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The Muscular System
OVERVIEW
INSTRUCTOR:
UNIT: Explanation of Animal Anatomy and Physiology Related to Nutrition, Reproductive
Health, and Management of Domesticated Animals.
LESSON:
The Muscular System
IMS REFERENCE: #8646-D
TOPIC NOTES
THE MUSCULAR SYSTEM
INTRODUCTION
The purpose of the animal’s muscular system is to move the animal and the materials
within the animal’s body. The muscles are important to the locomotion and life support
of animals. They are the lean portions of the carcasses of meat animals used for human
consumption. Muscles are similar to all tissues in that they consist of cells. In muscles,
the cells are in fibrous form. Muscle fibers usually occur in bundles or sheets and contain
large amounts of protein. Muscles are classified as voluntary and involuntary. The animal
can control voluntary muscles (e.g., leg muscles) but not involuntary muscles (e.g., heart
muscles). All voluntary muscles are striated, but involuntary muscles may be smooth
muscle, striated muscle, and cardiac muscle.
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ANATOMY OF VOLUNTARY MUSCLES
Muscle action and structure range from simple to very complex. As to function, muscles
can be categorized as flexor and extensor. The muscle responsible for decreasing the
angle between two bones when the muscle contract is a flexor, such as the biceps of the
forearm. Conversely, the muscle on the opposite side of the same two bones that is
responsible for increasing the angle between them is an extensor. The triceps, which are
responsible for extending the elbow, are a good example of an extensor. Muscles
involved in the action of more than one joint may have different classifications depending
on the joint involved. For example, the gastrocnemius muscle (a large muscle in the calf
of the leg) is both a flexor of the stifle joint and an extensor of the hock.
Muscles are agonists or antagonists relative to their involvement in a desired action.
Muscles producing a desired action are agonists; muscles producing the opposite of a
desired action are antagonists. Using the forelimb as an example, if the desired action is
extension of the elbow, the triceps are the agonists and the biceps are the antagonists.
However, the roles reverse if the desired action is a flexed elbow.
Muscles also may be adductors or abductors. Muscles that pull parts of the limbs toward
the middle of the animal’s body are adductors. The pectoral muscles of the front leg are
strong adductors and move the animal’s body forward if the foot is fixed on the ground.
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Muscles that tend to pull the limbs away from the middle of the body are abductors. The
deltoid muscles of the front leg are abductors; they also are flexors of the shoulder joint.
Smooth or striated muscles surrounding the openings in the body are sphincter muscles.
The muscles around the valve-like opening (pyloric sphincter) between the stomach and
small intestine are examples of smooth sphincter muscles. Muscles responsible for
closing the eyelid are examples of striated sphincter muscles. Cutaneous muscles are
responsible for movement of the skin. They are located just under the skin and joined to
the connective tissue covering the major skeletal muscles. Cutaneous muscles enable a
horse to twitch its skin when a fly lands on it. The scientific name of a muscle is usually
partially descriptive of the muscle and is determined by its action, attachments, shape,
position, direction, function, kind of tissue, or a combination of these factors. The name
is usually derived from two or more factors. For example, transversus implies movement,
and thoracis implies position. Thus, the transversus thoracis muscle in the thorax area of
the body moves during the process of breathing. The name of a muscle may also reflect
the muscle’s shape, such as triangular, quadrilateral, fanshaped, long, and short.
Voluntary (striated) muscle fibers can be sheet, bundle, spindle-shaped, and feather-like
arrangements. In the penniform (feather-like) arrangement, a tendon represents the quill
of the feather and the muscle fibers connected to it represent the vanes. If the muscle
fibers extend from only one side of the tendon, they are unipennate. When the fibers
extend from two sides of the tendon, they are bipennate. If they extend from three or
more sides of the tendon, they are multipennate. A pennate arrangement causes a muscle
to have the greatest power but decreases its potential for contraction. A parallel
arrangement of muscle fibers allows for the greatest contraction but does not produce the
greatest strength.
