Digestive system
...ed in birds, what the earliest function(s) of feathers was(were), when endothermy arose in some archosaurs, which group of theropods was ancestral to birds, how theropod ecology changed with the acquisition of flight, why some bird groups survived the Cretaceous extinction of other dinosaurs, etc. Without its feathers, Archaeopteryx looks much like a small coelurosaur such as a dromaeosaurid or troodontid. The facts are resoundingly in support of a maniraptoran origin for birds; certainly a theropodan origin at the very least. So when you see a hawk diving to snatch a dove, or an egret darting for fish, or an ostrich dashing across the African savanna, know that you are gaining some insight into what the extinct dinosaurs were like. However, do note that extant (living) birds are quite different from extinct dinosaurs in many ways, so it's not safe to assume that all dinosaurs are the same. For that matter, extant birds are quite different from Jurassic and Cretaceous birds. Time passes, the environment changes... life evolves. Extant birds have been separated evolutionarily from the other coelurosaurian dinosaurs for some 150 million years, so they do look, act, and function quite differently, but science has shown us that they are closely linked by their common evolutionary history. (2) Here's a typical avian digestive system: Source: http://numbat.murdoch.edu.au/Anatomy/avian/fig4.1.GIF The major components of the avian digestive system are the alimentary canal plus several accessory structures. The 'canal' includes the oral cavity, pharynx, esophagus (which includes a crop in some birds), stomach (proventriculus & gizzard), small intestine, & large intestine. The large intestine then empties into the cloaca. Important accessory structures include the beak, salivary glands, liver, & pancreas. Buccal, or oral, cavity: Contains few mucous glands & taste buds. salivary glands are well-developed in many birds. In some, like woodpeckers, the 'sticky' saliva aids in capturing prey. In others, like swifts, saliva is used in nest building (see photo below). salivary glands are reduced in aquatic species (because aquatic prey like fish require little additional lubrication to be easily swallowed). Esophagus: Tube that connects the oral cavity and the stomach. The muscular walls of the esophagus produce wave-like contractions (peristalsis) that help propel food from the oral cavity to the stomach. Large in diameter compared to other vertebrates, especially in birds that swallow large prey, e.g., cormorants, herons, & raptors may serve for temporary storage of food: temporary distension - fish-eating species, birds of prey, & some fruit eaters (like Cedar Waxwings) crop out-pocketing of the esophagus that's particularly well-developed in seed-eaters like pigeons & doves (Columbiformes) and gallinaceous birds (grouse and pheasants) specialized for production of 'milk' that pigeons & doves feed to their young. Crop 'milk' is rich in proteins, fats, & vitamins and is produced by proliferation & sloughing off of epithelial cells that line the crop. The avian stomach is divided into 2 parts: Proventriculus - also called the glandular stomach; receives food from the esophagus & secretes mucus, HCl, and pepsinogen. Pepsinogen is converted into pepsin (a proteolytic, or protein-digesting, enzyme) by the HCl. Ventriculus (or gizzard): the avian equivalent of teeth very muscular (but less so in birds that eat meat, insects, nectar, and other 'soft' foods used primarily to grind & break-up food (such as seeds) may, in seed-eating birds, contain grit (small stones ingested by the birds to help grind the food) lined with a tough, abrasive keratin-like layer of koilin, known as the cutica gastrica. Grinding action may, particularly in seed-eating birds, be assisted by grit and stones deliberately ingested. Intestine: The intestine is the chief organ of digestion & absorption, it receives bile from the liver and pancreatic juice from the pancreas, and its divided into a small intestine & a large instestine. Small intestine: Short & slightly coiled in meat-eating birds (e.g., raptors) but longer & highly coiled in herbivores (e.g., seed eaters) and omnivores. Villi Lined with numerous structures called villi (pictured below). Villi are projections that increase the amount of surface area available for absorption. Inside each villus are blood vessels and lymph vessels that absorb nutrients for transport throughout the body. Large intestine: The large intestine is relatively short, its primary function is to absorb water and electrolytes, it contains a minimum of non-digestible waste (to help keep birds as light as possible), has out-pocketings called caeca. Caeca are best developed in some waterfowl, gallinaceous birds (like chickens & grouse), and ostriches, and Caeca aid in the digestion of plant material (bacteria in the caeca help enzymatically digest the material). Cloaca: Cloaca receives waste from the large intestine & materials from the urinary & reproductive systems, it is divided into three sections: coprodaeum - receives waste from the large intestine, urodaeum - receives urine from the kidneys (via the ureters) and sperm & eggs from the gonads, and proctodaeum - stores (temporarily) and ejects material; closed posteriorly by the muscular anus. The bursa of Fabricius is located on the dorsal wall. The bursa is most prominent in young birds and serves as the area where B-lymphocytes (the white blood cells that produce antibodies) are generated (T-lymphocytes are generated in the Thymus). Once produced, the B-lymphocytes migrate to lymphoid tissue in other parts of the body & the bursa of Fabricius atrophies. Accessory organs: Liver - produces bile that is transported to the small intestine via the bile duct. Bile emulsifies fats (or, in other words, breaks fats down into tiny particles). Emulsification is important because it physically breaks down fats into particles than can then be more easily digested by enzymes (which are produced by intestinal cells and the pancreas). Pancreas - produces pancreatic juice that is transported to the small intestine. This 'juice' contains a bicarbonate solution that helps neutralize the acids coming into the intestine from the stomach plus a variety of digestive enzymes. The enzymes help break down fats, proteins, and carbohydrates. The pancreas also produces the hormones insulin and glucagon which regulate blood sugar levels (cells that produce these two hormones make up the 'islets of Langerhans', one of which is represented by the light-colored, circular structure in the photomicrograph below). (3) Digestive system of a grasshopper Digestive system of an insect consists basically of a long tube that extends from the mouth to the anus. The tube has three main divisions. They are (1) the foregut; (2) the midgut, or stomach; and (3) the hindgut, or intestine. After food has been chewed or sucked up by the mouthparts, it enters the foregut through the mouth. The food moves along the tube until it reaches an enlarged area called the crop, where it is temporarily stored and partly digested. Then the food passes into the gizzard, which has thick muscular walls that contract and grind the food into small bits. The gizzard of some insects has teeth that help break up and grind the food. The food next passes into the midgut, where most digestion takes place. Nourishing parts of the food are absorbed into the blood, and wastes and undigested parts move into the hindgut. Insects have a system of 2 to over 150 Malpighian tubes attached to the digestive system where the midgut and hindgut join. The tubes float in the blood and absorb waste materials, which then pass through the tubes into the hindgut. All wastes and extra water that enter the hindgut leave the body through the anus. The digestive system of the bird The digestive system of the bird is a little different than grasshoppers. Birds have an enlargement of the esophagus, called the crop that stores and moistens food. Birds regurgitate food from the crop to feed baby chicks. A muscular organ called the gizzard kneads and crushes food before it enters the stomach. Sometimes stones that...