Digestive System

The oral portion of the ruminants is the entrance of the digestive system. Cattle use their tongue for prehension, which is the act of bringing food into the mouth. Mastication is the act of chewing food. The roof of the oral cavity has ridges called palatine rugae, which are important in species like cows that eat roughages because it allows for abrasive impact that can happen during feeding. Cows don't have incisors in their mouth, instead there is a thickened platform-like, fibrous structure called the dental pad.

The tongue is a very vital organ of the mouth for chewing and swallowing food. It has papillae on the surface that help give it it's rough texture. Cattle have filiform, conical, fungiform, and vallate papillae, the fungiform and vallate having the taste buds. Conical papillae are unique to ruminants and are alveolations, like small mounds, on the torus lingua of the tongue. Filiform papillae are spiky structures and help with prehension of food, being located on the apex of the tongue. Fungiform papillae are also on the apex of the tongue and look similar to mushrooms. Vallate papillae are surrounded by small cups and are located on the posterior one-third of the tongue, or the torus lingua.

The esophagus is a muscular tube that expands when food is present and has stratified squamous epithelium to support the necessary functions. The outside covering is a fibrous membrane called serosa and within this is a muscular layer, submucosa, and mucosa. The muscular layer has two different layers, one is the outer longitudinal layer and the other is the inner circular layer. These muscles are important for peristalsis, which is the squeezing action to move contents through the esophagus. The outer longitudinal layer contracts to make the tube shorter and the inner circular layer squeezes the content. Cows also have a muscular support right below the mucosa called the muscular mucous membrane to protect against shear forces from roughages that are abrasive. Ruminants have a skeletal muscle esophagus that gives them the ability to move food back and forth while chewing their cud.

The ruminant stomach has four different compartments, including the rumen (paunch or herbarium), reticulum (honeycomb), omasum (book stomach), abomasum (true stomach). The rumen almost entirely fills the left side of the abdominal cavity and acts like a large fermentation vat. It ferments food and breaks down fiber, and the microorganisms synthesize proteins. The reticulum is most cranial compartment of the forestomach and with its honeycomb appearance, foreign objects usually get caught here. The omasum is a spherical organ that is filled with muscular laminae that lie in sheets like pages of a book. This compartment absorbs water and other components of the digestive contents. The abomasum is the glandular part of the ruminant stomach and it produces hydrochloric acid and other digestive enzymes that break down proteins.

The ruminal reticular groove, or sulcus ruminoreticularis is where the esophagus empties into the rumen. This is relevant in neonates and animals that are suckling milk. The reflex of suckling causes the closure of the groove, so that whatever is coming into the esophagus is shunted to go straight to the omasum and avoids the rumen and reticulum. This is because you would not want the milk to be fermented, so it goes to a located with a low pH and the the rennin can curdle it and hold it there for some time.

The pylorus connects the stomach to the duodenum, the first section of the small intestine and opens and closes during digestion. It is a valve that allows partly digested food to pass from the stomach to the small intestine. The three parts of the small intestine includes the duodenum, jejunum, and ileum. A lot of food digestion takes place here, as well as the breakdown of food into much simpler molecules for absorption. The duodenum mixes the food with enzymes from the pancreas and bile from the gallbladder in order to break down food. The jejunum absorbs nutrients and water, and the ileum absorbs bile acids, fluids, and vitamin b-12.

The liver is the largest gland in the body and its main digestive function is the secretion of bile. It also plays a role in metabolism, processing drugs to be excreted, lipid mobilization and digestion, glucogenesis, and removing material from blood. Bile carries away wastes and breaks down fats during digestion. The gallbladder is a storage sac and concentration site for bile. Cholecystokinin stimulates the gallbladder to contract and release the bile. The common bile duct carries bile from the gallbladder to the duodenum of the small intestine.

The pancreas is located in the duodenal loop and is important in digesting food with the release of pancreatic juice. It opens into the duodenum and releases several digestive enzymes with different functions. The pancreatic duct joins the common bile duct to form the ampulla of vater, which is a small opening that enters to the duodenum of the small intestine. This is where the pancreatic and bile duct release their secretions into the intestine. The sphincter of oddi is the smooth muscle that surrounds the end portion of the common bile duct and the pancreatic duct. It functions to regulate bile and pancreatic juice flow into the duodenum.

The three portions of the large intestine include the cecum, colon, and the rectum. The main function of this section of the digestive tract is absorption of water. The cecum is the area where the small and large intestine meet and is where some previously undigested fiber is broken down. The colon is where the most water absorption occurs in the large intestine and there is some microbial digestion still occurring. The rectum is where feces is formed and then released from the anus, leaving the digestive system.

The peritoneum holds the organs of the digestive tract in place. It is a broad serous membranous sac surrounded by connective tissue.