《生物学》课程参考书籍文献（英文版，Chapter 49 - Chapter 60）Part XIII Animal Form and Function、Part XIV Regulating the Animal Body

Chapter 50 www.mhhe.com/raven6e biocourse. com Summary Questions Media resources 50.1 A skeletal system supports movement in animals There are three types of skeleton: hydrostatic 1. What are the two major On Science article: skeletons. exoskeletons and endoskeletons components of the extracellular matrix in bone? what structural Bone is formed by the secretion of an organic matrix loes each componen by osteoblasts; this organic matrix becomes calcified. have? How do the two components combine to make resistant to fract 50.2 Skeletal muscles contract to produce movements at joints. Freely movable joints surround the articulating bones 2. what are the three types of Bioethics case study: with a synovial capsule filled with a lubricating fluid. joints in a vertebrate skeletor Sports and fitness keletal muscles can work toget Give an example of whe ere ea er as svnei ts or oppose each other as antagonists type is found in the body 3. What is the difference tween a skeletal muscle,s origin and its insertion 50.3 Muscle contraction powers animal locomotion. A muscle fiber contains numerous myofibrils, which 4. Of what proteins are thick ience article: consist of thick filaments composed of myosin and and thin filaments composed thin filaments of actin 5. Describe the steps involved There are small cross-bridges of myosin that extend in the cross-bridge cycle. What out toward the actin the cross-bridges are activated nctions does ATP perform in by the hydrolysis of ATP so that it can bind to actin and undergo a power stroke that causes the sliding of 6. Describe the steps involved Muscle contraction he myofilaments In excitatlon-contracton coupling. What functions do Detailed straited When Cat*+ binds to troponin, the tropomyosin shifts acetylcholine and Ca++ perf position in the thin filament, allowing the cross- this process bridges to bind to actin and undergo a power stroke 7. How does a somatic motor The release of Ca** from the sarcoplasmic reticulum neuron stimulate a muscle fiber · Activity: Muscle is stimulated by impulses in the muscle fiber to contracta Muscle cell function produced by neural stimulation 8. What is the difference · Body musculature and are resistant to fatigue; fast-twitch fibers can pro- tetanus a muscle twitch and Slow-twitch fibers are adapted for aerobic respiration between Head and neck uscles vide power quickly but produce lactic acid and fatigue 9. Why can't a myocardium produce a sustained contraction Upper limb muscles Lower limb muscles 10. How does smooth muscle Muscle characteristics Cardiac muscle cells have gap junctions that permit iffer from skeletal muscle in he spread of electric impulses from one cell to the terms of thick and thin filament organization, the role of Ca++ in Cardiac and smooth muscles are involuntary and reg- contraction, and the effect of ulated by autonomic nerves the contractions are au- stretching on the muscle's ability to contract tomatically produced in cardiac muscle and some smooth muscles 11. what do all modes of locomotion have in common? Animals have adapted modes of locomotion to three different environments: water. land. and air. 1016 Part XIlI Animal Form and function

1016 Part XIII Animal Form and Function Chapter 50 Summary Questions Media Resources 50.1 A skeletal system supports movement in animals. • There are three types of skeleton: hydrostatic skeletons, exoskeletons, and endoskeletons. • Bone is formed by the secretion of an organic matrix by osteoblasts; this organic matrix becomes calcified. 1. What are the two major components of the extracellular matrix in bone? What structural properties does each component have? How do the two components combine to make bone resistant to fracture? • Freely movable joints surround the articulating bones with a synovial capsule filled with a lubricating fluid. • Skeletal muscles can work together as synergists, or oppose each other as antagonists. 2. What are the three types of joints in a vertebrate skeleton? Give an example of where each type is found in the body. 3. What is the difference between a skeletal muscle’s origin and its insertion? 