Fetal Pig

Digestive

 
                                               
       
       
       
       
       
       
       
       
                                       
  Introduction

SALIVARY GLANDS

Turn the fetal pig on its side. 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. The platysma and other muscles associated with the skin are also removed. Expose the salivary glands and ducts. This is to be done very carefully, since the structures to be studied lie right below the skin.

In the cheek area note the thick masseter muscle. This is the major muscle used in chewing. It originates upon the zygomatic arch (cheek bone), and inserts upon the mandible. When it contracts, it brings the jaws together.

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, or Stensen's 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.

Note: do not confuse the branches of the facial nerve leading to the facial muscles with the parotid duct. The dorsal buccal branch and the ventral buccal branch of the facial nerve also cross the masseter muscle from beneath the edges of the parotid gland. The parotid duct is generally thicker than the nerve branches. 

Small buccal glands lie beneath the skin of the lips.

Submaxillary Gland (Mandibular 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, Wharton’s 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 submaxillary gland. Its duct parallels that of the submaxillary and both open under the tongue, along its side.

ORAL CAVITY

With your scissors cut through the corner of the mouth on each side in a posterior direction. Continue cutting through the angle of the jaw. Expose the entire tongue. The interior of the oral cavity may now be examined.

Vestibule — This refers to the area between the lips and the teeth.

Tongue — This elongated muscular structure is readily visible upon the floor of the mouth. It is attached vertically along much of its length by a membrane, the lingual frenulum, and posteriorly to the hyoid bone. The surface of the tongue is covered by variously shaped projections known as sensory papillae. The greatest number of large fibrous papillae are to be seen at the anterior edge of the tongue. Microscopic taste buds are found at the sides and base of the papillae.

     Palate: This structure forms the roof of the mouth. It is a partition which separates the oral from the nasal cavity.

     Hard Palate: This is the bony anterior portion of the palate. A series of transverse ridges, the palatine rugae, cross the roof of the mouth.

     Soft Palate: This is the posterior continuation of the palate. It is a muscular structure with bony support. It divides the oropharynx ventrally from the nasopharynx dorsally. In man there is a finger-like process, the uvula, which hangs down from its center posteriorly. It is absent in the pig.

Slit the soft palate longitudinally and observe the nasopharynx. The Eustachian tubes pass from the latero-dorsal wall of the nasopharynx to the middle ear. At the anterior end you will find the openings of the internal nares. They are continuous with the external nares, or nostrils.

Near the entrance to the nasopharynx find the isthmus of fauces, the opening from the oral cavity into the oropharynx.

Epiglottis — This cone-shaped flap of cartilage is located at the top of the larynx (voice box) near the base of the tongue. It protects the glottis, the slit-like opening to the trachea. During swallowing and eating the epiglottis prevents food from entering the trachea.

Trachea — This tube is commonly called the windpipe. It is topped by the epiglottis and larynx. It is kept open by rings of cartilage which extend around the trachea at intervals. They are incomplete dorsally. The trachea branches to form two bronchi which enter the lungs.

Esophagus — This muscular tube, located dorsal to the trachea is also known as the gullet. Unlike the trachea, however, it is collapsed. Food is pushed forward in the esophagus by the rhythmic contractions of its walls, a process knows 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 trachea and esophagus use two wooden probes. With one, penetrate the glottis and pass into the trachea. Move the probe up and down and observe the movement of the trachea. With the second probe enter the esophagus dorsal to the glottis. Move it up and down and observe the corresponding movement of the esophagus.

ABDOMINAL CAVITY

The muscular diaphragm separates the upper from the lower ventral body cavity. The upper is the thoracic; the lower is the abdominal cavity. We shall study the abdominal area first and later consider the thorax in relation to the study of the heart and circulatory system.

With your fingertips locate the lower edges of the ribs. Your fingertips will be tracing an arc, an inverted letter “V”.   (Click here for mapping illustration)  Make the cuts in the order of the numbers indicated, beginning with No. 1. Do not make incisions No.5 and No.6 until you have completed the observations of the abdominal viscera and you are ready to observe the thoracic organs. This will prevent the thoracic area from drying out prematurely.

