The digestive system is a long tube inside the rat, with the mouth as the opening at the anterior end and the anus as the opening at the posterior end. The process of digestion, the enzymatic breakdown of complex food substances into their simpler components, occurs in the lumen (cavity) of the digestive tube. The small molecules resulting from digestion are then absorbed by the cells lining the gut and transferred to all the other cells of the body via the circulatory system.
Within the cells, these molecules may be burned to release energy for cellular activity, built into the structural elements of the cell, or stored for later use.
The undigested material passes along the gastrointestinal tract and out of the anus as feces. The mouth is the most anterior part of the digestive system. Within the mouth, the food is ground up by chewing and mixed with saliva, which contains carbohydrate-splitting enzymes and lubricating mucus. Incisors are the four front most long, sharp teeth in the mouth of a rat.
The incisors are especially designed for gnawing. If you make an incision on one side of the body from the region of the shoulder to the angle of the jaw, and continue cutting along the lower jaw you will reveal the salivary glands.
There are three pairs of salivary glands. The largest lies just behind the ear and extends to the ventrolateral surface of the neck. The other glands are more ventral and extend anteriorly under the lower jaw. The saliva, as previously mentioned, contains enzymes, which begin the digestion of carbohydrates, and mucus, which moistens food and sticks it together to facilitate swallowing. The tongue plays a big role in the swallowing response. The food moves from the mouth into a chamber shared by the respiratory system called the pharynx and on into the esophagus.
The esophagus can be seen under the trachea which is a tube recognized by its cartilage rings in the neck region. The other organs of the digestive system are located within the body cavities. All the organs of the body cavity, particularly those of the digestive system, are called the viscera. These organs are supported from the dorsal body wall by mesenteries. The wall of the body cavities and the organs are lined with a thin, moist membrane, the peritoneum. The liver is a large, reddish brown mass that lies immediately posterior to the diaphragm, the muscle dividing the thoracic and abdominal cavities.
The liver has a great number of functions. However, its role in digestion is to produce bile, a substance that emulsifies fats breaks them into minute droplets, making them easier to digest. In humans, the bile is stored in the gall bladder before being released into the small intestine. However, the rat lacks a gall bladder. Therefore, the bile is released through a duct directly into the small intestine, where it acts. The stomach is a muscular organ. It is located on the left side of the rat’s upper abdomen. Food enters through the mouth and travels to the stomach from the esophagus.
The esophagus pierces the diaphragm and is next to the trachea. It is different from the trachea because it does not have cartridge rings. It looks like a tube and moves food from the mouth to the stomach. At the top, it receives food from the pharynx and at the bottom; it discharges it into the first portion of the stomach. As food reaches the end of the esophagus, it goes into the stomach through a valve called the lower esophageal sphincter. To digest food, the stomach secretes acids and enzymes that break the food down. The stomach is lined with layers of muscle tissue called rugae.
The stomach muscles contract periodically, churning food to assist digestion. The pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine. Most of the digestion and the absorption of the products of digestion take place in the small intestine. Glands in the wall of the small intestine secrete enzymes for the breakdown of both proteins and carbohydrates. Secretions of the pancreas enter the small intestine and contain enzymes for the breakdown of fats, carbohydrates, and proteins. The pancreas is an irregular mass of brownish glandular tissue in the mesentery dorsal to the stomach.
It produces a hormone, insulin, which passes directly into the circulatory system and is not involved with digestion. The cecum is a large sac where the small and large intestine meet. This sac is often confused with the large intestine. It is the point at which the small intestine becomes the large intestine. Rats and rabbits, will produce a special feces formed from the cecum product. They will then ingest these feces again, to digest it a second time. This behavior is called coprophagy. Running from the cecum, the colon ascends, crosses the abdominal cavity, and descends again.
The colon connects posteriorly with the poorly differentiated ”’rectum”’ of the rat. The rectum connects the colon and the anus. The primary function of the large intestine is to absorb most of the water of the digestive secretions, conserving it for use within the body. Cardiovascular system The rat heart is small so that the details of its structure are difficult to observe. The heart is located inside the rib cage of the rat. The heart is a very vital organ to the rat. The Pulmonary circulations carry blood through the lungs for oxygenation and then back to the heart.
