Gastrointestinal System

The digestive system is a very simple, yet efficient development in the Gallus gallus domesticus. Because of its simplicity, it is light in weight which is a great evolutionary adaptation for flight. The system includes the mouth and beak, tongue, crop, proventriculus, gizzard, small intestine, pancreas, intestinal cecum, liver, gallbladder, large intestine, and cloaca. It plays a vital role in the metabolism of carbohydrates, lipids, and proteins. 

Mouth and Beak

The class Aves is distinguished by a modification of the jaws into a beak. Beaks are featherless, toothless, and are coated by a layer of keratinous epidermis. The shape helps aid in foraging behaviors and is adapted to the specific diet of individual species. Gallinaceous birds (domestic poultry and game birds) for example, have pointed beaks adapted to help pick up individual food items. The mouth as a whole is referred to as the bill and is unique in that the palate is incompletely fused, resulting in an opening between the oral and nasal cavities known as the choanal cleft. The bill consists of the beak, a layer of blood vessels and nerves, and connective tissue that acts as a glue to keep the beak attached to the bones. The upper portion of the bill consists of the premaxilla bone and the maxillary beak and is referred to as the maxillary rostrum. The lower portion is comprised of the mandibular bone and mandibular beak and is referred to as the mandibular rostrum. The beak grows an average of 1 to 3 inches a year but is worn down by everyday activities such as feeding, climbing, chewing, grooming, and rubbing. 

Tongue

The tongue is located in the mouth and is supported by the hyoid bone. It is made up of thick, horn epithelium and contains rows of large papillae found on the posterior end of the tongue. It conforms to the shape of the oral cavity but is normally triangular in shape. Though it has numerous touch receptors, it has very few taste buds resulting in a very poor sense of taste. The main purposes of the tongue is to help the bird distinguish its food by the feel of it as well as to push back the feed towards the esophagus so that the bird can swallow. 

Esophagus

The esophagus is defined as the flexible, muscular tube that allows food to pass through the pharynx to the stomach and connects the mouth to the rest of the digestive tract. In the chicken, mucous glands in the esophagus secretes a lubricant that helps soften food, allowing for easy transport of food from the mouth to the crop, and then from the crop to the proventriculus. The opening is located caudal to the laryngeal mound. Stratified squamous epithelium can be found lining this tube and allows for it to be significantly stretched. 

Glottis 

The glottis is the known as the opening of the upper portion of the larynx. It can found at the opening of the trachea, caudal to the base of the tongue. It is made up of elastic cartilage (the epiglottis) and is covered by a mucous membrane.The glottis has two folds that aid in swallowing by coming together and covering the larynx so that food does not pass through the trachea, resulting in choking. It also contains the true vocal cords, making it an important vocal apparatus for the bird. 

Crop

The crop is a large, distensible dilation that can be found cranial to the furcula on the left side of the neck. It is essentially an enlarged portion of the esophagus, and like the esophagus, is lined by stratified squamous epithelium. The crop acts as a temporary storage unit for food and water and will store these until the digestive system is ready for it to pass. This allows the chicken to quickly consume large quantities of food and then store it to digest later. When full, it protrudes and is easily visible. When empty, it sends messages to the brain signaling hunger and causing the bird to eat more. Despite the secretion of amylase, a digestive enzyme, very little food is actually digested in the crop. Impaction of the crop is a serious condition that can occur when chickens very rapidly eat too much food due to a long period of time of having no food. If a chicken has an impacted crop, food cannot bypass it, causing the crop to swell and will eventually cut off the windpipe, resulting in suffocation. Impaction is more commonly seen in free-range chickens that live off of tough vegetation. 

                                

Proventriculus 

Connected to the esophagus and located after the crop in the digestive system is the proventriculus. Known as the "true stomach," the proventriculus is responsible for secreting mucous, hydrochloric acid and digestive enzymes such as pepsin to aid in digestion. These secretions break down the food which then leaves the proventriculus and travels to the gizzard to be ground up. 

