Zoology Notes: 012 Chapter 8

Chapter 8. The Digestive System Green plants build their tissues from inorganic materials by the photosynthetic process, using energy from the sun. The food of animals is obtained by eating plants or other animals. It serves two purposes, as a fuel to supply bodily energy and as a source of materials for growth and repair. The digestive system consists of a musculo-membranous tube extending from the mouth to the anus. Its functions are prehension, ingestion, grinding, digestion and absorption of food, and elimination of solid wastes. The digestive system reduces the nutrients in the food to compounds that are simple enough to be absorbed and used for energy and building other compounds for metabolic use. The main functions of digestion are ingestion, digestion and absorption of ingested food and elimination of solid wastes. The principal segments of the digestive system are the mouth and mouth (buccal) cavity, pharynx, esophagus, stomach, small intestine, and the large intestine. Accessory organs/glands (liver, pancreas, salivary glands, etc.) located outside the tubular portion of the alimentary tract originate as epithelial evaginations from the digestive tube. For the most part, these organs produce digestive enzymes that are used during the process of digestion. Food that is being digested does not directly enter any of the accessory organs. Instead, the accessory organs/glands produce their secretions and these are released into the alimentary canal where they do their job. Their chief function is to penetrate, and empty their secretory products into, the digestive tract. The Digestive Tract The alimentary canal, or gastro-intestinal (GI) tract is where food passes through directly. It is a one way tube with two openings; the mouth (entrance) and anus (exit). The Mouth and Mouth Cavity. The mouth is used primarily for grinding food and mixing it with saliva, but may also serve as a prehensile (grasping) mechanism and as a defensive and offensive weapon. The teeth and tongue are surrounded by lips, cheeks, and muscles to operate the jaws. Functions of the oral (buccal, mouth) cavity and associated structures include prehension (taking of food into the mouth), mastication (chewing), insalivation, bolus formation, and deglutition (swallowing). Lips. The lips are striated muscles covered with stratified squamous epithelium. The muscles of the lips cause opening and closing of the mouth cavity. It also functions as a prehensile organ in some animals. Cheeks. The cheeks, or side walls of the mouth, are made up of several accessory muscles of mastication, notably the buccinators, which also prevent food from escaping the chewing actions of the teeth. Teeth. Deciduous teeth (milk teeth) erupt first and are replaced by permanent teeth. The teeth function mechanically to separate food. It is composed of a crown, the exposed portion, which has an external covering of hard enamel and internal ivory substance, the dentin. The portion embedded in the jaw, the root, is covered by cementum. The central pulp contains blood vessels and nerves. Periodontal tissue, a tough fibrous membrane, lines the tooth socket supplying nourishment and acting as shock absorbers. The teeth are also an important indication of the age of some animals. Types of Teeth: 1. Incisors – are used for cutting 2. Canines (eye teeth, bridle teeth, tusks, tushes) – are used for grasping 40

3. Premolars and Molars (cheek teeth) – are used for grinding Tongue. The floor of the mouth contains the tongue, a highly movable structure that is composed of muscle fibers and is covered with numerous taste buds. The tongue mixes the food with saliva and keeps the food between the teeth when chewing before it pushes the food backward for swallowing. It also has importance as a prehensile tool in some animals. Palate. The palate or the roof of the mouth consists of two parts: 1. Hard palate – anterior portion 2. Soft palate – posterior portion. The soft palate is composed of muscles ending in a free projection called the uvula. The opening of the pharynx lies behind the uvula. Salivary glands. There are three major pairs of salivary glands:

Fig. 8.1. Teeth anatomy.

