[Music] the gastrointestinal tract of an avian is unique in comparison to those of mammals it contains an intestinal tract that is shorter than that of mammals the digestive tract begins with the mouth chickens don't have teeth or jaw muscles instead they use a lightweight beak to swallow food particles whole these particles are later broken down by the ventriculus or the gizzard which is located within the body cavity the upper beak is covered in hard keratin next is the tongue which is used for collecting moving and swallowing food the tongue which is covered in lateral barbs is used to brush food to the back salivary glands are less developed in birds but are present at the roof of the mouth they secrete mucus which lubricates the food and allows it to move down the esophagus taste buds are located in the upper beak and are also in small numbers on the anterior of the tongue chickens have up to 300 taste buds behind the tongue is the laryngeal mound that contains a narrow slit that opens into the glottis of the larynx its function is to aid in moving food toward the esophagus during swallowing [Music] the esophagus of a chicken is a thin-walled expansive tube that transports food from the mouth of the pharynx to the stomach as food is moved towards the back of the tongue the koana reflexively closes in the pharyngeal phase the infundibular mound and glottis close the tongue is moved backward and the esophagus is moved forward decreasing the distance between the oral cavity and the esophagus the avian esophagus allows birds to swallow their food whole its walls are made up of four layers mucosal submucosal muscle tunic and the serosa layer which contains only smooth muscle cells in addition the esophagus is divided into two sections the cervical near the beak and the thoracic region near the crop the esophagus is lined with epithelium tissue which contains mucous glands these glands are more numerous in the thoracic esophagus while being absent in the cervical region food is moved through the esophagus via the excitement of the myenteric plexus nerve rather than the muscles in the chicken digestive system the crop is an extension of the esophagus it's a thin walled pouch at the base of the thoracic section of the esophagus the walls and lining of the crop are made up of the same four layers as the esophagus the mucosal submucosal muscle tunic and the cerosal layer the main function of the crop is to hold food in storage until it can be released to the stomach or the pro-ventriculus the crop fills only after the pro-ventriculus is full food can remain in the crop for up to 12 hours until it is transferred via peristalic action no actual digestion takes place in the crop as there are no digestive chemicals secreted there to begin the process however softening and swelling of the feed will occur during the storage time this can lead to a condition called sour crop where the contents of the crop ferment and a bacterial yeast infection can occur after the food leaves the crop via peristolic action it moves into the proventriculus also known as the true stomach the pro-ventriculus is where actual digestion begins the walls of the pro-ventriculus are thick and composed of five layers a muscle layer itself composed of three individual layers a thick layer of glandular tissue and the inner mucous membrane layer the glandular tissue produces secretions which begin the process of digestion these secretions include hydrochloric acid which lowers the ph of the digestive system and food mixture the enzyme pepsin which helps break down protein and the hormone gastrin which stimulates gastric and pancreatic juice production the acid and digestive enzymes in the proventriculus break down food more significantly than the enzymes secreted by the salivary glands but at this point the food has not yet been ground only softened before it moves to the gizzard the gizzard or ventriculus is the muscular stomach where the grinding of food takes place after being softened by the proventriculus food moves into the gizzard it's a round organ with two lobes one larger than the other each surface of the gizzard is covered by a shiny layer of tendinous tissue which is thicker at the center and thinner out by the edges under this outer layer are located very powerful masses of red muscle the inner surface is lined with a creamy colored thick horny tissue raised in ridges this surface acts as the bird's teeth and is made up of a strong keratinized material called coilin that is able to withstand the potentially damaging effects of the hydrochloric acid and pepsin coming from the pro-ventriculus and the muscular action of grinding food as the coiling wears down from the grinding action glands in the gizzard produce more coiling that passes to the surface where it hardens to replace the worn tissue though the gizzard consists of very powerful muscles it alone can't pulverize everything the typical bird eats grit and small rocks are picked up as birds peck and feed this mineral matter accumulates in the gizzard and the particles help pulverize the seeds and grain once the food is ground it passes into the small intestine feed passes from the gizzard into the small intestine where digestion and absorption occur the small intestine of a mature chicken is more than four and a half feet in length and has three distinct areas the duodenum or the duodenal loop where the pancreas lies in the center the jejunum and the ilium collectively the jejunum and ilium are called the lower small intestine as food leaves the gizzard and enters the duodenum hormones are released which signal the pancreas to release digestive hormones and bicarbonate this acts to dilute the hydrochloric acid from the proventriculus the enzymes trypsin and chymotrypsin digest proteins amylase breaks down carbohydrates and lipase breaks down fat insulin and glucagon hormones that maintain blood glucose are also produced in the pancreas all these combine with bile from the gallbladder in the common bile duct to continue the digestive process the saturated and ground-up food mixture known as chyme moves through the jejunum and then through the nearly four feet of ilium where most of the released nutrients are absorbed inside the small intestine villi flexible finger like protrusions line the inner wall providing needed surface area for adequate absorption of nutrients like glucose calcium and vitamin b12 and remaining products of digestion not absorbed elsewhere the large intestine at four inches in length is actually shorter than the small intestine it runs in a nearly straight line below the vertebra and ends at the cloaca sometimes this section is referred to as the colon the last of water reabsorption occurs in this organ creating and storing undigested waste material that is eventually excreted through the cloaca the sica are two blind pouches located where the small and large intestines join some of the water remaining in the digested material is reabsorbed here additionally any remaining coarse materials are broken down in the sika through fermentation during this fermentation process the bacteria in the zika produce several fatty acids as well as the eight b vitamins thiamine riboflavin niacin pantothenic acid pyridoxine biotin folic acid and vitamin b12 because the zika are located so close to the end of the digestive tract however few of those produce nutrients are absorbed and available to the chicken the large intestine connects to the front part of the cloaca this organ is a flexible cavity that represents the junction of the digestive and urinary tracts in hens the reproductive tract also joins at the cloaca here digestive waste mixes with urinary waste and passes out of the body through the vent the cloaca in chickens is also flexible enough to accommodate an egg passing through it from the reproductive system [Music] you