Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Shoulder of the Pig (Sus scrofa)

Text by Lazaro Gonzales, Jr.

Introduction:

Dissecting the fetal pig to identify musculature and external features is approached differently than most fetal pig dissection books explain.  This is due to development of the internal structures being the common focus of fetal pig dissection.  Many of the external structures of the fetal pig to be identified are difficult to distinguish because the fetal pig is still in development.  This also causes variation from specimen to specimen.  Be careful to not cut too deeply when making incisions as the scalpel will easily damage subcutaneous tissue and musculature.  The bilateral symmetry of the fetal pig allow for errors or practice on one side and clean dissection on the other.

Surface Anatomy and Cutaneous Structures:

The most basic external divisions of the fetal pig include: 1) the head, 2) the neck, 3) the trunk, and 4) the tail [1,2].

Head.  Two large pinnae are found on either side of the head.  This larger size allows for better hearing.  A feature unique to the pig is its snout, which protrudes from the face and is capped with a disc-shaped piece of cartilage surrounding external nares.  The eyes have a nictitating membrane that functions to keep the eye lubricated and is located just deep to the palpebra (eyelids) [2,3]. 

Neck.  This is a short, muscular region.  The platysma is located on the ventral aspect of the neck.  It is musculature that is similar in function to the panniculus carnosus, which is described later.  Platysma is an extension of 2^nd arch musculature and the panniculous carnosus is from the posterior arches.

Trunk.  The trunk is further divided into the thorax and abdomen.  It is a common misconception that the presence of mammary papillae is an indication of a female pig.  Both sexes have two rows of teats.  The difference between a male and female is determined externally by the location of the urogenital orifice.  Females’ urinary orifice lies ventral to the anus where as the males’ urinary orifice is caudal to the umbilicus [1,2,3,4,5].  Attached to the thorax is the pectoral girdle from which the brachium (upper arm), antebrachium (forearm), andmanus (hand) extend.  The hind limb is formed by the thigh, crus (shin), and pes (foot).  Pigs are unguligrades meaning they walk on the tips of only two of the four digits on their hands and feet [1].  This causes confusion in identifying the joints in unguligrade limbs.  The most proximal joint is the shoulder or hip followed by the elbow or knee then the wrist or ankle.  

Tail.  It is the least complex region of the pig and does not have a known function.  In adults the tail may become curled.  In the fetal pig the tail is not haired and is the same color as the rest of the pig.

Discussion:

The fetal pig does not lend itself to clear identification of the cutaneous tissue.

Superficial fascia and deep fascia were nearly impossible to distinguish from each other and from other tissue.  Much of it will be damaged during skinning even if the utmost care is taken.  A thin sheet of muscle deep to the skin is the panniculus carnosus.  The common function of this muscle is shivering for warmth and moving the skin to shake off biting insects.  The segmental arteries and veins were easily seen due to the red and blue latex injected into the circulatory system.  Nerves were slightly more difficult to identify as they are thin and white.  It was sometimes difficult to distinguish between nerves and extraneous tissue, but nerves are usually more elastic. 

General Dissection:

I began dorsally with a shallow incision starting approximately midway between the head and trunk and continued along the spine to the caudal region.  I checked every few inches to make sure the incision was not too deep.  I found the cut where the latex was injected into the fetal pig’s throat and continued it dorsally until it intersected the first incision.  I made the same incision as the initial cut on the ventral side of the fetal pig stopping approximately three centimeters (cm) short of the umbilicus and curving around it keeping an approximate radius of two cm.  I cut dorsally intersecting the parallel dorsal incision at the caudal region.  The scalpel was used to initially separate the skin from the subcutaneous tissue.  Once a separation was begun it was easiest to clamp the skin with a hemostat and continue the separation with a blunt probe.  Another helpful technique for separating out layers or structures is using the sharp scissors closed in a scraping motion.  Segmental veins and arteries were visible piercing the skin from below; it is ok to cut through these.  The most difficult separation was on the ventral side near the umbilicus.  The skin around the arm was cut off around the wrist.  The skin was then disposed of and identification of musculature and subcutaneous structures took place.  The fetal pig will not have fully developed musculature.  For identification of muscles I used a finger to smooth out the tissue and find the direction of the muscle striations.  Where these change  direction is a good indication of new musculature or a different section of the same musculature.  Once the borders of the muscle were found I used a blunt probe and scissors to separate it from surrounding tissue.  It was necessary to reflect some muscles in order to view musculature deep to them.  I severed the muscles near their midlines so that it could be pieced back together. 