Voluntary muscle fibers vary in length depending on the length of the muscle that they
are a part of and whether they are pennate or parallel in structure. Some parallel muscle
fibers may extend the entire length of the muscle. In general, male animals have larger
muscle fibers than do females. Animals on full feed have larger fibers than do animals on
restricted diets. Exercise causes muscles to increase in volume. This increase is caused by
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an increase in the size of existing muscle fibers rather than an increase in the number of
muscle cells. At birth, an animal has the maximum number of muscle cells that it is
genetically capable of having. Therefore, if the nerve supply to a muscle is damaged, that
muscle will atrophy because it cannot regenerate. A voluntary muscle fiber consists of a
multinucleated body enclosed by a thin membrane (sarcolemma). A muscle fiber also
contains many smaller fibers with cross striations (myofibrils) that run parallel to the
elongated axis of the muscle fiber. The muscle fiber is filled with protoplasm
(sarcoplasm, in this case). The sarcoplasm contains delicate filaments (endoplasmic
reticulum) that are involved in the metabolism of the fiber as well as in the reaction to
nerve messages. The sarcolemma is the main contributor to the elasticity of the muscle
fiber. It acts as the vehicle of attachment to the tendenous tissue of other muscle fibers or
to the tendon that connects to the skeletal system. Connective tissue (endomysium)
surrounds individual muscle fibers. Bundles of muscle fiber are enclosed in connective
tissue called perimysium. The entire muscle is then enclosed in connective tissue called
epimysium. Some muscles may appear connected directly to the bone by a fleshy
attachment. Actually, the muscle fibers connect to short tendons that attach to the
periosteum of the bone or that may even slightly penetrate the surface of the bone. Dense,
regular, connective tissue makes up tendons whose fibers are arranged in parallel
bundles. Most tendons connect spindle-shaped or pennate muscles to bones. However,
some tendons appear as flat sheets and are usually associated with flat muscles, such as
the heavy fibrous sheet that covers the loin area.
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Many voluntary muscles connect to bones by tendons, while some are attached to
cartilage, ligaments, fascia (connective tissue), or skin. One of the attachments is usually
less movable than the other. The least movable attachment is called the origin while the
other attachment is referred to as the insertion. A good way to describe the attachments is
using the bicep muscles of the upper arm as an example. The origin of the muscle is
attached to the scapula that moves less than the radius of the lower arm to which the
insertion of the bicep muscles is attached. Muscles contract, bringing their origin and
insertion closer together. In this process, they move one or both of the bones to which
they are attached. Synovial structures reduce friction and allow for free movement of
body parts. These structures include capsules, bursae, and synovial sheaths. Each consists
of an inner layer of connective tissue membrane that produces synovial fluid. Joint
capsules usually produce just enough fluid to promote the smooth operation of the joints.
Sometimes these capsules produce excess synovial fluid caused by inflammation of the
joint that results in swelling and pain (arthritis).
A bursa is a synovial sac located between two body structures to reduce friction. A bursa
is located just under the skin at the elbow to reduce friction between the skin and the ulna
when the elbow moves. Another bursa is located at the hock between the skin and the
tendon at the point of the hock. Excess fluid can sometimes be produced by the bursa,
thus resulting in bursitis. Bursae give adequate protection for those structures that move
only slightly. Tendons move a long distance (several inches) and need protection for the
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entire length of the tendon. Therefore, a synovial sheath encloses those types of
structures. A synovial sheath is similar to a bursa wrapped around a tendon, resulting in
two layers of synovial membrane with synovial fluid between the two layers being
wrapped around the tendon. Inflammation of the synovial sheath is synovitis.
PHYSIOLOGY OF VOLUNTARY MUSCLES
The physiology of muscle action is quite complex. Energy for muscle contraction comes
mainly from nonprotein sources such as from the breakdown of glycogen and body fats.
