Gross Anatomy

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Exercise 20.1

Using the models and your textbook identify and label all of the organs and structures of the digestive system in Figures 20.1, 20.2, and 203.

The oral cavity has as its walls the cheeks, for its roof the anterior hard palate and posterior soft palate, and for its floor muscles of the throat and neck. Hanging from the posterior border of the soft palate is the uvula.

When food is taken into the oral cavity it is broken into smaller particles by the action of the teeth and tongue (mechanical digestion) and then mixed with saliva which begins the process of chemical digestion. Saliva is secreted by three pairs of salivary glands (discussed below).

The teeth from anterior to posterior on each side are two incisors, one cuspid, two premolars (bicuspids), and three molars (including the often extracted third molar or wisdom tooth).

Molar Root Canal Rooting

Figure 20.1 Tooth

Alveolar process Cementum Crown Dentin Enamel Gingiva Neck Periodontal ligament Pulp cavity Root Root canal

Each tooth is held firmly in place by the periodontal ligaments within the sockets formed by the alveolar processes of the jaw bones. The alveolar processes are covered with gingivae or gums. Each tooth typically consists of three principal portions: the crown is the exposed part of the tooth, the root(s) which extend(s) into the socket, and the neck which connects the crown and the root(s). The bulk of the tooth is composed of a calcified connective tissue called dentin. Within the dentin is a pulp cavity containing pulp with nerves and blood vessels. Narrow extensions of the pulp cavity extend down the root(s). These narrow extensions are the root canals. Dentin is held to the ligaments by cementum, another bone-like substance. A final very hard substance called enamel covers the crown.

The tongue is composed of skeletal muscle covered with mucous membrane. The superior surface of the tongue is covered with numerous papillae (sing. - papilla). These small bumps are classified into three types: filiform, fungiform, and circumvallate. The filiform papillae are located in parallel rows on the anterior two thirds of the tongue. They are whitish, contain no taste buds, and provide some small amount of friction for the movement of food by the tongue. The fungiform papillae are mushroom-shaped (hence their name) and are distributed among the filiform papillae. Most of them contain taste buds. The circumvallate papillae are located in a V-shaped row on the posterior surface of the tongue. All of them contain taste buds.

Sour Cheek Dissection
Figure 20.2 The Tongue

Bitter receptors Circumvallate papilla Filiform papilla Fungiform papilla Salt receptors Sour receptors Sweet receptors

The parotid salivary glands are located in the cheeks anterior to the ears. Saliva is secreted into ducts which empty into the roof of the mouth from these glands.

The submandibular salivary glands are located in the floor of the mouth just inside the body of the mandible. Anterior to the submandibular glands are the sublingual salivary glands located underneath the tongue.

After food has been chewed and mixed with saliva it is passed into the oropharynx through an opening called the fauces by the action of the tongue and other

Pancreatic Duct Alimentary Canal
Figure 20.3 Alimentary Canal and Accessory Structures

Ampulla of Vater Anal sphincter Appendix Ascending colon Body of Stomach Cardia Cardiac sphincter Cecum Common bile duct Cystic duct Descending colon Esophagus Fundus Gall bladder Hepatic duct Ileocecal valve Liver Mesentery Oropharynx Pancreas Pancreatic duct Parotid salivary gland Pyloric sphincter Pylorus Rectum Sigmoid colon Sublingual salivary gland Submandibular salivary gland Tongue Transverse colon muscles. The pharynx or throat is the tube joining the oral cavity, nasal cavity, esophagus, and larynx. It is divided into three regions: the nasopharynx, oropharynx, and laryngopharynx.

The esophagus is a tube extending from the pharynx to the stomach. Food passes through the esophagus by the action of its muscular wall.

The opening into the stomach is through the cardiac valve, a sphincter muscle. The stomach has four regions. The first region, where food enters from the esophagus, is the cardia. The upper, rounded portion to the left of the cardia is the fundus. The bulk of the stomach is the body and the inferior, narrow portion is the pylorus. The food passes into the small intestine through another sphincter muscle, the pyloric valve. The wall of the stomach has many wrinkles, termed rugae (pronounced roo-jee), which assist in the churning of food.

The small intestine is a muscular tube approximately twenty-one feet long. It is divided into three regions: the upper ten inches form the duodenum which joins to the stomach at the pyloric sphincter, the jejunum which accounts for the next eight feet of the intestine, and the final twelve feet, the ileum. (Note the spelling of "ileum": "ilium" is part of the os coxa; "ileum" is a portion of the small intestine.)

Liquids enter the duodenum through ducts from the pancreas, liver, and gall bladder. The pancreas is located posterior to the body of the stomach. It secretes digestive juices which aid in the chemical breakdown of food.

The liver is composed of two principal lobes, the right lobe and the left lobe, separated by the falciform ligament. Another ligament, the round ligament, extends from the falciform ligament to the umbilicus. The liver excretes bile, a waste product, and bile salts which aid in the digestion of lipids. Bile and bile salts are stored in the gall bladder until needed.

