Thyroid Gland

Thyroid Gland: the oval shaped gland is found in the neck ventral to the trachea and just below the larynx.

Bronchi

The trachea branches into each lung. These two tubes are called bronchial tubes. Inside the lungs these branch into smaller bronchioles that end with a grape-like cluster of air sacs or alveoli where oxygen and carbon dioxide are exchanged with capillaries.

Diaphragm

Since the pig is already open from the digestive system dissection, examine the diaphragm, a sheet of muscle that stretches across the abdominal cavity and separates it from the thoracic cavity where the lungs are located and in the center is the heart. It can be found right above the stomach and liver and under the lungs and heart. Once you locate this, the respiratory and circulatory is right on top. You may need to cut through the pig's sternum and expose the chest cavity (thoracic cavity) to view. You will need to cut up under the pig's throat and neck and make two more lateral incisions in order to fold back the flaps of shin covering the throat to expose the thoracic.

Pulmonary Veins

Pulmonary Veins: Consists of four vessels that return form lungs to the left atrium of the heart.

Pulmonary Arteries

Pulmonary Arteries: The pulmonary arteries carry deoxygenated blood from the heart to the lungs. These are the only arteries that carry deoxygenated blood. It begins at the base of the right ventricle then branches into two pulmonary arteries (left and right), which deliver de-oxygenated blood to the corresponding lung.

On the ventral surface of the heart passing dorsally and to the left is the large pulmonary artery. It originates in the right ventricle. Try tracing it its two branches to the lungs.

Pulmonary Trunk

Pulmonary Trunk: Is the artery that goes from the right ventricles to the lungs. It has two main branches (right and left pulmonary arteries).

Aorta

Locate the aorta, the largest systemic artery of the body. It leaves the left ventricle, curves to the left, dorsal to the pulmonary artery, and continues dorsally in a posterior direction along the left side of the vertebral column. The proximal curved portion of the aorta is called the aortic arch, while the next segment of the aorta within the thorax is known as he thoracic aorta.

Caroited Arteries

Carotid Arteries: Supplies blood to the neck and face from the top of the heart. Can be found in the neck and behind the larynx.

Pericardium

Pericardium: is a membrane like the pleura. The pericardium consists of two parts; the visceral pericardium is the thin and shiny membrane that is on the surface of the heart, and the parietal pericardium is the thicker membrane.

Vena Cava

Anterior vena cava: locate the trunk of this systemic vein above the heart. In the pig the term anterior vena cava is more correct and therefore preferred to superior vena cava. All anterior veins lead into it.

Posterior vena cava: This is the major vein returning blood from the lower extremities and from the abdominal area. Again, this designation in the pig is preferred to inferior vena cava. It can clearly be seen rising from the diaphragm, which it has penetrated, to enter the heart at the right atrium together with the superior vena cave.

Anterior to the heart, you can locate a large vein that enters the right atrium. This vein is the anterior vena cava, which brings blood to the right atrium from the anterior part of the body. Now lift the heart to view its dorsal surface. To do this you must use your scissors to carefully cute through all the major arteries attaching the heart to the mediastinum and then removing the heart with your hands to observe the posterior vena cava that carries blood from the posterior part of the body and empties it into the right atrium.

Mediastinum

Mediastinum: The mediastinum is a structure in the thorax, surrounded by loose connective tissue. It is the central compartment of the thoracic cavity. It contains the heart, the great vessels of the heart, esophagus, trachea, phrenic nerve, cardiac nerve, thoracic duct, thymus, and lymph nodes of the central chest. The mediastinum lies between the right and left pleura in and near the median sagittal plane of the chest. It extends from the sternum in front to the vertebral column behind.

Coronary Vessels

Coronary Vessels: The vessels on the heart that supply blood to the heart muscle. The vessels are found in the interventricular groove and this groove shows the split between the right and left ventricle.

Ventricles and Atria

Remove the pericardium, which is a thin membrane that surrounds the heart, with a blunt probe to expose the structure of the heart more closely. The right side of the heart pumps blood to the lungs, while the left side of the heart pumps blood to all other parts of the body. Locate the right and left sides of the heart. The structures visible on the heart are the two atria, the ventricle which have two chambers. Each side of the heart has an upper and a lower chamber. Upper chambers are called atria and receive blood, while lower chambers are called ventricles and pump blood out of the heart. You can easily locate the right and left atria and ventricle by looking at the heart.