50.2 Skeletal muscles contract to produce movements at joints. • A muscle fiber contains numerous myofibrils, which consist of thick filaments composed of myosin and thin filaments of actin. • There are small cross-bridges of myosin that extend out toward the actin; the cross-bridges are activated by the hydrolysis of ATP so that it can bind to actin and undergo a power stroke that causes the sliding of the myofilaments. • When Ca++ binds to troponin, the tropomyosin shifts position in the thin filament, allowing the cross￾bridges to bind to actin and undergo a power stroke. • The release of Ca++ from the sarcoplasmic reticulum is stimulated by impulses in the muscle fiber produced by neural stimulation. • Slow-twitch fibers are adapted for aerobic respiration and are resistant to fatigue; fast-twitch fibers can pro￾vide power quickly but produce lactic acid and fatigue quickly. • Cardiac muscle cells have gap junctions that permit the spread of electric impulses from one cell to the next. • Cardiac and smooth muscles are involuntary and reg￾ulated by autonomic nerves; the contractions are au￾tomatically produced in cardiac muscle and some smooth muscles. • Animals have adapted modes of locomotion to three different environments: water, land, and air. 4. Of what proteins are thick and thin filaments composed? 5. Describe the steps involved in the cross-bridge cycle. What functions does ATP perform in the cycle? 6. Describe the steps involved in excitation-contraction coupling. What functions do acetylcholine and Ca++ perform in this process? 7. How does a somatic motor neuron stimulate a muscle fiber to contract? 8. What is the difference between a muscle twitch and tetanus? 9. Why can’t a myocardium produce a sustained contraction? 10. How does smooth muscle differ from skeletal muscle in terms of thick and thin filament organization, the role of Ca++ in contraction, and the effect of stretching on the muscle’s ability to contract? 11. What do all modes of locomotion have in common? 50.3 Muscle contraction powers animal locomotion. www.mhhe.com/raven6e www.biocourse.com • On Science Article: Running improperly • Bioethics case study: Sports and fitness • On Science Article: Climbing the walls • Straited muscle contraction • Muscle contraction action potential • Detailed straited muscle • Actin-myosin crossbridges • Activity: Muscle contraction • Muscle cell function • Body musculature • Head and neck muscles • Trunk muscles • Upper limb muscles • Lower limb muscles • Muscle characteristics • Walking

5l elin ng Body Activities: Digestion Concept outline 51.1 Animals employ a digestive system to prepare food for assimilation by cells Types of Digestive Systems. Some invertebrates have a gastrovascular cavity, but vertebrates have a digestive tract that chemically digests and absorbs the food. Vertebrate Digestive Systems. The different regions of he gastrointestinal tract are adapted for different functions 51.2 Food is ingested, swallowed, and transported to The Mouth and Teeth. Carnivores. herbivores and omnivores display differences in the structure of their teeth Esophagus and Stomach. The esophagus delivers food to the stomach, which secretes hydrochloric acid and 51.3 The small and large intestines have very different The Small Intestine. The small intestine has mucosal FIGURE 51.1 folds called villi and smaller folds called microvilli that Animals are heterotrophs. All animals must consume plant absorb glucose, amino acids, and fatty acids into the blood. material or other animals in order to live. The nuts in this The Large Intestine. The large intestine absorbs water, hipmunk's cheeks will be consumed and converted to body ions, and vitamin K, and excretes what remains as feces tissue, energy, and refuse. Variations in Vertebrate Digestive Systems. Digest systems are adapted to particular diets 51.4 Accessory organs, neural stimulation, and P Plants and other photosynthetic organisms can produce the organic molecules they need from inorganic com- endocrine secretions assist in digestion ponents. Therefore, they are autotrophs, or self-sustaining Accessory Organs. The pancreas secretes digestive Animals are heterotrophs: they must consume organic mol enzymes and the hormones insulin and glucagon. The liver ecules present in other organisms(figure 51.1). The mole produces bile, which emulsifies fat; the gallbladder stores cules heterotrophs eat must be digested into smaller mole the bile cule der to be absorbed into the animals body. Neural and Hormonal Regulation of Digestion. these products of digestion enter the body, the animal can Nerves and hormones help regulate digestive functions use them for energy in cell respiration or for the construc 51.5 All animals require food energy and essential tion of the larger molecules that make up its tissues. Th process of animal digestion is the focus of this chapter Food Energy and Energy Expenditure. The intake of food energy must balance the energy expended by the body in order to maintain a stable weight. nerals, and specific amino acids and fatty acids for health 1017