Most food absorbed in the digestive system passes to the liver through the portal vein before moving on to the rest of the body. Carbohydrates, proteins, vitamins, minerals, and other nutrients enter the liver and are either stored, processed, or sent on to the body via the hepatic veins.

Poisons arc detoxified or stored in the liver. Other functions of the liver include the destruction of dead/dying red blood cells (a major component of bile) and the activation of vitamin D.

Bile from the liver passes through the hepatic duct to the gall bladder. The gall bladder is emptied by the cystic duct which becomes the common bile duct where it joins the hepatic duct. The hepatic duct joins with the pancreatic duct from the pancreas. The short tube connecting the common bile and pancreatic ducts to the duodenum is the ampulla of Vater.

The wall of the small intestine is characterized by the presence of many finger-like extensions called villi (sing. - villus). The villi greatly increase the surface area of the small intestine for absorption of food into the blood.

Each villus contains blood vessels which transport small molecules from the intestine to the liver for processing and a lymph vessel called a lacteal, which transports lipids into the lymphatic system (see chapter thirty).

The ileum empties into the large intestine or colon through the ileocecal valve, another sphincter. The large intestine has four parts, the ascending, transverse, descending, and sigmoid colons.

The ascending colon, the portion into which the ileum empties, ascends up the right side of the abdominal cavity to the liver. The lower end of the ascending colon is the cecum. Extending inferiorly from the cecum is a narrow tube the appendix or vermiform appendix.

The transverse colon crosses the body to the descending colon which descends down the left side of the body to the sigmoid colon. (The Greek letter "sigma" is "S "-shaped, like the colon).

The sigmoid colon empties waste into the rectum where it is stored until eliminated. Two inferior anal sphincters control the release of waste from the rectum.

The organs of the digestive system are held in place by a two-layered serous membrane, the peritoneum. The layer of the peritoneum which is closest to the organs of the abdominal cavity is the visceral peritoneum. The layer which is closest to the body wall is the parietal peritoneum. As was discussed in chapter one, the peritoneum is a single membrane which is folded so as to appear to be two membranes (see Figure 1.8).

The peritoneum has several divisions. The mesentery contains blood vessels for, and holds the small intestine. The mesocolon performs the same functions for the large intestine.

Other extensions of the peritoneum form the falciform ligament, greater omentum, and lesser omentum. The falciform ligament attaches the liver to the anterior abdominal wall and to the diaphragm. The lesser omentum suspends the stomach and duodenum from the liver. The greater omentum drapes over and folds behind the small intestine.

Exercise 20.2

Study the model of the digestive system. Be able to identify the following structures on the model: appendix, ascending colony cardiac sphincter, cecum, common bile duct, descending colon, duodenum, esophagus, falciform ligament, gall bladder, hard palate, hepatic artery, ileocecal valve, ileum, jejunum, liver, pancreas, pancreatic duct, pharynx, portal vein, rectum, round ligament, sigmoid colon, soft palate, spleen (not part of the digestive system), stomach (including: cardia, fundus, body, pylorus, and rugae), transverse colon, and uvula.

Study the models of teeth. Identify: alveolar process, cementum, crown, dentin, enamel, neck, pulp, pulp cavity, root, and root canal.

Identify the following on the model of the pancreas, duodenum, and gall bladder: common bile duct, cystic duct, duodenum, hepatic duct, gall bladder, pancreas, pancreatic duct, portal vein, and spleen.

Be able to identify the following on the model of the liver: common bile duct, cystic duct, falciform ligament, gall bladder, hepatic duct; left lobe, portal vein, right lobe, and round ligament.

On the model of the intestinal villus identify: lacteal, mucosa, muscularis, serosa, and submucosa (discussed later).

On the model of the lower jaw identify: alveolar process, bicuspid (premolar), body of mandible, buccinator\ caries (tooth decay), cementum, crown, cuspid, dentin, enamel, gingiva, incisor; masseter, molar, neck, pulp, ramus of mandible, and root

Exercise 20.3

Carefully open the abdominal cavity of the cat by making two transverse and one longitudinal incision in the shape of a capital "I". Reflect the muscles to examine the digestive system. Do not open the thoracic cavity.

Carefully pull the lower end of the greater omentum from behind the small intestine. Note how it forms a pouch to hold the intestine. The greater omentum is also called the fatty apron. Why? Leave the greater omentum attached to the stomach.

Identify the stomach, small intestine, three parts of the large intestine (the cat does not have a sigmoid colon), rectum, liver, falciform ligament, gall bladder, and the pancreas. (The spleen is also visible, but it is part of the lymphatic system.)

Identify the cardia, fundus, body, and pylorus of the stomach.

Cut the stomach open longitudinally. Note the rugae and the pyloric sphincter.

Expose the pancreatic duct by cutting away a portion of the pancreas. The duct lies within the pancreas.

Remove a two inch section of the jejunum. Slit it open and dispose of the contents. Examine the wall with a magnifying glass for villi.

Cut open the cecum to reveal the ileocecal valve.

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