The Alimentary Canal

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Digestive System
Production and hosting by Elsevier B. Until the early s, the medical community generally believed that ulcers were caused by several factors, including stress and a poor diet. Small intestine - duodenum. Glands contributing digestive juices include the salivary glands , the gastric glands in the stomach lining, the pancreas , and the liver and its adjuncts—the gallbladder and bile ducts. The digestive system also separates and disposes of waste products ingested with the food. Q Using the numbers from the diagram above, list, in order, the structures each mouthful of food or drink passes through on its way through the digestive process.

Introduction to Digestive System:

Mitochondria - Turning on the Powerhouse

The metabolism of carbohydrate, fat and protein is closely linked to the digestive system. The digestive system consists of the alimentary canal along which the food passes after eating to where the residual wastes are eliminated from the body, together with the liver and the pancreas.

The digestive system is responsible for the ingestion of food, its breakdown into its constituent nutrients and their absorption into the blood stream, and the elimination of wastes from this process.

The liver produces bile and is associated with the metabolism of nutrients together with a number of other functions. The main function of the pancreas is the production of digestive enzymes and special compounds called hormones. The alimentary canal is a long tube-like organ that starts at the beak and ends with the vent or cloaca in the abdominal region.

Generally the alimentary canal has layers of muscle that run lengthwise and around it and is lined with mucous membranes. Glands that produce important digestive juices are found in different locations of the canal. The nutrients from the food, after digestion, are absorbed through the wall of the alimentary canal into the circulatory system for transport to the liver or other parts of the body.

The waste remaining is eliminated from the body via the cloaca or vent. There are no teeth. The so called egg tooth found on the end of the beak of newly hatched chickens is an aid to their escape from the egg at hatching and disappears after a day or two.

The hard palate that forms the roof of the mouth, presents a long, narrow median median — along the middle slit that communicates with the nasal cavity. The hard palate has five transverse rows of backwardly pointing, hard, conical papillae.

Numerous ducts of the salivary glands pierce the hard palate to release their secretions into the mouth cavity. A thick layer of stratified squamous epithelium covers the free surface. The salivary glands run the whole length of the hard palate, the groups of glands merging to form one mass of glandular tissue under the epithelium. Lymphoid tissue is found in most glands. The pharynx is continuous with, and follows, the mouth.

The combined cavity of the mouth and the pharynx is often referred to as the oropharynx. The common opening for the two eustachian tubes is located in the middle of its dorsal wall roof.

The tongue is long and pointed and conforms to the shape of the beak in which it operates. The epithelium of the tongue is thick and horny, especially towards the tip. A transverse row of simple, large and horny papillae with their tips directed towards the rear of the mouth cavity are located on the posterior end. The hyoid bone provides the framework to support the tongue. The entoglossal bone extends longitudinally in the median plane.

Small patches of lymphatic tissue are located throughout the corium. Mucous glands are located in the tongue with short ducts directed towards the rear. Some believe that there are taste buds located on the tongue, but this belief is not universally held.

In any case, the sense of taste appears to be very weak if at all present. The oesophagus is wide and is capable of being significantly stretched. It connects the mouth region to the crop in close association with the trachea. The crop is a large dilation of the oesophagus located just prior to where the oesophagus enters the thoracic cavity.

The crop provides the capacity to hold food for some time before further digestion commences. Inside the thoracic cavity, the oesophagus enters or becomes the proventriculus which is a very glandular part of the digestive tract often called the glandular stomach. The wall of the oesophagus is composed of four layers of tissue, the innermost being mucous membrane. The mucous membrane is an important barrier to the entry of microbes and the mucous it produces is a lubricant that aids the passage of the food along the alimentary canal.

The structure below the crop is similar to that above except there is less lymphoid tissue below the crop. The crop structure is similar to that of the oesophagus except there are no glands present in fowls. Ducks and geese have glands in the crop mucous membranes. The glandular stomach, or proventriculus, is relatively small and tubular.

The wall is very thick and is composed of five layers:. The glands form the greater part of the thickness of the organ. Simple single glands group to form lobules each of which converges into a common cavity near the surface. The cavities converge to form a common duct that leads to the surface through the apex of a small papilla see figure below.

These glands produce a number of juices or enzymes that are used in the digestion or breaking down of food into its constituent nutrients. The mucous membrane is raised into folds and between these folds are numerous simple tubular glands that produce hydrochloric acid as well as lymphoid tissue.