Atria

Atria: The heart is divided into four chambers that are connected by heart valves. The upper two heart chambers are called atria. Atria are separated by an interatrial septum into the left atrium and the right atrium. The lower two chambers of the heart are called ventricles. Atria receive blood returning to the heart from the body and ventricles pump blood from the heart to the body. The heart is located between the lungs an under the sternum and is protected by the rib cage.

Visceral Pleura

Visceral Pleura: the thin, shiny membrane on the surface of the lung. The pleura cavity is the space located between the visceral and parietal pleura.

Parietal Pleura

Parietal Pleura: This is the serous membrane found within the thorax. The parietal pleura lines the inner walls and can easily be found by locating where the lungs are and the rib cage, and then looking at the filmy thin layer of tissue laying on top.

Lungs

Lungs: is the essential respiration organ. The two lungs are located near the backbone on either side of the heart in the chest cavity. Their main function is to transport oxygen from the atmosphere into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere. This exchange of gases is accomplished in the mosaic of specialized cells that form millions of tiny, exceptionally thin-walled air sacs called alveoli.

In the thoracic cavity, carefully separate the pericardium or sac surrounding the heart and the diaphragm from the body wall. Locate the two, spongy lungs that surround the heart. The tissue that covers and protects the lungs is called pleura.

Trachea

Trachea: A tube-like portion of the respiratory tract that connects the larynx with the bronchi. Also known as the windpipe, the trachea is located in the chest cavity and in the neck. This allows air to be transported down into the lungs where it is cycled into the bloodstream.

Find the trachea: a large air tube that lies anterior to the lungs. The trachea is easy to identify because of the cartilaginous rings that help keep it form collapsing as the animal inhales and exhales.

Larynx

Larynx: At the anterior (toward head) of the trachea, you can find the hard light colored larynx (or voice box). The larynx allows sounds to be produced. Once you cut up into the throat with scissors and remove the two flaps to expose what is under, the larynx is easily identifiable due its width compared to the small ringed trachea and light pinkish color. Its uppermost segment is the triangular flap of tissue, the epiglottis, which protects the opening to the trachea.

Thymus

Thymus: The thymus is a pinkish-gray color, soft, and lobulated on its surfaces. The thymus is made up of two identical lobes and is located anatomically in the anterior superior mediastinum, in front of the heart and behind the sternum. The thymus provides an inductive environment for development of T-lymphocytes from hematopoietic progenitor cells. In addition, thymic stromal cells allow for the selection of a functional and self-tolerant T-cell repertoire. Therefore, one of the most important roles of the thymus is the induction of central tolerance.

The Respiratory System

The Respiratory System: The organ system that carries out gas exchange and supplies the body with oxygen and disposes it of carbon dioxide. 

Esophagus

Esophagus — This muscular tube is located dorsal to the trachea. Food is pushed forward in the esophagus by the rhythmic contractions of its walls in a process known as peristalsis.

The esophagus extends posteriorly and dorsally within the thorax, then passes through the diaphragm into the abdominal cavity where it ends at the stomach

In order to find the esophagus use a probe to move the diaphragm away from the heart towards the abdominal cavity. Then with a pair of scissors, cut the left and right edges of the diaphragm slightly, just enough for you to completely pull it away from heart. Then gently use your probe to left the heart slightly to expose the esophagus underneath.

To view it from the abdominal cavity, locate the stomach and move the diaphragm up towards the heart. Then to the left corner of the stomach towards the liver and up against the diaphragm, you should be able to view the connection point of the esophagus to the stomach.

Urinary Bladder

Urinary Bladder — in the fetal pig the urinary bladder is an elongated sac in the lower ventral abdominal cavity. It lies between the prominent umbilical arteries and is seen when the portion of the body wall supporting the umbilical cord is folded down. You can use your probe to separate it out from the umbilical cord by sticking it underneath the point of connection and breaking through the connective tissue to free it from the skin of the umbilical cord.

Large Intestine

Large Intestine: the large intestine is joined at the bottom of the small intestine. Follow the coils of the small intestine. The end of the ileum enters the large intestine. At this juncture the cecum, a short blind sac about 2 cm. long, is formed. This organ takes about sixteen hours to finish the process of the digestive system. Food is no longer broken down at this stage of the GI tract. The colon absorbs vitamins such as K, B12, thiamine and riboflavin. The large intestine also compacts feces and stores fecal matter in the rectum until it can be released by the anus in defecation.

