Figure 1: The Digestive System
The Digestive System consists of The gastrointestinal tract and the accessary digestive glands, liver, gallbladder, and pancreas.The gastrointestinal tract is a tube-like structure that extends from the mouth to the anus. It consists of the mouth, esophagus, stomach, small intestine, which consists of duodenum, jejunum and ileum, and large intestine, which consists of cecum, colon, and rectum.

Figure 2: Ruminant Stomach
A ruminant stomach consists of four compartments, rumen, occupying the left side of the abdominal cavity, and reticulum, omasum, and abomasum, all of which reside in the right side of the abdominal cavity in ruminants.

Figure 3: Histology of the gastrointestinal tract
The gastrointestinal wall consists of four layers, mucosa, submucosa, muscularis, and serosa. The mucosa, the closest layer to the lumen of the gut, which is in direct contact with the food, consists of three layers, epithelium (enterocytes and endocrine cells), lamina propria, and muscularis mucosae. The mucosa is followed by the submucosa, then two muscle layers, an inner, thick, circular and an outer, thin, longitudinal, followed by the outer most layer the serosa. On the enterocytes of the mucosa there are villi i.e., sensors to sense the internal environment of the gut. Between the two muscle layers there are two nerve plexuses, myenteric and submucosal. The myenteric plexus is located between the two muscle layers and the submucosal is located above the submucosa. The two plexuses comprise what is known as the enteric nervous system or the intrinsic innervation of the gut. The gut is supplied by an artery (A), a vein (V) and a lymphatic (L) vessel. Also, the gut is supplied by extrinsic innervation, the vagus (1), a parasympathetic nerve, and the splanchnic (2), a sympathetic nerve. Each of these nerves consists of a motor portion (efferent, sends orders from the central nervous system to the gut) and a sensory portion (efferent, receives information from the gut and sends them to the central nervous system). The cell bodies of the preganglionic motor part of the vagus are in the dorsal vagal complex (DVC) brainstem / hindbrain in a nucleus known as the motor nucleus of the vagus or DMV.The postganglionic cell bodies are located within or close to the myenteric ganglia. The cell bodies of the sensory portion of the vagus are in the nodose ganglia (NG).There are three vagal afferenets, intravillus arbors (I), which sense the lumen of the gut, intraganglionic laminar endings (II), which communicate with the plexuses of the enteric nervous system, and intramuscular arrays (III), which sense muscle tension. The cell bodies of the preganglionic splanchnic nerve are in the thoraco-lumbar area of the spinal cord, and the postganglionic cell bodies are in the celiaco-mesenteric ganglia (CMG).The cell bodies of the sensory portion of the splanchnic nerve are in the dorsal root ganglia (DRG).

Figure 4: The systems that control of the gut
The systems that control the diffrent functions of the gastrointestinal tract are divided into two categories, (1) intrinsic, located inside the wall of the gut and consists of nerves, the enteric nervous system or ENS, and endocrine secretions (gut hormones such as Secretin, gastrin, CCK, GIP, motilin), and (2) extrinsic, located outside the gastrointestinal tract and consist of nerves (vagus and splanchnic) and endocrine secretions (aldosterone).

Figure 5: Varicosities of Enteric Neurons
Diffrence between regular neuron (top picture) and enteric neuron (lower picture). Regular neurons secrete their neurotransmitters from their synaptic terminals. Enteric neurons secrete their neurotransmitters from bulges on their axons known as varicosities. Those structures allow the enteric neurons to spread their neurotransmitters over a longer and a wider area.

Figure 6: Morphology of the enteric neurons
The morphology of the enteric neurons can be divided into Dogiel type I neurons (C), which have small cell bodies, short one or two dendrites, and they are motor in function. Dogiel type II neurons (A, B, D), which have large cell bodies, long dendrites, and they are mostly sensory in function.

Figure 7: Small intestinal smooth muscle contraction
At any point, denotated by the vertical line, during small intestinal contraction the gut forms two segments, propulsive segment, and a receptive segment. Each of these segments has an inner circular muscle and an outer longitudinal muscle. For contraction to take place, so the food can move forward i.e., in front of the propulsive segment, the circular muscle in the propulsive segment must contract, using acetylcholine and substance P as the neurotransmitters, while the longitudinal muscle must relax, using vasoactive intestinal peptide and nitric oxide as their neurotransmitters. The opposite actions occur in the receptive segment. e circular muscle relaxes while the longitudinal muscle contracts.