The muscular stomach or gizzard is located immediately after the proventriculus, partly between the lobes and partly behind the left lobe of the liver. It has a flattened, rounded shape somewhat like a convex lens, with one side slightly larger than the other. Each surface is covered by a glistening layer of tendinous tissue which is thicker at the centre and becoming thinner towards the edges.

Under this outer layer there are located very powerful masses of red muscle. The inner surface is lined with a creamy-coloured, thick, horny tissue raised in ridges. The gizzard almost always contains quantities of hard objects such as gravel or other grit that aids in the disintegration of food, which is the primary function of the gizzard. Each layer contains a certain tissue type that plays a crucial role in the breakdown of food. The mucosa also known as mucus membrane is the innermost layer of tissue.

The basic structure of the alimentary canal, including its four basic layers. In certain regions of the alimentary canal, the mucosa may perform one or all three of these functions. Digestive mucosa is made up of three sublayers: The mucus it produces protects certain digestive organs from being digested by enzymes working within the same cavity, it also eases food passage along the GI tract. The lamina propria, which underlies the epithelium, is loose areolar connective tissue.

Its capillaries nourish the epithelium and absorb digested nutrients. Its isolated lymphoid follicles which are a part of MALT ,. Large collections of lymphoid follicles occur in the pharynx tonsils and appendix. External to the lamina propria is the musularis mucosae , a layer of smooth muscle cells that produces local movements of mucosa. The submucosa , just external to the mucosa, is areolar connective tissue containing a rich supply of blood and lymphatic vessels, lymphoid follicles, and nerve fibers which supply the surrounding tissues of the GI tract wall.

Its elastic fibers enable the stomach to regain its normal shape after temporarily storing a large meal. Instead, the tongue tip is a dynamic liquid-trapping device that dynamically traps nectar by rapidly changing their shape during feeding. In addition, the tongue—fluid interactions are identical in both living and dead birds, demonstrating that this mechanism is a function of the tongue structure itself, and therefore highly efficient because no energy expenditure by the bird is required to drive the opening and closing of the trap.

These results rule out previous conclusions from capillarity-based models of nectar feeding and highlight the necessity of developing a new biophysical model for nectar intake in hummingbirds. Hummingbird tongue tips twist to trap nectar. How the hummingbird tongue really works with videos. Close encounters with possible prey.

You want to live 10—20 years. You are peering under leaves, poking into rolled ones, searching around stems, exploring bark crevices and other insect hiding places. Abruptly an eye appears, 1—5 cm from your bill. The eye or a portion of it is half seen, obstructed, shadowed, partly out of focus, more or less round, multicolored, and perhaps moving. Now, a safe few meters away, are you going to go back to see whether that was food?

Associated body patterns often suggest other head and facial features, which in turn enhance the eye-like nature of the spots. None of these patterns exactly matches the eyes or face of any particular species of predator; but, even when quickly and partially glimpsed, all give the illusion of an eye or face.

These false eyes are mimicking the eyes and faces of such predators of insect-eating birds as snakes, lizards, other birds, and small mammals, as perceived at close range by the insectivorous birds in their natural world. Note the distended throat of this American Kestrel. Pigeons generally lay two eggs one day apart, which hatch 18 days after they are laid. A similar substance is produced by flamingos and male Emperor Penguins.

The normal function of the crop is food storage. Pigeon 'milk' also contains IgA antibodies and antioxidants carotenoids. The avian stomach is divided into 2 parts:. Photomicrograph 50X of a cross section through the proventriculus showing folds of mucous membrane P ; deep proventricular glands GP ; capsule connective tissue around the glands arrow head ; muscle layer m ; serosa connective tissue with blood vessels S , and the lumen L From: Photomicrograph X of longitudinal section of the gizzard showing folds of mucous membrane lined by simple prismatic epithelium P ; simple tubular glands Gs in the lamina propria constituted by connective tissue Lp ; secretion of glands S that are continuous with the cuticle or koilin ; C , part of muscle layer m , interpersed with bundles of connective tissue Tc From: Photomicrograph X of the koilin of an Eclectus Parrot Eclectus roratus.