The blood enters the heart where it is pumped into the lungs for oxygenation. It is then distributed around the body only to return to the heart once again to start the process over. The rat circulatory system is almost identical to humans. The right side of the heart circulates blood to the lungs. The powerful contraction of the muscles of the left ventricle drives the blood out of the heart through the ”aorta” to the rest of the body. The effectiveness of these contractions is increased by the presence of valves that prevent the backflow of blood. These valves prevent blood from flowing back into the ventricles from the arteries.
The circulatory system performs the essential duties of transporting oxygen and nutrients to metabolizing body tissue and carries off carbon dioxide and other metabolic waste that will eventually leave the body. The rat has a closed circulatory system, which means that the blood remains within a system of vessels through which it is pumped by the heart. A vessel that carries blood away from the heart to a capillary bed is an ”artery”. A ”vein” carries blood in the reverse direction, from the capillaries back to the heart. The aorta is the largest artery in the rat’s circulatory system.
Its purpose is to carry the oxygenized blood that is being pumped out of the heart. The aorta arises from the left ventricle of the heart, forms an arch, and then goes down to the abdomen. Once in the abdomen, the aorta branches off into two smaller arteries. These arteries carry blood to all the veins and eventually back to the heart. The renal arteries stem off of the abdominal aorta. The purpose of the renal arteries is to supply the kidneys with blood. The heart pumps oxygenized blood into the aorta, which pushed the blood into the renal arteries to be taken to the kidneys.
Before reaching the kidneys, the artery divides into 4 or 5 smaller branches. Because of the anatomy of the rat’s body, the right renal artery is usually longer than the left renal artery. The arteries can usually be distinguished from veins because they are stiffer and lighter in color than veins. The vena cava veins returns blood from the body to the right atrium. The Pulmonary vein enters the left atrium with blood from the lungs. ?The Internal and external jugular veins is located in the neck region. These carry blood from the head region back toward the heart.
Muscular System There are three different muscle tissues: skeletal, cardiac, and smooth. They all have different organizations. Skeletal muscle tissue cells are long, cylindrical, and are striated. Cardiac muscle tissue cells are short and branched. Smooth muscle tissue is not striated, cells are short, and spindle- shaped. The biceps brachii lie on the upper arms between the shoulder and the elbow on both arms of the rat. These muscles are anterior to the shoulder and have two anchors in the shoulder region. The biceps brachii’s main function is to help the rat flex the forearm.
These muscles allow the rat to walk as well as use their arms to perform any type of activity. The center of the biceps brachii is thick and wide, while the ends are thinner and narrow. The masseter muscle is the primary muscle involved in chewing. It also acts to clench the teeth and raise the jaw. The sternomastoid is responsible for turning the head to the opposite side; it also helps extend the head. Pectoralis superficialis or Pectoralis profundus adducts the forelimb, and moves the arm toward the midline of the chest. The rectus abdominis helps compress the abdomen.
The spinodeltoid abducts the humerus and pulls the humerus forward. The spinodeltoid is the muscle right about the arm. The latissimus dorsi pulls the humerus backwards, and rotates the scapula backward. The large triangular muscle that lies caudal to the shoulder and arm and fans out over the back is the latissimus dorsi. The triceps are the largest muscle, which covers most of the caudal, lateral, and medial surface of the humerus. The triceps arises by three heads: a long head from the caudal, a lateral head from the proximal part of the humerus, and a small medium head (which is very difficult to see) from he proximal two-thirds of the humerus.
This muscle is the primary extensor of the forearm. The brachialis arises from the proximal part of the humerus and inserts the ulna. It flexes the forelimb, and flexes the forearm at the elbow. The gluteus medius is the largest of the gluteal muscles in the rat. It is seen in the lateral surface of the ilium and from the sacrum. The bicep femoris expands to form a broad muscle that is inserted along most of the length of the tibia. A group of three trapezius muscles covers the face of the shoulder.
Different parts of the complex are inserted on different parts of the shoulder, which is the cranial cleidocervicalis. The middle muscle of the shoulder is the cervical trapezius and the caudal is the thoracic trapezius, which is the upper part of the spine. These muscles help hold the shoulder in place. The deltoid complex is a triangular muscle mass lying ventra to the trapezius group. In rats, but not in humans, it consists of two parts: a spinodeltoid and cleidobrachialis.