1017 51 Fueling Body Activities: Digestion Concept Outline 51.1 Animals employ a digestive system to prepare food for assimilation by cells. Types of Digestive Systems. Some invertebrates have a gastrovascular cavity, but vertebrates have a digestive tract that chemically digests and absorbs the food. Vertebrate Digestive Systems. The different regions of the gastrointestinal tract are adapted for different functions. 51.2 Food is ingested, swallowed, and transported to the stomach. The Mouth and Teeth. Carnivores, herbivores, and omnivores display differences in the structure of their teeth. Esophagus and Stomach. The esophagus delivers food to the stomach, which secretes hydrochloric acid and pepsin. 51.3 The small and large intestines have very different functions. The Small Intestine. The small intestine has mucosal folds called villi and smaller folds called microvilli that absorb glucose, amino acids, and fatty acids into the blood. The Large Intestine. The large intestine absorbs water, ions, and vitamin K, and excretes what remains as feces. Variations in Vertebrate Digestive Systems. Digestive systems are adapted to particular diets. 51.4 Accessory organs, neural stimulation, and endocrine secretions assist in digestion. Accessory Organs. The pancreas secretes digestive enzymes and the hormones insulin and glucagon. The liver produces bile, which emulsifies fat; the gallbladder stores the bile. Neural and Hormonal Regulation of Digestion. Nerves and hormones help regulate digestive functions. 51.5 All animals require food energy and essential nutrients. Food Energy and Energy Expenditure. The intake of food energy must balance the energy expended by the body in order to maintain a stable weight. Essential Nutrients. Food must contain vitamins, minerals, and specific amino acids and fatty acids for health. Plants and other photosynthetic organisms can produce the organic molecules they need from inorganic com￾ponents. Therefore, they are autotrophs, or self-sustaining. Animals are heterotrophs: they must consume organic mol￾ecules present in other organisms (figure 51.1). The mole￾cules heterotrophs eat must be digested into smaller mole￾cules in order to be absorbed into the animal’s body. Once these products of digestion enter the body, the animal can use them for energy in cell respiration or for the construc￾tion of the larger molecules that make up its tissues. The process of animal digestion is the focus of this chapter. FIGURE 51.1 Animals are heterotrophs. All animals must consume plant material or other animals in order to live. The nuts in this chipmunk’s cheeks will be consumed and converted to body tissue, energy, and refuse

51.1 Animals employ a digestive system to prepare food for assimilation by cells. Types of Digestive Systems cialized in different regions for the ingestion, storage, fra mentation, digestion, and absorption of food. All higher Heterotrophs are divided into three groups on the basis animal groups, including all vertebrates, show similar sp of their food sources. Animals that eat plants exclusively cializations( figure 51.3) are classified as herbivores; common examples include The ingested food may be stored in a specialized region cows, horses, rabbits and sparrows. Animals that are of the digestive tract or may first be subjected to physical meat-eaters, such as cats, eagles, trout, and frogs, are fragmentation. This fragmentation may occur through the carnivores. Omnivores are animals that eat both plants chewing action of teeth(in the mouth of many vertebrates), and other animals. Humans are omnivores, as are pigs or the grinding action of pebbles(in the gizzard of earth bears and crows worms and birds). Chemical digestion then occurs, break Single-celled organisms (as well as sponges) digest ing down the larger food molecules of polysaccharides and their food intracellularly. Othe er anim als digest the eIr disaccharides, fats, and proteins into their smallest sub food extracellularly, within a digestive cavity. In this units. Chemical digestion involves hydrolysis reactions that case, the digestive enzymes are released into a cavity that liberate the subunit molecules--primarily monosaccha is continuous with the animals external environment. In rides, amino acids, and fatty acids--from the food. These coelenterates and flatworms(such as planaria), the diges oroducts of chemical digestion pass through the epithelial tive cavity has only one opening that serves as both lining of the gut into the blood, in a process known as ab- mouth and anus. There can be no specialization within sorption Any molecules in the food that are not absorbed this type of digestive system, called a gastrovascular cavity, cannot be used by the animal. These waste products are ex- ecause every cell is exposed to all stages of food creted. or defecated from the anus tion(figure 51.2) Specialization occurs when the digestive tract, or al mentary canal, has a separate mouth and anus, so that Most animals digest their food extracellularly. The transport of food is one-way. The most