Note the regular, columnated structure of the koilin layer K and its association with the glandular epithelium E of the ventriculus From: De Voe et al. A, koilin, B, crypts, C, glands that secrete koilin, D, epithelial surface, E, desquamated epithelial cells, 2 Mucosa of the gizzard.

A, koilin, B, secretion in gland lumens and crypts, and 3 Koilin layer. A, secretion column, B, koilin-layer surface, C, horizontal stripe indicating a 'pause' in secretion of the koilin, D, cellular debris.

Eglitis and Knouff Vultures of the seas -- Animals are primarily limited by their capacity to acquire food, yet digestive performance also conditions energy acquisition, and ultimately fitness. Optimal foraging theory predicts that organisms feeding on patchy resources should maximize their food loads within each patch, and should digest these loads quickly to minimize travelling costs between food patches.

GPS-tracking of 40 Wandering Albatrosses from the Crozet archipelago during the incubation phase confirmed foraging movements of between — km, giving the birds access to a variety of prey, including fishery wastes.

Using miniaturized, autonomous data recorders placed in the stomach of three birds, the first-ever measurements of gastric pH and temperature in procellariformes were obtained. Such low stomach pH gives Wandering Albatrosses a strategic advantage because it allows a rapid chemical breakdown of ingested food and rapid digestion.

This is useful for feeding on patchy, natural prey, but also on fishery wastes, which might be an important additional food resource for Wandering Albatrosses. It is likely that this physiological characteristic evolved as a response to a diet largely composed of squid, and to a patchy distribution of this food resource resulting in large, infrequent meals.

The strategy of Wandering Albatrosses is to cover long distances rapidly and at low costs to increase the probability of encountering dispersed prey patches whose distribution is unpredictable. Knots with large gizzards consumed far more molluscs with shells than the birds with smaller gizzards. Birds with smaller gizzards simply couldn't feed fast enough.

By allowing them to crush more shell per gizzard-full, larger gizzards gave birds the edge. Thus, even though it is energetically costly for the knots to maintain a larger gizzard, when the bird needs to get the most out of its crunchy diet, it's a price worth paying. So, the birds' gizzards enlarge as they fatten for migration. Because the molluscs' shells stay the same size as the molluscs shrink, the amount of shell a bird must process to eat its fill also increases.

But with their larger gizzards, the birds can still make the most of even the crunchiest winter diet! Within 14 days, they showed a doubling of the size of their gizzards.

Red Knots have strong muscular gizzards for feeding on molluscs. A shift back to a mussel diet induced about a doubling in gizzard mass in just a few days. As the knots were fed progessively smaller mussels day 22 to day 46 that are easier to crush, gizzard mass again declined.

A switch back to a soft food pellet diet caused a further decline in gizzard mass. Finally, a switch back to a mussel diet again cause a rapid increase in gizzard mass From: Piersma and Drent Ostrich Struthio camelus stomach.

Note how particle size of material in the gizzard ventriculus is smaller than in the proventriculus due to the grinding action of the muscular walls plus small pebbles gastroliths. The capacity to reduce particle size is related to the metabolic demands of a species. Therefore, particle size reduction is often considered the key digestive difference between ecto- and endotherms that allows endotherms to rely on shorter digesta retention times without losing digestive efficiency, and hence facilitate the high level of food intake necessary to meet their increased metabolic requirements.

In contrast, adaptations for chewing intrinsically increase the weight of the head. The use of the gizzard system has the potential advantages that intake rate is not limited by chewing, that no investment in dental tissue is necessary, and that dental wear is not a determinant of senescence as observed in mammals. The absence of age-dependent tooth wear might even be a contributing factor to the slower onset of senescence in birds as compared to mammals.

On the other hand, the use of a gizzard requires the intake of suitable grit or stones—an action that represents, in the few studies where this has actually been quantified in birds, a relevant proportion of feeding time Fritz et al. Gastrointestinal tracts of a carnivorous hawk, an omnivorous chicken, and 4 herbivorous birds.

Note larger size of crop in omnivore and herbivores, and particularly in hoatzin. Ceca are small in hawks and relatively large in grouse. Although ceca are relatively small in Hoatzins , Emus, and Ostriches, an expanded foregut Hoatzins , a much longer midgut Emus , or a much longer colon Ostriches compensates for this From: Stevens and Hume Over-reliance on the passive pathway provides metabolic advantages and ecological constraints.