      · Cecum: the cecum connects the ileum with the ascending colon of the large intestine.

      · Ileocecal Valve: the ileocecal valve is situated at the junction of the small intestine (ileum) and the large     intestine. This passage limits the reflux of the colonic contents into the ileum.

      · Ascending Colon: the ascending colon is the first section of the large intestine. The main function of this section is to remove the water and other nutrients from waste material and recycle it back into the body by the blood stream. The waste material is then moved upwards towards the transverse section of the colon by peristalus (muscle contraction).

      · Transverse Colon: the transverse colon is the second section of the large intestine and connects the ascending colon to the descending colon. The transverse colon expulses the left over water and nutrients that came from the ascending colon into the blood stream and uses peristalus (muscle contraction) and carries the waste to the descending colon.

      · Descending Colon: the descending colon is the third section of the large intestine and is connected to the transverse colon and the sigmoid colon. The descending colon expulses of any left over water and nutrients into the blood stream and is moved to the sigmoid colon by peristalus (muscle contraction).

      · Sigmoid Colon: the sigmoid colon is the fourth section of the large intestine that is connected in between the descending colon and the rectum. The sigmoid colon also absorbs water and nutrients into the blood stream for recycling.

      · Rectum: the rectum is the fifth and final section of the large intestine that is connected between the sigmoid colon and the anus. It is the posterior dorsal portion of the large intestine and descends along the midline through the pelvic girdle to the onus, the intestinal opening to the exterior. The rectum, just like the rest of the large intestine, is a temporary storage place for fecal matter. When it becomes full the pressure from the fecal matter forces the anal canal apart making the fecal matter enter the canal and uses peristalic waves (muscle contraction) to propel the fecal matter out of the rectum forcing the anal muscles up.  

  · Anus: the anus is the intestinal opening to the exterior of the body. It is found underneath the pig's tail and expels the feces (waste) out of the body using peristalus (muscle contraction).

The spiral colon is a compact coiled mass clearly visible upon the left ventral surface. It is shorter, darker, and thicker than the small intestine. It is the major portion of the large intestine.

Anus

Liver

Liver — This dark brown organ dominates the upper abdomen. The falciform ligament, a ventral peritoneal membrane attaches the liver to the diaphragm and to the ventral body wall. The coronary ligament attaches the dorsal portion of the liver to the central tendon of the diaphragm.

The falciform ligament lies in a cleft of the liver which divides it into right and left halves. Five lobes can be differentiated. The four principal lobes may be seen from the ventral aspect, they are the right lateral, right central, left central and left lateral. A very small lobe, the caudate lobe, may be seen when the intestinal coils are moved to the left. It is attached to the posterior surface of the right lateral lobe.

Gall Bladder — Lift the right central lobe of the liver and expose the gall bladder embedded within a depression in its dorsal surface. This sac-like structure stores bile secreted by the liver and releases it into the duodenum. Bile is transported by the cystic duct from the gall bladder. It is joined by the hepatic duct from the liver to form the common bile duct which enters the duodenum.

Pancreas

Pancreas — Lift the main portion of the small intestine. Expose the stomach and duodenum. Observe the pancreas, a lobulated glandular structure lighter in color than the neighboring intestines. Its main portion lies in the loop of the duodenum. An elongated portion may be observed extending to the left, toward the stomach and spleen. Parts of the gland may also be seen along the dorsal body wall extending to the right of the duodenum and along the dorsal midline. The pancreatic duct enters the duodenum. It is small and difficult to find since it is embedded in glandular material.

Spleen

Spleen — this dark-colored elongated organ can readily be seen in the left side of the abdominal cavity without moving any other organs. It lies to the left of the stomach, along its greater curvature, and extends toward the right. It is tied to the stomach by a portion of the greater omentum, a specialized fold of the peritoneum, known as the gastrosplenic ligament.

Kidneys

Kidneys — this reddish-brown bean-shaped organ lays embedded retroperitoneal, or behind the parietal peritoneum, in the dorsal body wall, one on each side. To locate them you must lift the small and large intestines to reveal the dorsal body underneath, and then use your probe to break through the mesentery cover them in order to view them completely. You will be able to identify where they are by seeing an oval shaped lump through the mesenteries on both the left and right sides of the dorsal body.

Small Intestine

Small Intestines: the small intestines are located under the liver and the stomach. The small intestines contain millions of villi which contains microvilli so that the nutrients from the stomach and duodenum can be absorbed efficiently with such a large surface area.