primitive digestive digestive tract, with a one-way transport of food and tract is seen in nematodes(phylum Nematoda), where it is specialization of regions for different functions, allows simply a tubular gut lined by an epithelial membrane food to be ingested, physically fragmented, chemically Earthworms(phylum Annelida) have a digestive tract spe digested and absorbed Nematode Mouth Food Wastes Mouth Anus Earthworm rop Gizzard Pharynx Anus Body stalk Intestine Salamander Stomach Cloaca Gastrovascular Anus FIGURE 51.2 FIGURE 51.3 The gastrovascular cavity of Hydra, a coelenterate. Because The one-way digestive tract of nematodes, earthworms, and there is only one opening, the mouth is also the anus, and no vertebrates. One-way movement through the digestive tract pecialization is possible in the different regions that participate allows different regions of the digestive system to become in extracellular digestion specialized for different functions 1018 Part XIII Ar I Form and function

cialized in different regions for the ingestion, storage, frag￾mentation, digestion, and absorption of food. All higher animal groups, including all vertebrates, show similar spe￾cializations (figure 51.3). The ingested food may be stored in a specialized region of the digestive tract or may first be subjected to physical fragmentation. This fragmentation may occur through the chewing action of teeth (in the mouth of many vertebrates), or the grinding action of pebbles (in the gizzard of earth￾worms and birds). Chemical digestion then occurs, break￾ing down the larger food molecules of polysaccharides and disaccharides, fats, and proteins into their smallest sub￾units. Chemical digestion involves hydrolysis reactions that liberate the subunit molecules—primarily monosaccha￾rides, amino acids, and fatty acids—from the food. These products of chemical digestion pass through the epithelial lining of the gut into the blood, in a process known as ab￾sorption. Any molecules in the food that are not absorbed cannot be used by the animal. These waste products are ex￾creted, or defecated, from the anus. Most animals digest their food extracellularly. The digestive tract, with a one-way transport of food and specialization of regions for different functions, allows food to be ingested, physically fragmented, chemically digested, and absorbed. 1018 Part XIII Animal Form and Function Types of Digestive Systems Heterotrophs are divided into three groups on the basis of their food sources. Animals that eat plants exclusively are classified as herbivores; common examples include cows, horses, rabbits and sparrows. Animals that are meat-eaters, such as cats, eagles, trout, and frogs, are carnivores. Omnivores are animals that eat both plants and other animals. Humans are omnivores, as are pigs, bears, and crows. Single-celled organisms (as well as sponges) digest their food intracellularly. Other animals digest their food extracellularly, within a digestive cavity. In this case, the digestive enzymes are released into a cavity that is continuous with the animal’s external environment. In coelenterates and flatworms (such as Planaria), the diges￾tive cavity has only one opening that serves as both mouth and anus. There can be no specialization within this type of digestive system, called a gastrovascular cavity, because every cell is exposed to all stages of food diges￾tion (figure 51.2). Specialization occurs when the digestive tract, or ali￾mentary canal, has a separate mouth and anus, so that transport of food is one-way. The most primitive digestive tract is seen in nematodes (phylum Nematoda), where it is simply a tubular gut lined by an epithelial membrane. Earthworms (phylum Annelida) have a digestive tract spe- 51.1 Animals employ a digestive system to prepare food for assimilation by cells. Gastrovascular cavity Body stalk Tentacle Mouth Food Wastes FIGURE 51.2 The gastrovascular cavity of Hydra, a coelenterate. Because there is only one opening, the mouth is also the anus, and no specialization is possible in the different regions that participate in extracellular digestion. Nematode Earthworm Salamander Mouth Mouth Mouth Pharynx Pharynx Esophagus Intestine Intestine Intestine Anus Anus Anus Crop Gizzard Liver Pancreas Stomach Cloaca FIGURE 51.3 The one-way digestive tract of nematodes, earthworms, and vertebrates. One-way movement through the digestive tract allows different regions of the digestive system to become specialized for different functions

