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Biology
Rat #2: Muscles and Internal Organs
5.9.12
Muscle Introduction:
The design and location of muscles allows for the complex motion of vertebrates. Muscles are
usually arranged on the skeleton in opposing pairs: one muscle contracts to move the body in
one way and the contraction of the opposing muscle causes the body to move back to the
original position. For example, after you contract your bicep to bend your arm at the elbow,
you must contract the opposing muscle (triceps) to straighten your arm and stretch the first
muscle.
Motion produced by muscular contraction also depends on the design of the skeletal system.
When muscles contract, they use bones as levers for resistance and motion. As you examine
the muscles of a rat, you need to refer to the rat skeleton handout and locate the bones where
each muscle originates and attaches.
Muscles are attached to bones by connective tissue called tendons that attach to spines, knobs,
ridges, and depressions on bones. The end of the muscle attached to the bone that moves
during contraction is called the insertion. The end attached to the bone that does not move
during contraction is called the origin. Movement caused by the contraction of a muscle is the
muscle’s action.
Muscle Procedure:
1. Begin by examining and identifying the muscles on your skinned rat by picking the stringy
fascia away from the surface of the body. The more connective fascia that you remove, the
more clearly you will see the muscle.
2. Use a blunt probe to gently separate the various muscles. Work cautiously until you
develop a sense of where two muscles overlap.
3. Refer to the pictures frequently as you separate each muscle.
4. Find the following muscles. Know their action and locate their origins and insertions.
- External oblique
- Pectoralis major
- Internal oblique
- Flexors
- Spinotrapezius
- Gluteus superficialis
- Rectus abdominus
- Gracilis
- Biceps brachii
- Triceps brachii
- Latissimus dorsi
- Gastrocnemius
Organ Procedure:
1. Make a small incision with your scalpel just below the sternum (breast bone). Do not cut
too deeply into the body cavity.
2. With your scissors, snip through the abdominal wall toward the pelvic area. Cut just to the
right of the white line on the surface of the abdomen until you reach the genitals.
3. Cut from the genitals laterally to each hind limb.
4. Return to the chest, and snip the diaphragm loose from the rib cage. The diaphragm is a
thin layer of muscle separating the thoracic cavity from the abdominal cavity.
5. Open the thoracic cavity by cutting away the sternum (breastbone) and continuing with two
lateral cuts to each arm.
The Thoracic Organs:
The heart is centrally located in the thoracic cavity. The two dark-colored chambers at the top
of the heart are the atria (singular = atrium), which receive blood from the veins. The light
brown muscular areas below the atria are the ventricles, which are the pumping chambers of
the heart. The heart is covered by a thin membrane known as the pericardium.
Air enters the lungs from the bronchial tubes branching from the trachea. Posterior to the
lungs and heart lays the diaphragm. Cramps of the muscular diaphragm sometimes occur when
you run and can cause a sharp pain in your side when you try to breathe rapidly. The
diaphragm can also develop a twitch, which may occur every 10 – 20 seconds (hiccups).
The Abdominal Organs:
The coelem is the body cavity within which the internal organs are suspended. The abdominal
cavity and viscera are covered by a membranous tissue called the peritoneum.
The liver is the dark-colored organ suspended just under the diaphragm. Rats do not have a
gallbladder. The many functions of the liver include producing bile to aid in fat digestion,
storing glycogen (a high-energy molecule), transforming nitrogenous wastes into less harmful
substances, and transforming digested nutrients into various molecules needed by the body.
For these functions, the liver is strategically located near the digestive system to receive a rich
supply of blood directly from the intestinal tract. The liver has four lobes, or parts.
The esophagus pierces the diaphragm and carries food from the mouth to the stomach, which
is on the left side of the rat just under the diaphragm. The functions of the stomach include
food storage, physical breakdown of food, and initial enzymatic digestion of protein. The
entrance of the esophagus to the stomach is guarded by the cardiac sphincter. Sphincters are
circular muscles that control the flow of enclosed fluids by contracting and closing the opening
between cavities. The stomach is lined with ridges called rugae. The stomach is attached to
the small intestine via the pyloric sphincter.
The spleen is similar in color to the liver and is attached to the greater curvature of the
stomach. It is associated with the circulatory system and functions in the formation, storage,
and destruction of blood cells.
The pancreas is the brownish, flattened gland suspended in membranous tissue near the
junction of the stomach and small intestine. This gland has exocrine and endocrine functions.
Look in your book to see what the terms endocrine and exocrine mean. The pancreas produces
digestive enzymes, which go to the intestinal tract through small ducts. The pancreas also
secretes and releases insulin into the bloodstream to regulate carbohydrate metabolism.
The small intestine is the slender, coiled tube that receives partially digested food from the
stomach. The small intestine continues digestion and nutrient absorption. It consists of three
sections: duodenum, jejunum, and ileum. The duodenum receives enzymes from the digestive
glands. It begins at the pyloric sphincter of the stomach and continues under the stomach.
The duodenum has the small fingerlike folds of the intestinal wall called villi. Villi increase the
surface area of the intestines so that absorption of nutrients is increased. Absorption continues
as food travels through the jejunum and ileum towards the ileocolic sphincter, which controls
the movement of food into the large intestine.
The colon (large intestine) is the large greenish tube extending form the small intestine to the
anus. The colon is where the final stages of digestion and water absorption occur. It contains
bacteria that aid the digestion of cellulose and other foods difficult to digest.