The chief source of energy for muscle contraction is from the breakdown of adenosine
triphosphate (ATP). However, if food intake is insufficient, energy for muscle contraction
may also come from protein sources. Striated muscle fibers consist of bands of light and
dark fibers from which they get their name. The lighter bands, called isotrophic bands,
contain small filaments of the protein actin. Darker bands, called anisotrophic bands,
consist of filaments of the protein myosin. Approximately six times as many actin
filaments occur compared to myosin filaments. A small, dark band within the lighter area
of muscle fibers appears to be an extension of the sarcolemma. It is responsible for
transmission of the nerve message to the inside of the muscle fiber. A lighter area within
the darker band contains only myosin, but the remainder of the darker band consists of
myosin and actin. Muscle contraction involves longitudinal movement of the actin
filaments relative to the myosin filaments. Neither filament shortens, but movement of
the filaments relative to each other causes the contraction. When the muscle fiber
contracts to 65% of its resting length, the lighter band disappears. One theory of muscle
contraction involves the folding of protein molecules to produce contraction. Adenosine
Triphosphate (ATP) is the primary source of energy for muscle contraction. Myosin and
calcium ions form an enzyme (adenosine triphosphatase) that splits the ATP molecule
into adenosine diphosphate and phosphoric acid. During this process, energy is released
that the muscle fibers use to contract. If ATP is unavailable, the muscles become stiff,
which may be caused by the locking of the myosin and actin filaments. Rigor mortis is a
hardening of the muscles when an animal dies.
Muscles relax if sufficient ATP is present but is not being split. A substance called
alphaglycerophosphate in conjunction with magnesium ions and the enzyme activity of
myosin may cause the cessation of ATP splitting and subsequent muscle relaxation. The
recovery phase that the muscle undergoes after it has relaxed involves the breakdown of
glycogen into lactic acid and then the oxidation of the lactic acid. The presence of oxygen
is necessary for the oxidation of the lactic acid so energy can be created for future muscle
contraction. The maximum contraction that can be expected of a muscle is about one-half
of its resting length. Whenever a muscle fiber is stimulated to contract by a nerve
impulse, it contracts to its maximum ability. Therefore, the greater the contraction of a
muscle, the greater the individual muscle fibers are stimulated to contract. A branch of a
voluntary nerve, or motor neuron, controls each voluntary muscle fiber. A motor neuron
and its many branches connected to muscle fibers are called a motor unit.
The nerve impulse that stimulates muscle contraction does not travel beyond the nerve
ending. The electrical stimulus provided by the nerve is passed to the muscle by a
chemical reaction that passes through the nerve and muscle membranes. An electrical
action potential is produced by a change in the balance of ions inside and outside of the
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nerve ending membrane. The resulting chemical ions on the outside of the nerve
depolarize the muscle fiber membrane resulting in muscle fiber contraction.
Only 25% of the energy produced for muscle contraction is actually used by muscles.
The remainder of the energy generates heat. This heat is important in maintaining body
temperature when the environmental temperature decreases. Two general types of heat
are involved - initial heat or heat given off during muscle contraction and heat of
recovery given off when the muscles are rebuilding energy.
ANATOMY AND PHYSIOLOGY OF INVOLUNTARY MUSCLES
The organ walls of many body systems, such as digestive, reproductive, circulatory, etc.,
consist of involuntary muscles. In addition, most secretory organs have involuntary
muscles to force secretions out of the body. Involuntary muscles move food through the
digestive tract. Involuntary muscles in the uterus aid the sperm in reaching the
descending ovum in the reproductive tract, as well as aiding in the birth process.
Involuntary muscles cause heart and blood vessels to contract, thus regulating blood flow.