1. Parotid glands – are located in the subcutaneous regions of the cheek, anterior and inferior to the ears, and open just opposite the second upper molar 2. Submaxillary glands – are located in the floor of the mouth close to the angle of the jaw and open laterally to the point where the frenulum attaches to the tongue 3. Sublingual glands – are located under the mucous membrane of the floor of the mouth lateral to the tongue and open either near the tongue or into the submaxillary duct Pharynx. The pharynx is the portion of the digestive tract that serves as a passageway for both food and air. It permits an individual to breathe through the mouth when the nasal passages are obstructed. The epiglottis, a little flap of flesh and cartilage at top of esophagus, prevents food from entering the windpipe or trachea. Esophagus. The esophagus is a muscular canal that carries food from the lower part of the pharynx to the stomach. Muscles of the canal produces wave contractions, moving the food down the stomach. Mucous glands in the wall of the esophagus secrete mucus that helps the movement of food by keeping the surface of the esophagus moist & slippery. Stomach. The stomach is located just behind the left side of the diaphragm. Its parts include the cardia, the body, and pylorus. The cardia and pylorus have sphincter muscles (cardiac sphincter and pyloric sphincter) that control the passage of food through the stomach. Immediately surrounding the cardia is an area of stratified squamous epithelium called the esophageal region. Other regions of the stomach are the cardiac gland region, the fundic gland region, and the pyloric gland region. The stomach’s functions are: 1. digestion of food 2. mechanical churning of food until chyme (murky, semi-fluid mass of partially digested food) is formed 3. permitting the food to empty slowly into the duodenum at a rate suitable for proper digestion and absorption in the small intestine Small intestine. The small intestine is the principal region for digestion & absorption. It extends from the distal end of the pyloric sphincter to the cecum, the first portion of the large intestine. It is divided in to 3 portions: the duodenum (shortest, widest, and most fixed portion), jejunum, and ileum. The functions of the small intestines are:

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1. Secretion – The Crypts of Lieberkuhn are small tubular depressions between the villi of the small intestine and represent a major source of enzymes produced by the mucosa of the small intestine. 2. Motility – Intestinal content is moved towards the rectum by waves of contraction called peristalsis. 3. Digestion – with the help of digestive enzymes 4. Absorption – Villi (finger-like projections) contain blood capillaries & small lymphatic vessels used for absorption of nutrients. Large Intestine. The large intestine extends from the end of the ileum to the anus and is divisible into the: cecum (the first portion to which the appendix is attached), colon (ascending, transverse, and descending), rectum and anus. Its functions include being the storehouse for feces transferred into the rectum by peristaltic movement, reabsorption of water, and synthesis of nutritional factors with the help of natural microbial flora. Rectum and Anus. Feces is temporarily stored in the rectum. The anus is the terminal segment of the digestive tract. Feces is eliminated (egestion) from the body through the anus. Accessory Organs of the Digestive System Pancreas. The pancreas is a large, lobulated gland resembling the salivary gland. It has both exocrine and endocrine functions. Pancreatic juice, a digestive juice, is the product of the exocrine pancreas. The pancreatic ducts connect the pancreas to the duodenum. Liver. The liver is the largest gland in the body & one of the most complex organs. It produces bile, a greenish yellow digestive fluid used for the breaking down of fats. The many complex functions of the liver are: 1. Excretion – of waste products 2. Secretion – of bile 3. Storage – of glycogen, lipids, vitamins A, D, E, K as well as B12 4. Synthesis – of fibrinogen, globulins, albumin, prothrombin) 5. Phagocytosis – of foreign particulate matter 6. Detoxification – of lipid-soluble drugs 7. Conjugation – of toxic substances, steroid hormones 8. Esterification – of free fatty acids to triglycerides 9. Metabolism – of carbohydrates, fats, proteins, hemoglobin, drugs 10. Hemopoiesis – in the embryo and potentially in the adult

Fig.8.2. The digestive tract and accessory organs.

Gall Bladder. All domestic animals except the horse have a gall bladder. The gall bladder is a saclike structure attached to the inferior surface of the liver and serving as a reservoir of bile. Bile consists mainly of water, cholesterol, pigments, inorganic salts, salts of bile acids and phospholipids.

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