows the membranes to slide freely across each other, much like two wet panes of glass (easy to slide, hard to separate), and allows them to maintain contact.  This ensures that the lungs will inflate when the thoracic cavity expands as a result of diaphragmatic contraction or expansion of the rib cage.

            When neonatal mammals inhale for the first time, their lungs inflate.  When they then exhale, the lungs don’t deflate all the way.  That’s because pulmonary surfactants reduce the surface tension of water (just like soap does – you can float a bottlecap on water until you add a surfactant like soap).  In this case the water is in the form of a film that coats each and every alveolus.  If it weren’t for these surfactants, the surface tension of this layer would collapse the delicate alveoli – causing the lungs to “collapse” after each breath.  This surfactant is produced by the lungs during the last part of pregnancy. 

Think about it

1.      Why does the trachea have cartilaginous rings?

2.      Why is it important for air to be moist when it enters the lungs?  Many desert mammals have extremely convoluted nasal cavities.  How might these large and complex nasal cavities conserve water during exhalation?

3.      When you catch a cold, you get a runny nose.  Is snot your body’s way of combating a viral invader, or is the virus simply using you to reproduce and spread itself? 

CIRCULATORY SYSTEM

        The circulatory (or cardiovascular) system is responsible for transporting nutrients, gases, hormones, and metabolic wastes to and from individual cells.  Actually, the loading and unloading take place in capillaries.  Oxygen is added to the blood (and carbon dioxide removed) in the capillaries of the lungs.  In the capillaries of the small intestine, nutrients are added to the blood, while in the capillaries of the kidneys the blood is cleansed of various metabolic wastes and excess ions.

         In mammals, the circulatory system is divided into a pulmonary circuit, which involves blood flow to and from the lungs, and the systemic circuit, which involves blood flow to and from the rest of the body.  Your pig has been doubly injected (red for arteries, blue for veins).  However, note that in reality, arteries and veins are defined by the direction of blood flow, not by the oxygen content of the blood contained therein.

1.  The Heart (Fig. 6)

             In living animals, the pericardial cavity is filled with fluid that acts as a shock absorber to protect the heart from injury.  The coronary artery and coronary vein lie in the diagonal groove between the 2 ventricles.  These vessels supply and drain the heart (the heart is a muscle and as such has the same requirements of any other organ).  When the coronary artery becomes obstructed, a heart attack may occur.  It is the coronary arteries that are "bypassed" in coronary bypass surgery.  Note that the atria have external flaps, known as auricles.

            In an adult mammal (fetal circulation will be discussed below), deoxygenated blood flows into the right atrium from the anterior and posterior vena cavae.  It then makes the following circuit:  right ventricle, pulmonary trunk, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aortic arch, aorta, and on into the systemic circulation.  On the heart model, trace this path and find the above as well as the following structures:

Fig. 6.  The heart and major arteries and veins.

3.  Major arteries of the systemic circulation, anterior to the heart (Fig. 7b)

            The first large vessel that branches from the aortic arch is the brachiocephalic trunk.  This artery soon branches into the right subclavian and the common carotid arteries (as well as sending vessels along the inner and outer walls of the rib cage).  The subclavian arteries carry blood to the forelimbs, the carotid arteries carry blood to the head.  The carotid branches into an internal carotid, which goes to the brain, and the external carotid, which goes to the face.  In desert-dwelling ungulates, the internal carotid forms an arterial "capillary" bed (rete) over the nasal passages and then reforms the carotid artery and delivers blood to the brain.  Because the nasal passages represent the intersection of hot dry outside air and moist internal body surfaces, a great deal of evaporative cooling takes place there.  Instead of expending energy (and water) to cool their entire bodies, these mammals can allow their bodies to heat up to brain-damaging temperatures while their brain's blood stays cool. 

            The second large vessel that branches from the aortic arch is the left subclavian artery. 

4.  Major arteries of the systemic circulation, posterior to the heart (Figs. 8, 9)

             The coeliac artery branches off the aorta to supply the stomach, spleen, and liver.  Huh?  but the coeliac is so tiny!  So the liver, the largest

Figure 7.  A. Major veins anterior to the heart. 

B. Major arteries of systemic circulation anterior to the heart.

organ in the body, is supplied by a mere branch of a rather small artery!  Well, it's more complicated than that.  First, the liver also gets blood from the hepatic portal vein (see below).  “But that's a vein" you say.  And you are correct.  But so much blood flows through it ... and the intestines aren't always a super metabolically active organ, so the liver can benefit from it (and it certainly benefits nutrient-wise).  The other trick is that despite its size, the liver is not particularly metabolically active.  At any given time, only a small proportion of its cells are doing anything.  So that, plus the fact that the liver is really weird in having sinusoids rather than proper capillaries, allows it to work as it does. 