Involuntary muscles are smooth or striated, spindle-shaped, and with centrally located
nuclei. The sizes of involuntary muscle cells vary considerably. For example, the
increased size of the uterine wall during pregnancy is caused by an increase in the
amount of smooth involuntary muscle. Involuntary muscles may increase in number
through mitosis, and nonspecific mesenchymal cells may develop into involuntary
muscle cells.
Each cell of smooth involuntary muscle is surrounded by connective tissue. The
connective tissue joins with other connective tissue associated with involuntary muscle.
Smooth involuntary muscle cells of hollow organs are organized in such a manner that
only a small fraction of them ever receive a direct nerve stimulus to contract. Those
receiving the nerve impulse for contraction spread the message from one muscle fiber to
another. In some situations, each smooth muscle cell appears to receive its own nerve
stimulation. Examples include the smooth muscles of the skin and eye.
Smooth muscles contract as a result of nerve impulses, chemicals, hormones, or electrical
stimulation. The hormones epinephrine and nonepinephrine stimulate sympathetic nerves
to relax and contract involuntary smooth muscle, respectively. Acetylcholine released by
the parasympathetic nerves also stimulates contraction of involuntary muscles. Smooth
muscle contraction is usually rhythmic (in a wave-like motion), such as peristalsis of the
intestines. Gradual stretching generally does not cause involuntary muscles to contract, as
is the case for the bladder. However, sudden or extreme stretching can cause involuntary
muscles to contract.
Cardiac muscle is striated involuntary muscle. Heart muscle fibers form a network by
connecting with each other.
Although the heart beats automatically, the autonomic nervous system influences the rate
and force of contraction.
Cardiac muscle contains many blood and lymph vessels. The large blood supply is
important as the heart begins beating early in prenatal life and continues until the animal
dies. Overexertion or high altitude causes heart muscles to increase in size, thus resulting
in heart problems.
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MUSCLES OF THE FRONT LIMB
The trapezius (a triangular, flat muscle) originates along the midline of the back. Its
points of attachment extend from the head to the area of the lumbar vertebrae. The
trapezius primarily inserts along the spine or raised ridge of the scapula (shoulder blade).
Portions of this muscle, in front of the scapula, swing the scapula forward. The
attachment to the midline behind the scapula swings the scapula backward. A heavier
muscle located under the trapezius is the rhomboid. This muscle also originates from the
midline and inserts on the deeper, inside face of the scapula.
The largest and most important muscle attaching the front leg to the body is the serratus
ventralis. It is a fan-shaped muscle originating from the raised protrusions of the cervical
vertebrae. Serratus ventralis inserts along the inside of the top of the scapula. The muscle
serves as a sling to support the trunk of the body between the front legs. It also swings the
lower part of the scapula backward and forward.
The brachiocephalicus muscle’s point of origin is the occipital bones of the skull and
protrusions of the cervical vertebrae. It extends over the point of the shoulder and inserts
on the outside of the top part of the humerus bone.
It is the shoulder’s main extensor and also acts as a flexor to pull the neck to the side. The
brachiocephalicus is a single part muscle in horses, but in cattle, sheep, and dogs, it has
three parts.
The latissimus dorsi (a wide, triangular muscle) originates from protrusions on the
thoracic and lumbar vertebrae. It inserts itself on the inside of the humerus. It is a strong
flexor of the shoulder and pulls the front leg backward. The primary muscles in the
brisket are pectoral muscles. They originate from the sternum and insert on the top part of
the humerus. The deltoid muscles originate from the scapula and insert on the humerus.
Muscles associated with the elbow are either flexors (located in front of the elbow) or
extensors (located in the back of the elbow). The main flexors of the elbow are the
biceps, which originate just in front of the joint surface of the scapula and insert on the
top end of the radius. In animals with a separate radius, they tend to rotate the forearm
outward. The triceps are the main extensors of the elbow. One part of the triceps
originates on the lower part of the scapula, and the other two parts originate on the inside
and outside of the humerus. All three parts insert on the point of the elbow at the top end
of the ulna.