Figure 8: Extrinsic Innervation of the gastrointestinal tract
Extrinsic innervation of the gut is provided by the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS).The SNS and the PNS are motor systems.The splanchnic nerve is sympathetic innervation of the gut, while the vagus nerve is the parasympathetic innervation of the gut. In each of these nerves, there are two types of fibers motor and sensory or different and different.The first sends orders from the central nervous system (CNS) to the gut and the second picks sensory information from the gut and sends them to the CNS. The PNS and the SNS is the eerent portion of each of the nerves.The vagus and the splanchnic, each as a unit, consists of two-neurons, preganglionic (PreG) and postganglionic (PostG). The cell bodies of the PreG sympathetic nerve are in the thoraco-lumbar area of the spinal cord. The cell bodies of the PostG sympathetic nerve are in the celiaco-mesenteric ganglia (CMG).The cell bodies of the sensory portion of the splanchnic nerve are in the dorsal root ganglia (DRG). The cell bodies of the PreG parasympathetic nerve are in the dorsal motor nucleus of the vagus (DMV) in the brainstem /hindbrain. The cell bodies of the PostG parasympathetic nerve are in or very close to the myenteric and submucosal plexuses of the enteric nervous system (ENS) in the gut. The cell bodies of the sensory portion of the vagus are in the nodose ganglia (NG). e PreG nerves of both the PNS and SNS secrete acetylcholine (ACH).The PostG nerve of the PNS continues to secrete ACH, while the PostG nerve of the SNS secretes epinephrin and norepinephrine (Epi/NorEpi).

Figure 9: Anatomy of the Rumen
An open view of a ruminant stomach showing two of its compartments, the reticulum, and the rumen with all its sacs, and pillars.

Figure 10: Anatomy of the pancreas
The pancreas is an accessary gland of the digestive system located beside the duodenum, and consists of three types of tissues, Islands of Langerhans, which secrete the endocrine secretions of the pancreas including insulin and glucagon, acinar cells, which secrete an exocrine secretin of the pancreas, including digestive enzymes such as lipase, amylase and trypsinogen, and ductal cells, which also secrete an exocrine secretion including water and bicarbonates.

Figure 11: Fat absorption
Absorption of fat occurs when the end-products of fat digestion like cholesterol (C), fatty acid (FA), monoglyceride (MG) and glycerol, cross enter the enterocyte from the intestinal lumen and reach the blood stream or the lacteal or the lymphatic stream. Glycerol crosses by simple difuusion from the intestinal lumen to the lumen of the enterocyte, then from the enterocyte to the blood stream. Cholesterol also enters the enterocyte by simple difuusion. Fatty acids and MG require fatty acid binding proteins (FABP) to enter the enterocyte from the intestinal lumen. In the enterocyte, FA and MG convert into triglycerides (TG). Finally, cholesterol, TG, phospholipid (PL) along with a protein named apolipoprotein, through a re-esterification process, form a molecule called chylomicron. Chylomicron then enters the lymphatic vessel to end in the thoracic duct before it reaches the venous circulation.

Figure 12: A photomicrograph of the small intestine following a myotomy / myectomy procedure (line) showing the severed and the intact layers of the gut. While the serosa and the two muscle layers are severed, the submucosa, the holding layer of the gut, is still intact. Any leakage through this layer may lead to peritonitis and severe post-operative complications. Therefore, the submucosal layer must be securely sutured. In addition, the serosa must also be kept clean and thoroughly irrigated during intestinal / abdominal surgery.

Figure 13: Celiaco-Mesenteric Ganglionectomy or removal of the celiaco-mesenteric ganglia, the sympathetic innervation of the gut. In A, the celiaco-mesenteric ganglia (denoted by the star) are located between the aorta (1), celiac artery (2) and the cranial mesenteric artery (3). In B, the ganglia have been removed.

Figure 14: Intra-arterial catheterization
This procedure has been developed to deliver very small amounts of combinations of gut hormones and peptides through external catheters to specic areas in the gastrointestinal tract. Such areas demonstrated that they contain sites of actions for those substances e.g., CCK, gastrin releasing peptide, glucagon like peptide 1. A depicts the normal anatomy of the vessels supplying the gastrointestinal tract, 1 aorta, 2 celiac artery which supplies the stomach and the upper duodenum, and 3 is the cranial mesenteric artery (CMA) which supplies most of the gut. 4 represent the renal artery. B depicts ligation of the CMA to control blood flow. C represents puncturing of the CMA. D represents placing a catheter in the CMA artery and loosening of the ligation.