5.  The hepatic portal system (Figs. 8, 9)

            In a normal circulatory pathway, blood takes the following path:  artery – capillary bed – vein.  In a portal system, the blood travels in the following manner:  artery – capillary bed – portal vein – capillary bed – vein.  Portal systems are found in many different parts of the body and carry blood from the capillaries of one organ to the capillaries of another organ.  In the case of the hepatic portal system, nutrient-rich blood from the mesenteric veins flow into a single mesenteric vein, which joins with the lienogastric (gastrosplenic) vein from the spleen and stomach and becomes the hepatic portal vein.  This vein now carries blood to the liver, where it breaks into a second capillary bed.  Here the products of digestion pass into liver cells.  This ensures that the liver has "first shot" at toxins from the diet as well as glucose, amino acids, and lipids.  Capillaries in the liver then converge into the hepatic veins, which empty into the caudal vena cava for transport back to the heart.  If the intake of toxins (such as alcohol) exceeds the liver's ability to filter them from the blood, the excess enters the general circulation and on to other organs (like the brain).

Figure 8.  Illustration of hepatic portal system depicting associated veins and organs.

Figures 9 and 10.  9. Hepatic portal system.  10A. Fetal and renal circulation.  10B. Posterior circulation.

Female Reproductive System (Fig. 11)

            In the female, the opening of the urogenital sinus / vaginal vestibule lies directly ventral to the anus.  It is bounded laterally by low folds, the labia, which come together ventrally to form a protruding genital papilla.  The clitoris, a small body of erectile tissue on the ventral portion of the urogenital sinus, may be visible.  The clitoris is homologous (similar in structure and developmental origin) to the male penis.  In the male, the tissues of the penis develop around and enclose the urethra, while in the female the urethra opens posteriorly to the clitoris.

Figure 11.  Kidneys, excretory system, and female reproductive system.

            Within the body of the female, the urethra is bound by connective tissue to the vagina.  Gently separate this tissue.  The vagina and the urethra join together about 1 cm from the exterior body opening to form the urogenital sinus / vaginal vestibule.  This structure is not present in adult females--separate external vaginal and urinary openings begin to develop after birth as the urogenital sinus shrinks.  How might this occur? 

            Trace the vagina anteriorly to the cervix, a slightly constricted region of tissue which leads to the uterus (did you know that 99% of cervical cancers in humans are due to viral infection?).  The cervix acts as a sphincter to separate the vagina from the uterus.  It's usually closed.  In fact, the female mammalian reproductive system has many safeguards against sexually transmitted disease:  an acidic vagina, antibacterial mucus, and lots of white blood cell activity.  Why are such safeguards especially important in humans?  The uterine body branches anteriorly into two uterine horns (pigs and many other mammals have a bicornate uterus; humans have a simplex uterus).  Another feature of uterine horns is the production of litters (incidentally, pigs are the only ungulates that produce litters).  Trace the uterine horns to the oviducts, where fertilization normally takes place.  These tubes are much smaller than the horns and lie extremely close to the ovaries.  The ovaries are the sites of egg production and the source of female sex hormones, estrogen and progesterone.  Every egg (actually primary oocyte) that a female pig (or human) will ever produce is already present in the ovary at the time of birth. 

Male Reproductive System (Fig. 12)

     Bear in mind that the testes, the site of sperm and testosterone production, are found in the scrotum in older fetuses, but may remain undescended within the body cavity in younger fetuses.  

      Locate the epididymis, a tightly coiled tube along one side.  Sperm produced in the testis mature in the epididymis until ejaculation.  Unlike females, male mammals are not born with a lifetime supply of gametes.  Sperm are produced only after puberty, but then continue to be produced for the rest of the life of the male.  Cells within the testis (but not those that give rise to sperm) are responsible for the production of testosterone.  Evidence suggests that sperm may not be recognized as “self” by the immune system and must therefore be protected.  Not only is there a blood/testis barrier (just like there is a blood/fetus barrier in females), but also the immunosuppressive characteristics of testosterone are no accident.  The testosterone produced within the testes by the interstitial cells that physically surround the spermatogenic cells provide a strong defense.

            The slender elongated structure that emerges from each testis is the spermatic cord.  It goes through the inguinal canal (actually an opening in the abdominal wall connecting the abdominal cavity to the scrotal cavity).  It is through this canal that the testes descend.  The spermatic cord consists of the vas deferens (plural vasa deferentia), the spermatic nerve, and the spermatic artery and vein. The vasa deferentia are severed in a vasectomy.

            Locate the seminal vesicles on the dorsal surface of the urethra where the two vasa deferens enter.  The seminal vesicles are responsible for 60% of the volume of the seminal fluid.  They release fructose to provide energy for the swimming sperm and prostaglandins and clotting factors to aid in the mass movement of the ejaculate up the female reproductive tract. 

            Situated between the bases of the seminal vesicles is the prostate gland.  This gland produces bicarbonate, an alkaline substance, to neutralize the acidic environment of the vagina.  The bulbourethral (Cowper's) glands lie on either side of the juncture of the penis and urethra--their precise function is poorly understood, though they also produce an alkaline solution.  The urethra joins the penis just posterior to the Cowper's glands.  The retractable penis extends through the tissue of the "flap" that holds the bladder to the urogenital opening.  Use your finger to feel the penis within the flap.  Carefully pick away the tissue in this area to separate the penis.

Think about it

1.       Why do most male mammals have testes in an external sac (scrotum)?  (Hint: there is some evidence that men that wear briefs produce fewer viable sperm than men that wear boxers).  Why might some mammals pull their testes back into their body in the non-breeding season?

2.      What is the difference between the ejaculate of a vasectomized man and a man who has not undergone this procedure?  Do vasectomized men continue to produce testosterone, and if so, by what path does it get into the general circulation?