The main extensor of the carpus or knee joint is the extensor carpi radialis. It originates
on the bottom end of the humerus and inserts on the top end of the cannon bone. It is the
most common muscle of the forearm. The primary flexor of the carpus is the flexor carpi
radialis.
The extensor of the digits in the foreleg is the common digital extensor. This muscle is
the extensor of all the digits; for example, the double digits in cattle and the four digits in
swine. The main digital flexors in all animals are the deep and superficial digital flexors.
MUSCLES OF THE REAR LIMB
Although the hip joint is a ball-and-socket joint, the primary movements are extension
(backward movement) and flexion (forward movement). The main extensor muscles of
the hip, commonly called hamstrings, extend from the pin bones behind the hip joint to
the tibia or fibula. The muscles in this group include the biceps femoris, semitendinosus,
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and semimembranous muscles. The vertical grooves visible in the thighs of an animal’s
hindquarters are the divisions of these three muscles.
Another strong extensor of the hip joint is the middle gluteus muscle. The muscle
originates from the wing of the pelvic girdle and inserts on the femur. Important flexors
of the hip are the iliacus and psoas major muscles. They are located in front of the femur.
The iliacus’ point of origin is the wing of the pelvic girdle. The psoas major’s point of
origin is the extension of the lumbar vertebrae. The animal’s tenderloin consists of the
psoas major and psoas minor.
Abductors (muscles that move the hindleg away from the middle of the body) run
lengthwise over the hip joint.
They serve as levers or pulleys in moving the leg away from the body. For example, the
deep gluteus muscle extends from the front of the pelvic girdle to the top of the femur.
When the muscle pulls the top of the femur toward the center of the body, the remainder
of the leg moves backward.
Muscles that serve as adductors in pulling the leg toward the middle of the body are
located on the inside of the thigh. They extend from the pelvic girdle to the femur or the
tibia. The gracilis muscle has the greatest inward pull on the hindleg. The largest muscle
on the inside of the thigh is the adductor muscle.
Because the stifle acts basically as a hinge joint, the muscles associated with it are mainly
extensors and flexors.
Much of the stifle’s extension is accomplished by the quadriceps femoris. The muscle
consists of four parts, with the longest part originating from the pelvic girdle and the
other three originating from the femur shaft. All four muscle parts insert on the patella.
The patella fastens to the front of the tibia. When the quadriceps pull on the patella, the
hock extends. The hamstring is the main flexor of the stifle.
Flexor and extensor muscles are primary movers of the hock. Extensors of the hock
primarily attach to the point of the hock by the tendon of achilles. The gastrocnemius and
superficial digital flexor originate at the femur’s bottom and backside. The tibialis
anterior and peroneus muscles are the primary flexors of the hock. They are on the front
surface of the hock and insert on the tarsus and metatarsus.
Flexors and extensors of the hind digits are very similar to those for the front digits. The
long digital extensor originates from the lower part of the femur and inserts on the third
phalanx. This muscle, in order to correlate with the number of digits in the species,
ranges from one part in the horse and two parts in cattle and sheep to four parts in the
dog, cat, and pig. Superficial and deep flexors of the hindleg are similar to those in the
foreleg, except the superficial flexor also attaches to the point of the hock.
MUSCLES OF THE TRUNK, NECK, AND HEAD
The longissimus dorsi is the loin muscle extending along either side of the protrusions of
the vertebrae to the head.
The muscle consists of many bundles of muscle fibers that extend lengthwise between the
vertebrae or the protrusions of the vertebrae. In domestic animals, the muscle serves for
extension and lateral flexion of the spinal column.
It also allows slight twisting of the vertebral column, as is seen in a bucking bull.
The same general arrangement of muscles in the neck exists in the animal’s back.