Use your scalpel to cut the musculature along the line you have traced with your fingertips and indicated as No. 1 on the photo. Do not cut too deeply. The skin and muscles of the fetal pig are very thin and soft. A sharp scalpel in an untrained hand may lead to the destruction of internal organs and possible injury to the student.

Continue with incision No. 2. This will bring you just above the umbilical cord. Cut around the cord (incision No. 3) to avoid injury to the umbilical arteries, umbilical vein, the urinary bladder and the penis (in males). Extend incision No. 4 to the rear body wall.

After the muscle layers have been cut you will find a fine membrane, the peritoneum, which lines the inside of the abdominal cavity. The portion of this serous membrane that you see is the parietal peritoneum; the visceral peritoneum covers the abdominal viscera. Cut through the peritoneum; fold back the entire ventral abdominal wall to expose the organs below. You will note that the muscular wall below the umbilical cord cannot be lifted. This is because of the umbilical vein which passes into the liver. It is necessary to cut the vein at this time to expose the abdominal cavity.

Some specimens may contain excess preservative fluid, coagulated blood, or dye that has escaped from the blood vessels. In these cases it is first necessary to wash out the abdominal cavity. Hold the pig under a moderate flow in the sink and rinse gently. Use paper towels to soak up excess water.

Identify the following structures:

Diaphragm — This dome-shaped muscular wall separates the thoracic from the abdominal cavity. It is also the most important muscle for breathing, permitting inhalation and exhalation. Three major vessels pass through the diaphragm between the thorax and the abdomen. These are the aorta, posterior vena cava, and the esophagus.

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.

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.

Open the stomach with your scissors by cutting along the greater curvature of the stomach, on the left side. Wash out the contents of the stomach. Note the cardiac sphincter which 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.

The green debris found in the stomach and elsewhere in the digestive tract is called meconium. Since the animal is still in the fetal state it does not represent food actually eaten. It is a combination of bile-stained mucus, epithelial cells sloughed off from the skin and lining of the digestive tract, and amniotic fluid swallowed by the fetus. It will be discharged in the first bowel movement of the newborn.

Small Intestine — 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. The second section of the small intestine is the jejunum, which makes up about half the length of this organ. The ileum is the final section. Open the jejunum or ileum, wash its contents and touch the inner surface with your fingertips. The velvety texture felt is due to the presence of numerous villi along the inner walls. Use a hand lens or a low power dissection microscope to observe them more clearly.

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. Note its shiny thin appearance. It is interlaced with narrow blood vessels, lymphatic vessels, adipose tissue, and lymph nodes. Some of the tiny blood vessels form the beginnings of the portal system, transporting digested food from the intestine to the liver. Cut through the mesentery to unravel the small intestine. Measure its length. How does it compare to the relative length of man’s intestine (about twenty feet)?

Large 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. In some organisms such as horses, this section is enlarged and houses microorganisms which can digest cellulose. Humans possess a vermiform appendix that projects from the end of a short cecum. Cut into the cecum at about the point where the ileum enters. Wash out its contents, look for and locate the ileocecal valve.

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. The posterior dorsal portion of the large intestine is the rectum. It descends along the midline through the pelvic girdle to the onus, the intestinal opening to the exterior. The colon of human beings is relatively shorter than that of the fetal pig and is not coiled.

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 human pancreas is much more compact. Its duct, the pancreatic duct, enters the duodenum. It is small and difficult to find since it is embedded in glandular material.

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.

Kidney — this reddish-brown bean-shaped organ lays embedded retroperitoneally, namely, behind the parietal peritoneum, in the dorsal body wall, one on each side. The adrenal gland is located near the anterior end of the kidney, but is separated from it, lying slightly media1 of the kidney. In humans the adrenal gland forms a cap upon the kidney.

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.

Reproductive Structures — Most of the female reproductive structures and some of the male’s are located in the abdominal cavity. The urogenital system and its associated structures will be studied and more fully discussed in a later chapter.