Cutaneous Musculature:

Shoulder Muscles:

Branchiomeric muscles appear to be from the shoulder girdle and function as shoulder muscles, but are actually remnants of the gill arches.  These muscles are the trapezius, cleidomastoid, and sternomastoid.  In mammals with clavicles the latter 2 muscles combine to form sternocleidomastoid.  The fetal pig does not have a clavicle.  The most superficial muscle is trapezius which can be seen as soon as the pig is skinned.  It is a large muscle that fans from the shoulder dorsally across the back attaching to the spine.  I stroked the striations of this muscle to find the 3 divisions of this muscle: spinotrapezius, acromiotrapezius, and clavotrapezius.  Cleidomastoid was much more difficult to find, but is also superficial.  Examine the neck region for thin musculature running from the back of the skull ventrally to the pectoral region.  Sternomastoid runs alongside the cleidomastoid on its ventral side.

The superficial muscles of the shoulder include: pectoralis superficialis, pectoralis profundus, deltoideus, and latissimus dorsi.  Pectoralis superficialis is on the ventral side of the thorax with striations running medial to lateral.  Pectoralis profundus is deep to pectoralis superficialis originating at supraspinatus and inserting on the humerus.  Latissimus dorsi is ventral to the trapezius on the lateral surface of the pig.  It is a broad, fan-shaped muscle similar to trapezius.  Deltoideus covers the shoulder and is almost a continuation of trapezius.

I reflected trapezius to reveal the deep shoulder muscles.  Rhomboideus is on the dorsal surface of the neck cranial to trapezius.  Deep to pectoralis profundus is teres major.  It is on the lateral side inserting deep to the biceps and the medial head of the triceps.  Serratus ventralis is deep to and between lattisimus dorsi and the lateral portion of pectoralis profundus. 

Branchiomeric Muscles [1,2,3,4,5]

Superficial Shoulder Muscles [1,2,3,4,5]

Deep Shoulder Muscles [1,2,3,4,5]

Comparative Anatomy:

One major comparative theme throughout the labs is comparison of developmental versus adult anatomy.  Other mammals available for comparison in this particular lab class include: Norway rat, opossum, beaver, cat, and rabbit.  The beaver has a very thick panniculus carnosus in comparison with the fetal pig.  Although little is known of the function of such thick musculature in the beaver it is possible it plays a role in swimming.  As will be the case in most of the limb musculature, the cat most closely resembles the fetal pig in shoulder musculature because they are cursorial mammals.  The forelimbs and hindlimbs are relatively equal as opposed to mammals such as the rabbit, which has smaller forelimbs.

References:

1. Odlaug, T.O.  1980.  Laboratory Anatomy of the Fetal Pig.  Wm. C. Brown Company Publishers, Dubuque, Iowa.

2. Walker, W.F.  1980.  Dissection of the Fetal Pig.  W.H. Freeman and Company, San Francisco, CA.

3. Field, H.E.  1944.  The Fetal Pig: An Introduction to Mammalian Anatomy.  Stanford University Press, CA.

4. Smith, D.G.  1998.  A Dissection Guide and Atlas to the Fetal Pig.  Morton Publishing Company, Englewood, CO.

5. Holstad, G.E.  1959.  The Fetal Pig: An Introduction to the Anatomy of the Fetal Pig.  Burgess Publishing Company, Minneapolis, MN.