The Digestive System

The Oral Cavity:

Since ruminants don't have dental incisors on the upper part of their mouth, they instead have a dental pad. The dental pad is a hard and keratinized structure that allows ruminants to break off and bring large amounts of plants or other feed into their mouths and hold it there when the jaw is shut(when the jaw is closed the lower incisors rest against the dental pad).

The tongue is basically a huge muscle consisting of different parts like papillae and taste buds. The tongue aids in early oral digestion by helping move food and form a bolus to then be swallowed and further digested.

The mouth also has an important role in early digestion via contact of food and oral enzymes such as salivary amylase, an enzyme that breaks down starch in the mouth.

The Esophagus:

After the oral cavity forms a bolus from the food, this bolus then travels from the pharynx to the stomach via the esophagus. The esophagus is a tube formed by muscle that bridges from the pharynx to the stomach. Along its path to the stomach, the esophagus passes through the diaphragm via the esophageal hiatus. The esophagus has lots of folds to aid in the accommodation of different sized food boluses, allowing for the tube stretch, and the esophagus is formed of stratified squamous epithelium which helps protect the tube from sharp hay or bit of food that could injure the tissue.

Since the fetal bovine calf is a ruminant the esophagus remains skeletal muscle or striated throughout its entirety, however in non-ruminants this esophagus starts as striated but then transitions to smooth. If you are having trouble finding the esophagus start with finding the trachea and the esophagus should be nearby.

The Sulcus Ruminoreticularis(Esophageal Groove):

The esophageal groove is formed by muscular folds of the mucosa by and near the cardia of the stomach. This groove goes from the cardia to the omasum. In the fetal calf that is being dissected, this groove plays a critical role in the suckling reflex. When the bovine calf suckles, a reflex is stimulated, and the two folds of the muscles on either side of the gastric groove contract to form a tube for the milk to pass. This allows the milk to skip the rumen and reticulum of the ruminant stomach and go straight to the other parts of the stomach, where milk is not at risk for spoiling.

The Rumen:

The rumen is part of the forestomach of the cow, and just like in other ruminants, the rumen is where the fermentation of food happens for cows. Inside the rumen there are microbes and bacteria that help to ferment and digest the large amount of food stored in the rumen. In particular, these microbes help to digest and breakdown cellulose which cows and humans alike cannot digest alone. The observed papillae in the rumen help to increase surface area as well as promote and support absorption.

The Reticulum:

The reticulum is another part of the bovine forestomach. The reticulum is easily identifiable due to its unique honeycomb-like appearance from its mucosa arrangement. The reticulum is important for filtering and separating materials ingested. The reticulum helps to separate digestible food particles from potentially harmful ingested materials like wood or maybe metal that the cow accidentally consumed.

The Omasum:

The omasum is the chamber of the ruminant forestomach that primarily functions in absorption, mainly of water. The omasum can be identified by its book-like folds and sheets of laminae which spread apart similar to pages of a book. Papillae can also be found here.

The Abomasum:

The abomasum of the stomach, nicknamed the "true stomach" is the stomach chamber where chemical digestion occurs. The abomasum functions similarly to that of non-ruminant stomachs. Various digestive enzymes and secretions aid in chemical digestion of the food here. The abomasum also is glandular.

The Pylorus & Pyloric Sphincter:

The Pylorus and its associated sphincter is found between the stomach and the small intestine, and it regulates the passage of the digested food gastric juice mixture, chyme, into the duodenum by closing, opening or narrowing to regulate this flow.

The Small Intestine:

The first part of the small intestine is the duodenum. Here in the duodenum which you can see comes off of the pylorus, the pancreas, and the pancreas and the liver are connected via ducts to aid in digestion. The gall bladder releases bile into the duodenum and secretions from the pancreas combine with the food in the duodenum to be further digested and move down the digestive tract. The duodenum also has a duodenal loop which contains an accessory organ, the pancreas.

The Jejunum is the second part of the small intestine and it comes off the duodenum. Out of three parts to the small intestine(Duodenum, jejunum, ileum) the jejunum is the longest.

The Ileum is the final part of the small intestine and is also the shortest. The Ileum is recognizable by the many groups of lymph nodules present, nicknamed Peyers Patches.

The Pancreas:

The Pancreas is an accessory organ/gland with a variety of functions, both exocrine and endocrine. To find the pancreas look for the duodenum, and from there you will see the duodenal loop containing the pancreas. The pancreas is responsible for secreting important enzymes into the duodenum via ducts to regulate and assist with the chemical digestion occurring here, but it also plays a role in secreting major hormones such as insulin into the blood. The pancreas itself has multiple ducts that help to transport bile and liquids. The main and accessory pancreatic ducts work to transport secretions and fluids from the pancreas into the duodenum. These fluids directly drain into the duodenum through the ampulla of Vater, and the sphincter of Oddi opens or closes to allow pancreatic fluids/secretions and bile into the duodenum when its needed to break food down in the duodenum.

The Gall Bladder:

The Gall Bladder can be found under the liver, and its primary function is to store bile. When food enters the duodenum of the small intestine, cholecystokinins signal to the gall bladder to release bile into the duodenum to help chemically break down the food. The gall bladder also has a duct called the cystic duct which joins with the hepatic duct of the liver to make the bile duct, better known as the common bile duct. Bile ultimately passes through the common bile duct to reach the duodenum.

The Liver:

The bovine liver is a digestive organ/gland that has 4 main lobes. The main function of the liver is to filter blood that comes into the liver via blood from the hepatic artery as well as the portal vein which brings blood from the viscera which is then cleansed by sinusoids before emptying with the rest of the deoxygenated blood from the liver into the caudal vena cava. The liver also produces and makes bile via its main cells the hepatocytes. The liver also is responsible for breaking down and detoxifying bad substances that cannot be digested or shouldn't be in the blood, like poisons. The liver has a variety of cells working within it, the main players being the resident immune cells of the liver, Kupffer cells, and hepatocytes. Bile in the liver leaves via hepatic ducts that then combines with the cystic ducts of the gall bladder to enter into the duodenum through the common bile duct.

The Large Intestine:

The colon is a structure of the large intestine responsible for the last step of water and nutrient absorption but also the creation of poop to be excreted by the body. It has ascending, transverse, and descending portions. A unique visible feature to helps you find the colon in cows is the spiral-shaped loops of the colon.

The cecum, a blind sac, is another part of the large intestine, and similar to the colon it functions in absorbing nutrients and salts prior to the creation of fecal matter.

The Rectum:

The rectum is sort of like the storage tube-space of feces. After fecal matter has been moved from the colon into the rectum it is ready to exit the body via the anus. In order to do so the smooth muscles of the rectum must contract when the rectum is full, these contractions result in the excretion of poop through the anus and out of the body.

The Peritoneum:

The peritoneum is basically a connection of serous folds(two main folds) lining the abdominal wall. Along with mesentery which connects and lines the intestines, the peritoneum helps stabilize and support these digestive organs and structure, as well as provide serous fluid to help things move around easier and not rub against each other. When you are performing the dissection you'll likely feel the serous fluid as slippery and maybe a bit sticky on your gloves, like snot. The peritoneum also serves as folds that structure and hold together necessary vasculature and ligaments.