Duodenum: the first portion of the small intestine is the duodenum. It is a continuation of the pyloric end of the stomach. It is a short “U” shaped tube, approximately 1 cm. long. The common bile duct and the pancreatic duct open into the duodenum. It uses enzymes to break down food in the small intestine and regulates the rate of which the stomach empties with hormonal pathways. Secretin and chloecytokinin are released from the cells in the duocanal epithelium in response to acidic and fatty stimuli when the pyloris opens and releases gastric chyme into the duodenum for further digestion.

Jejunum: The second section of the small intestine is the jejunum, which makes up about half the length of this organ. The jejunum lies between the duodenum and the ileum. The jejunum is specialized in the absorption of carbohydrates and proteins. What is left over from the stomach and the duodenum is soaked into the villi of the jejunum and is carried into the blood stream.

Ileum: the ileum is the final part of the small intestine and is connected to the jejunum and is separated from the cecum by the ileocecal valve. The ileum mainly absorbs vitamin B12 and bile salts as well as other digestion products that were not absorbed by the jejunum with the villi and microvilli that are a part of the folds in the ileum's inner wall.

The coils of the small intestine are held in place by a fine peritoneal membrane, the mesentery. It may be observed when lifting a coil of the small intestine and stretching the two ends. A fine, thin membrane, the mesentery, will be visible. It is responsible for the coiling observed.

Stomach

Stomach — This muscular pouch lies on the left side in the upper abdomen. It is the continuation of the esophagus. Find the esophagus and locate where it pierces the diaphragm to join the stomach. This is the cardiac end of the stomach. The fundus is the dilated anterior portion; the body is the main portion, while the pyloric region is the most posterior. This end joins the duodenum.

The cardiac sphincter controls the entrance of food into the stomach from the esophagus. The pyloric sphincter at the posterior end regulates the release of partially digested food (chyme) into the duodenum. Look along the inner walls of the stomach and note the rugae, or folds which help to churn and mix the food with digestive juices.

Mesenteries

The Mesenteries: make up the double layer extensions of the pericardium, they support most of the organs in the abdominal cavity. To locate the mesenteries, look for a layer of tissue that is lining the abdominal cavity. Some parts of the mesentery are attached to the organs to hold them in place.

Salivary Glands

To find the salivary glands, you must turn the fetal pig on its side first to get a better view of its cheeks. Then carefully cut through the skin along the side of the head between the ear and the mouth. Separate the skin from the underlying tissues and remove it to expose more of what is underneath. The platysma and other muscles associated with the skin are also removed to expose the salivary glands and ducts. This is to be don’t very carefully since the structure to be examined lie directly below the skin.

Parotid Gland — This is the largest of the salivary glands. It lies ventral to the pinna (external ear) and is recognized by its triangular shape and lobular texture. The parotid duct, can be seen emerging from the anterio-ventral edge of the gland by several roots. It then crosses the lower portion of the masseter muscle of the cheek, following the course of the prominent external maxillary vein. It continues to the mouth and opens opposite the upper fourth molar. You can find the opening by looking inside the cheek and tugging lightly on the duct with forceps.

Submandibular Gland — Most of this gland lies beneath and ventral to the parotid gland just posterior to the angle of the jaw. It is small and oval in shape. Separate the two glands. Its duct is hard to trace since it passes amongst some of the jaw muscles. It extends to the angle of the jaw, passes to the floor of the mouth, to the anterio-ventral connection of the tongue. At this point it opens into the mouth.

Sublingual Gland — This is the third salivary gland. It is flat and narrow and lies along the duct of the submandibular gland. Its duct parallels that of the submandibular and both open under the tongue, along its side.

The Digestive System

To examine the digestive system of the fetal pig, we cut down the center of the fetal pig, and around the umbilical cord. We made several horizontal cuts to make it easier to fold back the flaps of skin, to make the digestive system more accessible. Scissors are helpful when cutting through the ribs. Make sure to cut carefully and not too deeply, to avoid damaging the organs in the thoracic and abdominal cavity. 

Pull away the flaps of skin and muscle to expose the organs of the digestive system.

Video - Dissection Overview

Lets Take a Closer Look- this video shows an overview of the entire abdominal dissection pointing out the major parts included in it.