However, the neck has greater flexibility. The neck muscles that extend or raise the head
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include the splenius, complexus, rhomboids, dorsal oblique, and dorsal straight. Many of
these muscles originate from the vertebrae in the area of the withers and insert on the
occipital bones of the skull. A heavy ligament (ligamentum nuchae) also extends from the
withers to the skull to assist in raising the head.
Head flexion is attributed to the force of gravity. However, the sterno-cephalicus
originating from the sternum usually inserts on the mandible of animals and assists in the
flexion of the head by the neck. Other flexors of the head include the sternothyrolyoideus,
longus coli, and ventral straight muscles.
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Support of the digestive and reproductive organs is an important function of abdominal
muscles. Abdominal muscles assist in defecation (excretion of solid waste), urination,
and gestation (female). Abdominal muscles also assist in regurgitation and rapid
expiration of air during sneezing or coughing. They are arranged in layers with muscle
fibers extending fan-like in various directions. Many of these muscles have a broad
insertion at the midline of the abdomen called the linea alba (white line). The external
abdominal oblique muscle originates from the last few ribs and a heavy sheet of
connective tissue over the lumbar area of the back. The muscle extends back and
downward where it inserts to the ligament at the white line of the abdominal cavity.
Towards the rear of the abdominal cavity, this muscle forms the outside of the inguinal
canal through which the spermatic cord of the male extends into the scrotum.
Immediately under the external abdominal oblique muscle is the internal abdominal
oblique muscle. The muscle extends downward in a forward direction and also inserts at
the middle of the abdominal cavity. The muscle forms the inner wall of the inguinal canal
and inguinal ring to prevent protrusion of the intestines into the scrotum. The rear fibers
of the muscle extend through the inguinal canal and attach to the outer covering of the
testicles. This attachment, called the cremaster muscle, draws the testicles to the body.
The deepest abdominal muscle, the transversus abdominis, originates from the back and
goes directly down to the white line of the abdomen. The floor of the abdomen is formed
by the rectus abdominis, which originates from the cartilage of the ribs and sternum. It
extends straight back and inserts on the pubis of the pelvic girdle.
The muscles associated with respiration are either expiratory (pushing air out of the chest
cavity) or inspiratory (drawing air into the lungs). The main muscle of inspiration is the
diaphragm. The diaphragm is a convex-shaped sheet of muscle that separates the chest
cavity from the abdomen. When the muscle contracts, it pushes the abdominal organs
downward, thus increasing the capacity of the chest. Simultaneously, a vacuum develops
to draw air into the lungs. The external intercostal muscles extend between the ribs.
When they contract, they push the ribs upward and outward, thus increasing the capacity
of the thoracic cavity.
Expiration is primarily accomplished by the abdominal muscles pushing the abdominal
organs against the diaphragm, thus decreasing the thoracic capacity. The internal
intercostal muscles lie under the external intercostals between the ribs. When they
contract, the ribs are pushed inward and downward and force air from the lungs.
ACKNOWLEDGEMENTS
Kristy Corley, Graduate Technician, Department of Agricultural Education,
Texas A&M University, revised this topic.
Larry Ermis, Curriculum Specialist, Instructional Materials Service,
Texas A&M University, reviewed this topic.
Vickie Marriott, Office Software Associate, Instructional Materials Service,
Texas A&M University, prepared the layout and design of this topic.
Christine Stetter, Artist, Instructional Materials Service,
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Texas A&M University, prepared the illustrations for this topic.
REFERENCES
Anatomy and Growth and Development. [On-line]. Available:
http://www.ca.uky.edu/agripedia/
classes/asc106/growskel.htm. [2002, February]
Campbell, John R. and John F. Lasley. The Science of Animals That Serve Humanity. St.
Louis, MO:
McGraw Hill Book Company, 2001.
Frandson, R. D. Anatomy and Physiology of Animals. Philadelphia, PA. Lea & Fibiger,
1992.