It does provide birds with an absorptive process that can deal with rapid and large changes in intestinal sugar concentrations. The passive pathway is also energetically inexpensive to maintain and modulate. However, passive absorption through the paracellular pathway is dependent on concentration gradients. In the absence of a transport system that selects which materials to absorb, this non-discriminatory pathway may also increase vulnerability to toxins, and thus constrain foraging behavior and limit the breadth of the dietary niche of the birds.

Another problem is that when luminal sugar concentrations are lower than those in plasma, glucose may diffuse back into the lumen.

Cross-section of the intestine ileum of a Spotted Tinamou Nothura maculosa. Villi are lined with columnar epithelium EP , including goblet cells arrows that secrete mucus. The muscle layer includes longitudinal fibers MI on the perimeter, circular fibers Mc , and additional longitudinal fibers at the base of the villi muscularis muscosae; MM From: Chikilian and de Speroni Blue-headed Parrots at clay lick.

Meyer-Rochow and Gal determined that the pressures involved could be approximated if they knew the 1 distance the feces traveled, 2 density and viscosity of the material, and 3 shape, aperture, and height of the anus above ground.

How penguins choose the direction of defecation, and how wind direction factors into that decision, remain unknown. Avian Pancreas tissue Source: The Avian Digestive Tract. Avian geophagy and soil characteristics in southeastern Peru. Luminal morphology of the avian lower intestine: Histological aspects of the stomach proventriculus and gizzard of the Red-capped Cardinal Paroaria gularis gularis. Comparative study of the digestive system of three species of tinamou.

Crypturellus tataupa, Nothoprocta cinerascens , and Nothura maculosa Aves: Journal of Morphology Journal of Experimental Zoology Rictal bristle function in Willow Flycatcher. Dysplastic koilin causing proventricular obstruction in an Eclectus Parrot Eclectus roratus. Journal of Avian Medicine and Surgery Anatomy and physiology of the digestive system in fowl.

Pages in Proc. An histological and histochemical analysis of the inner lining and glandular epithelium of the chicken gizzard. American Journal of Anatomy An ecomorphological study of the raptorial digital tendon locking mechanism. Dietary and developmental regulation of intestinal sugar transport.

Digesta retention patterns in geese Anser anser and turkeys Meleagris gallopavo and deduced function of avian caeca. Comparative Biochemistry and Physiology A Histological and global gene expression analysis of the 'lactating' pigeon crop. Vultures of the seas: Evolution of the structure and function of the vertebrate tongue.

Journal of Anatomy Light and scanning electron microscopic study of the tongue in the cormorant Phalacrocorax carbo Phalacrocoracidae, Aves. Functional morphology of the tongue in the nutcracker Nucifraga caryocatactes. A tropical horde of counterfeit predator eyes. Instructed learning in the auditory localization pathway of the Barn Owl. The morphology of the bill apparatus in the Steller's Sea Eagle.

Wild Bird Society of Japan, Tokyo. Use of dung as a tool by burrowing owls. The integration of energy and nitrogen balance in the hummingbird Sephanoides sephaniodes. Does gut function limit hummingbird food intake? Physiological and Biochemical Zoology Pressures produced when penguins pooh—calculations on avian defaecation.

Scare tactics in a neotropical warbler: Gliding flight and soaring. Theoretical Ecology Series, vol. Modelling the flying bird C.

Structure, form, and function of flight in engineering and the living world. Phenotypic flexibility and the evolution of organismal design. Trends in Ecology and Evolution The hummingbird tongue is a fluid trap, not a capillary tube. Between air and water: Use of prey hotspots by an avian predator: Structure and mechanical behavior of a toucan beak. Movement and direction of movement of a simulated prey affect the success rate in Barn Owl Tyto alba attack.

Musculoskeletal underpinnings to differences in killing behavior between North American accipiters Falconiformes: Accipitridae and falcons Falconidae.

Journal of Morphology, online early. Le Bohec, and Y. Adjustments of gastric pH, motility and temperature during long-term preservation of stomach contents in free-ranging incubating King Penguins. Journal of Experimental Biology A tough nut to crack. Adaptations to seed cracking in finches. Cost-benefit analysis of mollusc-eating in a shorebird. Optimizing gizzard size in the face of seasonal demands. How do woodpeckers extract grubs with their tongues?

Why do woodpeckers resist head impact injury:

The Mucosa