Muscles of Neck

Muscles of the Shoulder & Neck:

Acromiodeltoid: This is an elongated slender muscle extending over the anterior surface of the shoulder to the forelimb. To find this muscle you must expose the lateral surface of the upper forelimb and shoulder area by pulling back the skin first. Then toward the very top of the shoulder near the brachiocephalic, you must once again find the point of separation and break through it using the probe to separate out the muscle. Then find the other separation point about a half an inch below and break through it to isolate the muscle.

Masseter muscle: This is located on the neck of the pig slightly above the muscles of the throat. To locate this muscle, you do not have to do anything other than see where your previous throat muscles are located and look at the area slightly above them on either the left or right of the pig and there it is.

Muscles of Shank

Muscles of the shank:

The muscles of the shank are exposed once the skin is removed and can be identified once separated from one another.

Medial View of the Gastonemius:

Lateral View of the Gastronemious:

Gastronemius: This is the large muscle in the calf. To isolate this muscle, use your probe to remove all fat and connective tissue around the shank and lower hind leg so that you will be able to clearly see the separation points of the muscles in the area. On the lateral side, look for a separating point that runs vertically from the middle of the hind leg and break through it with your probe. This should go through to other side separating the muscle from the bone and other muscles attached to it.

Soleus: This muscle lies deep in the gastronemius. Once the gastronemius is isolated, look inside towards the middle of it for a separation point under the fat and connective tissue and break through once found. Keep breaking through the connective tissue to the other separation point to isolate. You may pull out (do not detach) the muscle for a better view if needed.

Tibialis anterior: This is the most ventral muscle of the shank and lies upon the tibia. Once the skin is removed this muscle is easily identifiable and is located at the front of the shank under the knee and above the ankle. Find the two separating points in the front under the fat and connective tissue and break through them to expose the wide muscle of the tibia.

This group of deeper muscles of the lower hind leg originates from the tibia and the distal end of the femur:

Extensor digitorum longus: This muscle is shaped like a narrow band and ends as a long, tough tendon. It is located beside the tibialis major and can be found by locating the edge of the muscle from the separation point between it and the tibialis and continuing to break the connective tissue from underneath to the next separating point from its neighboring muscle.

*Be sure not to rip the muscle off on the other hand as it does turn to a thin tendon on the bottom.

Peroneus longus: This muscle can be located beside the extensor digitorum longus and can be found in the same way as well. All you have to do is follow the separation point from the first side that has already been exposed all the way through to other with your probe separating out the muscle from the others.

Peroneus tertuis: This is the last of the deeper muscles of the shank located next to the peroneus longus. It can be found by locating the last separation point by breaking through the connective tissue and isolating the thin muscle.

Muscles of the Neck and Throat

The muscles of the neck and throat include the Sternohyoideus, Sternothyroideus, and the Sternomastoideus.

Sternohyoideus

Sternohyoideus Close up

The Sternohyoideus is the most ventral. To isolate this muscle:

-Remove any connective tissue to reveal the underlying muscle.

-Locate the edges of the Sternohyoid and use blunt probe to gently separate from the muscles located under it.

-There is one band of the Sternohyoid muscle on either side of the Sternothyroideus.

Sternothyroideus

The Sternothyroideus is located under the Sternohyoid. To isolate this muscle:

-Cut and reflect the sternohyoid and use blunt probe to separate the muscle from the connective tissue surrounding it.

-Find the flaps of the Sternothyroid and use blunt probe to gently separate from the connective tissue a little.  

Sternomastoideus

The Sternomastoideus is the large “U” shaped muscle located in the ventral and lateral surfaces of the neck. To isolate this muscle:

-Use blunt probe to move any remaining connective tissue.

-Insert blunt probe into the neck, in between the Sternohyoideus and Sternothyroideus and the side of the head.

-Look for the “U” shaped muscle.

Muscles of the Lower Forelimb

Muscles of the Lower Forelimb include the Extensor carpi ulnaris, Extensor digitorum lateralis, Extensor digitorum communis, Extensot carpi radialis and the Brachioradialis. The muscles are located along the lower forelimb and work together to extend and flex the foot and the digits.

-To isolate these muscles remove the connective tissue from the lower forelimb with a blunt probe.

-Look for the muscle separations and use blunt probe to separate the different muscles. 

The muscle are labeled in order below:

Extensor carpi ulnaris

Extensor digitorum lateralis

Extensor digitorum communis

Extensot carpi radialis

Brachioradialis