Mellors, Robert C. Bone. [On-line]. Available:
http://edcenter.med.cornell.edu/CUMC_PathNotes/
Skeletal/Bone_01.html. [2002, February].
Muscles. [On-line]. Available:
http://www.ultranet.com/~jkimball/BiologyPages/M/Muscles.html. [2002,
February].
Stufflebeam, Charles E. Principles of Animal Agriculture. Englewood Cliffs, NJ: Prentice
Hall, Inc., 1983.
The Muscular System. [On-line]. Available:
http://www.msms.doe.k12.ms.us/biology/anatomy/
muscular.html. [2002, February].
GLOSSARY OF TERMS
Atrophy – A decrease in size of a tissue, organ, or other body part.
Autonomic – Involuntary or spontaneous; in reference to glands and smooth and cardiac
muscles.
Bursa – Sac or cavity, in reference to body joints.
Bursitis – Inflammation of a bursa.
Cardiac muscle – Muscle that makes up the wall of the heart.
Convex – Having a surface that curves outward.
Defecation – The process of eliminating feces from the body.
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Depolarize – To counteract the state of having positive and negative ends relative to
magnetic poles.
Dorsal – Of, toward, on, in, or near the back or upper surface of an organ, part, or
organism.
Enzyme – A protein molecule produced by the body that speeds up various chemical
reactions without being used up itself.
Excretion – Process of discharging waste materials from the bowels.
Expiratory – Of, relating to, or involving the expiration of air from the lungs.
Fascia – Thin sheet of fibrous connective tissue covering or binding together a muscle,
part, or organ.
Gestation – The period of life in a mammal from fertilization to parturition; duration of
pregnancy.
Hormones – Chemical substances, produced in the body by endocrine glands, that are
transported by the blood to other organ s to stimulate their functions.
Inspiration – Breathing in of air by the lungs.
Intercostal muscles – The muscular area among the ribs of an animal.
Ions – Molecules carrying an electrical charge, positive or negative, and usually formed
when salts, acids, or bases are dissolved in water.
Ligaments – Tissues connecting bones and/or supporting organs.
Mesenchymal cells – Embryonic connective tissue cells that give rise to the connective
tissue of the body and blood vessels.
Metabolism – Sum total of the chemical reactions changing energy to make it available
for body use.
Mitosis – Form of cell division producing two identical cells related to tissue growth and
maintenance.
Nuclei – Parts of a cell containing chromosomes enclosed by a special membrane.
Ovum – The female gamete or reproductive cell; the egg.
Oxidation – The combining of oxygen with another element to form one or more new
substances.
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Parasympathetic – Relating to the nervous system in the vascular walls.
Peristalsis – Specialized contraction in a wave-like motion that causes the materials
inside the digestive system to move along.
Prenatal – The period prior to birth in the development of a mammalian fetus.
Protoplasm – A semi-liquid, clear substance that is the essential living matter of all
animals and plants.
Quadrilateral – Having four sides.
Regurgitation – A process whereby the contents of the stomach are brought up from the
stomach through the esophagus to the mouth.
Rigor mortis – Stiffness that occurs in the muscles shortly after an animal dies.
Secretory organs – Organs that help force secretions out.
Smooth muscle – The muscle in the walls of all the hollow organs of the body (except the
heart).
Sphincter – Relating to a circular muscle in a tubular organ that acts as a valve in closing
the opening.
Striated muscle - The voluntary muscle attached to the skeleton; skeletal muscle.
Sympathetic – Pertaining to the autonomic nervous system that reduces the contraction of
smooth muscles in blood vessels.
Synovial fluid – A thick, adhesive fluid containing synovin, mucin, or small amounts of
mineral salts and found in joint cavities, bursa, and tendon sheaths.
Synovitis – Inflammation of a synovial membrane resulting in pain and swelling in a
joint.
Tendon – A type of connective tissue that connects muscle to bone.
Ventral – Relating to or situated on the lower surface of the body of an animal.
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