Gizzard

The second part of the stomach is referred to as the gizzard. The gizzard is a unique characteristic shared among birds that is a great adaptation for the avian diet of whole, intact seeds. Known as the "mechanical stomach," the gizzard is comprised of two sets of muscles that act as grinders. After food has passed through the proventriculus, it enters the gizzard whose main purpose is to act like mammalian teeth and the food is ground up, mixed, and mashed. Free-range chickens tend to eat stones which aid in grinding and remain in the gizzard until the acids eventually weaken them and they are small enough to pass through the digestive system. Commercial fed chickens typically do not need stones unless they are being fed whole grains. Though the gizzard has a thick layer of epithelium to protect its muscles, sharp objects such as nails or glass that may be accidentally ingested can put a hole in the gizzard muscle. Damaged gizzards will starve the chicken, eventually leading to death.                                                                

Small Intestine

Food and water are absorbed in the small intestine which is composed of the duodenum as well as the lower small intestine. The lower small intestine is comprised of the jejunum and ileum and it is attached and held in place by thin, connective tissues known as mesentery. Herbivorous birds, such as the chicken, have longer and more developed small intestines than carnivorous birds. The pancreas sends digestive enzymes to the duodenum to help digest proteins. Bile sent from the liver aids in digestion of lipids and helps the small intestines absorb vitamins A, D, E, and K. The duodenum digests anything else remaining and the small intestine absorbs the released nutrients through its membranes and releases them into the bloodstream. 

Pancreas

The pancreas occupies the space between the descending and ascending loops of the duodenum. It has two or three ducts that are hidden by tissue but pass secretions produced by the pancreas to the distal end of the duodenum. These secretions are a mixture of digestive enzymes that are responsible for the digestion of proteins. The pancreas is also responsible for the production of hormones, such as insulin and glycogen, associated with the metabolism of carbohydrates as well as the regulation of blood sugar. 

                                                

Intestinal Cecum

The ceca (plural for cecum) are located where the small and large intestines come together and are on average about 16-18 centimeters long in adults. The ceca begins with a narrow base originating at the ileo-colic-caecal junction, continues into thin, flexible tissue, and ends in a wide apex with thick walls. The ceca is responsible for reabsorption of water in fecal matter, the fermentation of any coarse materials, and the digestion of fiber. Upon fermentation and digestion, the ceca produce volatile fatty acids as well as thiamine, niacin, pantothenic acid, riboflavin, biotin, vitamin B12, folic acid, and pyridoxine. Very little of these nutrients are actually absorbed and utilized by the bird, however, due to the fact that the ceca is located at the end of the digestive tract. A healthy and well functioning ceca is emptied two to three times a day, producing mustard to dark brown colored droppings. 

Liver

Lying ventrally and caudally to the heart is the liver, a dark brown colored organ with two lobes. Its major responsibilities include forming bile, metabolizing carbohydrates, protein, and lipids, producing and destroying blood cells, detoxifying, and storing glycogen and volatile fat vitamins. The liver has two bile ducts leading to the distal duodenum and another duct connected to the gallbladder. 

Gallbladder

The gallbladder is found beneath the spleen and is responsible for the storage and secretion of bile into the small intestine. Stimulation of cholecystokinin occurs when food enters the digestive tract, causing the gallbladder to contract and release bile into the duodenum. The bile mixes with the partly digested food in the duodenum and aids in digesting the lipids found in the food. The gallbladder is important due to its ability to increase the concentration and therefore potency of bile during storage. 

Large Intestine

Unlike what its name suggests, the large intestine is actually relatively short in comparison to the small intestine at about 4 inches long. Also known as the colon, it runs from below the vertebrae to the cloaca and is important in the storage of waste material and reabsorption of any remaining water. 

Cloaca

The cloaca is the ending point of the digestive tract. It is found at the end of the large intestine and ends at an opening to the exterior part of the chicken's body. Its main purpose is to mix digestive wastes with urates and transport these wastes outside of the body. It is divided into three parts: the copradaeum, the urodaeum, and the proctodaeum. The copradaeum is an extension of the large intestine while the urodaeum is the middle section containing the open ducts of the uterus and genitals and the proctodaeum consists of the opening to the vent.