Vertebrate Digestive Systems In humans and other vertebrates, the digestive system con sists of a tubular gastrointestinal tract and accessory diges- Salivary gland (figure 51.4). The initial gastrointestinal tract are the mouth and the pharynx, which is the common passage of the oral and nasal cavities. The Salivary gland pharynx leads to the esophagus, a muscular tube that deliv ers food to the stomach, where some preliminary digestion occurs. From the stomach, food passes to the first part of the small intestine, where a battery of digestive enzymes Th then pass across the wall of the small intestine into the bloodstream. The small intestine empties what remains into the large intestine, where water and minerals are ab- sorbed. In most vertebrates other than mammals. the waste products emerge from the large intestine into a cavity Stomach called the cloaca(see figure 51.3), which also receives the Gallbladder products of the urinary and reproductive systems. In mam- mals, the urogenital products are separated from the fecal material in the large intestine; the fecal material enters the rectum and is expelled through the anus testine In general, carnivores have shorter intestines for their Cecum size than do herbivores. A short intestine is adequate for a Appendix Colon carnivore,but herbivores ingest a large amount of plant cellulose, which resists digestion. These animals have a long, convoluted small intestine. In addition, mammals Anus called ruminants(such as cows) that consume grass and FIGURE 51.4 other vegetation have stomachs with multiple chambers, where bacteria aid in the digestion of cellulose. Other her The human digestive system. Humans, like all placent bivores, including rabbits and horses, digest cellulose(with mammals, lack a cloaca and have a separate exit from the digestive tract through the rectum and anus the aid of bacteria) in a blind pouch called the cecum lo- cated at the beginning of the large intestine The accessory digestive organs(described in detail later in the chapter) include the liver, which produces bile(a green Blood vessel solution that emulsifies fat), the gallbladder, which stores and concentrates the bile, and the pancreas. The pancreas produces pancreatic juice, which contains digestive enzymes and bicarbonate. Both bile and pancreatic juice are secreted land outside into the first region of the small intestine and aid digestion. Nerve gastrointestinal The tubular gastrointestinal tract of a vertebrate has a characteristic layered structure(figure 51.5). The innermost Mucosa layer is the mucosa, an epithelium that lines the interior of the tract(the lumen). The next major tissue layer, made of connective tissue, is called the submucosa. Just outside the submucosa is the muscularis which consists of a double have a circular orientation, and those in the outer layer are steric layer of smooth muscles. The muscles in the inner layer rranged longitudinally. Another connective tissue layer, the Muscularis Submucosal serosa. covers the external surface of the tract. Nerves. in- entwined in regions called plexuses, are located in the sub- Gland in onnective tissue layer submucosa Serosa mucosa and help regulate the gastrointestinal activities FIGURE 51.5 The vertebrate digestive system consists of a tubular The layers of the gastrointestinal tract. The mucosa contains a gastrointestinal tract, which is modified in different ning epithelium; the submucosa is composed of connective animals, composed of a series of tissue layers. tissue(as is the serosa), and the muscularis consists of smooth Chapter 51 Fueling Body Activities: Digestion 1019

Vertebrate Digestive Systems In humans and other vertebrates, the digestive system con￾sists of a tubular gastrointestinal tract and accessory diges￾tive organs (figure 51.4). The initial components of the gastrointestinal tract are the mouth and the pharynx, which is the common passage of the oral and nasal cavities. The pharynx leads to the esophagus, a muscular tube that deliv￾ers food to the stomach, where some preliminary digestion occurs. From the stomach, food passes to the first part of the small intestine, where a battery of digestive enzymes continues the digestive process. The products of digestion then pass across the wall of the small intestine into the bloodstream. The small intestine empties what remains into the large intestine, where water and minerals are ab￾sorbed. In most vertebrates other than mammals, the waste products emerge from the large intestine into a cavity called the cloaca (see figure 51.3), which also receives the products of the urinary and reproductive systems. In mam￾mals, the urogenital products are separated from the fecal material in the large intestine; the fecal material enters the rectum and is expelled through the anus. In general, carnivores have shorter intestines for their size than do herbivores. A short intestine is adequate for a carnivore, but herbivores ingest a large amount of plant cellulose, which resists digestion. These animals have a long, convoluted small intestine. In addition, mammals called ruminants (such as cows) that consume grass and other vegetation have stomachs with multiple chambers, where bacteria aid in the digestion of cellulose. Other her￾bivores, including rabbits and horses, digest cellulose (with the aid of bacteria) in a blind pouch called the cecum lo￾cated at the beginning of the large intestine. The accessory digestive organs (described in detail later in the chapter) include the liver, which produces bile (a green solution that emulsifies fat), the gallbladder, which stores and concentrates the bile, and the pancreas. The pancreas produces pancreatic juice, which contains digestive enzymes and bicarbonate. Both bile and pancreatic juice are secreted into the first region of the small intestine and aid digestion. The tubular gastrointestinal tract of a vertebrate has a characteristic layered structure (figure 51.5). The innermost layer is the mucosa, an epithelium that lines the interior of the tract (the lumen). The next major tissue layer, made of connective tissue, is called the submucosa. Just outside the submucosa is the muscularis, which consists of a double layer of smooth muscles. The muscles in the inner layer have a circular orientation, and those in the outer layer are arranged longitudinally. Another connective tissue layer, the serosa, covers the external surface of the tract. Nerves, in￾tertwined in regions called plexuses, are located in the sub￾mucosa and help regulate the gastrointestinal activities. The vertebrate digestive system consists of a tubular gastrointestinal tract, which is modified in different animals, composed of a series of tissue layers. Chapter 51 Fueling Body Activities: Digestion 1019 Salivary gland Salivary gland Liver Esophagus Gallbladder Pharynx Cecum Appendix Anus Rectum Small intestine Pancreas Stomach Colon FIGURE 51.4 The human digestive system. Humans, like all placental mammals, lack a cloaca and have a separate exit from the digestive tract through the rectum and anus. Blood vessel Nerve Myenteric plexus Submucosal plexus Connective tissue layer Serosa Gland in submucosa Longitudinal layer Circular layer Muscularis Gland outside gastrointestinal tract Mucosa Lumen Submucosa FIGURE 51.5 The layers of the gastrointestinal tract. The mucosa contains a lining epithelium; the submucosa is composed of connective tissue (as is the serosa), and the muscularis consists of smooth muscles

51.2 Food is ingested, swallowed, and transported to the stomach. The mouth and teeth Canines Specializations of the digestive systems in different kinds of vertebrates reflect differences in the way these animals live Fishes have a large pharynx with gill slits, while air-breathing vertebrates have a greatly reduced pharynx. Many verte brates have teeth(figure 51.6), and chewing(mastication) breaks up food into small particles and mixes it with fluid se- cretions. Birds, which lack teeth, break up food in their two- Incisors chambered stomachs(figure 51.7). In one of these chambers, Molars Premolars the gizzard, small pebbles ingested by the bird are churned together with the food by muscular action. This churning grinds up the seeds and other hard plant material into smaller chunks that can be digested more easil Vertebrate Teeth FIGURE 51.6 Carnivorous mammals have pointed teeth that lack flat Diagram of generalized vertebrate dentition. Different grinding surfaces. Such teeth are adapted for cutting and vertebrates will have specific variations from this ge shearing. Carnivores often tear off pieces of their prey but pattern, depending on whether the vertebrate is an herbivore have little need to chew them, because digestive enzymes carnivore, or omnivore can act directly on animal cells. (Recall how a cat or dog gulps down its food. )By contrast, grass-eating herbivores, ch as cows and horses, must pulverize the cellulose cell walls of plant tissue before digesting it. These animals have large, flat teeth with complex ridges well-suited to grinding Esophagus uman teeth are specialized for eating both plant and Stomach animal food. Viewed simply, humans are carnivores in the front of the mouth and herbivores in the back(figure 51.8) Gizzard The four front teeth in the upper and lower jaws are sharp, chisel-shaped incisors used for biting. On each side of the ncisors are sharp, pointed teeth called cuspids(sometimes referred to as"canine"teeth), which are used for tearing od. Behind the canines are two premolars and three lars, all with flattened, ridged surfaces for grinding crushing food. Children have only 20 teeth, but these de ciduous teeth are lost during childhood and are replaced by 32 adult teeth The mouth Inside the mouth the tongue mixes food with a mucous so- lution, saliva. In humans, three pairs of salivary glands se- birds store food in the crop and grind it up in the gizzard Birds lack teeth but have a muscular chamber called the gizzard mucosal lining. Saliva moistens and lubricates the food so that works to break down food. Birds swallow gritty objects or that it is easier to swallow and does not abrade the tissue it pebbles that lodge in the gizzard and pulverize food before it asses on its way through the esophagus. Saliva also con- passes into the small intestine tains the hydrolytic enzyme salivary amylase, which initi ates the breakdown of the polysaccharide starch into the flow of about half a milliliter per minute when the mouth is saccharide maltose. This digestion is usually minimal in empty of food. This continuous secretion keeps the mouth numans, however, because most people dont chew their moist. The presence of food in the mouth triggers an in- creased rate of secretion, as taste-sensitive neurons in the The secretions of the salivary glands are controlled by mouth send impulses to the brain, which responds by stim- Is system, which in humans m ulating the salivary glands. The most potent stimuli are 20 Part XIlI Animal Form and function

The Mouth and Teeth Specializations of the digestive systems in different kinds of vertebrates reflect differences in the way these animals live. Fishes have a large pharynx with gill slits, while air-breathing vertebrates have a greatly reduced pharynx. Many verte￾brates have teeth (figure 51.6), and chewing (mastication) breaks up food into small particles and mixes it with fluid se￾cretions. Birds, which lack teeth, break up food in their two￾chambered stomachs (figure 51.7). In one of these chambers, the gizzard, small pebbles ingested by the bird are churned together with the food by muscular action. This churning grinds up the seeds and other hard plant material into smaller chunks that can be digested more easily. Vertebrate Teeth Carnivorous mammals have pointed teeth that lack flat grinding surfaces. Such teeth are adapted for cutting and shearing. Carnivores often tear off pieces of their prey but have little need to chew them, because digestive enzymes can act directly on animal cells. (Recall how a cat or dog gulps down its food.) By contrast, grass-eating herbivores, such as cows and horses, must pulverize the cellulose cell walls of plant tissue before digesting it. These animals have large, flat teeth with complex ridges well-suited to grinding. Human teeth are specialized for eating both plant and animal food. Viewed simply, humans are carnivores in the front of the mouth and herbivores in the back (figure 51.8). The four front teeth in the upper and lower jaws are sharp, chisel-shaped incisors used for biting. On each side of the incisors are sharp, pointed teeth called cuspids (sometimes referred to as “canine” teeth), which are used for tearing food. Behind the canines are two premolars and three mo￾lars, all with flattened, ridged surfaces for grinding and crushing food. Children have only 20 teeth, but these de￾ciduous teeth are lost during childhood and are replaced by 32 adult teeth. The Mouth Inside the mouth, the tongue mixes food with a mucous so￾lution, saliva. In humans, three pairs of salivary glands se￾crete saliva into the mouth through ducts in the mouth’s mucosal lining. Saliva moistens and lubricates the food so that it is easier to swallow and does not abrade the tissue it passes on its way through the esophagus. Saliva also con￾tains the hydrolytic enzyme salivary amylase, which initi￾ates the breakdown of the polysaccharide starch into the disaccharide maltose. This digestion is usually minimal in humans, however, because most people don’t chew their food very long. The secretions of the salivary glands are controlled by the nervous system, which in humans maintains a constant flow of about half a milliliter per minute when the mouth is empty of food. This continuous secretion keeps the mouth moist. The presence of food in the mouth triggers an in￾creased rate of secretion, as taste-sensitive neurons in the mouth send impulses to the brain, which responds by stim￾ulating the salivary glands. The most potent stimuli are 1020 Part XIII Animal Form and Function 51.2 Food is ingested, swallowed, and transported to the stomach. Molars Premolars Canines Incisors FIGURE 51.6 Diagram of generalized vertebrate dentition. Different vertebrates will have specific variations from this generalized pattern, depending on whether the vertebrate is an herbivore, carnivore, or omnivore. Mouth Esophagus Stomach Gizzard Intestine Anus Crop FIGURE 51.7 Birds store food in the crop and grind it up in the gizzard. Birds lack teeth but have a muscular chamber called the gizzard that works to break down food. Birds swallow gritty objects or pebbles that lodge in the gizzard and pulverize food before it passes into the small intestine