Functional Anatomy

Equine Science II

Four Basic Surfaces1. Dorsal refers to the upper surfaces of

the animal2. Ventral refers to the lower or

abdominal area3. Anterior or cranial refers to the head

or front4. Caudal or posterior refers to the tail

or rear

Anterior

Caudal

Ventral

Dorsal

Nine Body Systems

1. Skeletal System- includes bone and cartilage that give the body shape and protect internal organs

2. Muscular system- provides movement both externally and internally

3. Digestive System- converts feed into a form that can be used by the body for maintenance, growth and reproduction

4. Urinary System- maintains water and mineral balance while holding and excreting wastes when needed

5. Respiratory System- takes in oxygen and delivers it to the tissues and cells and picks up carbon dioxide from the tissues and cells to deliver to the environment.

6. Circulatory System- distributes blood throughout the body

7. Nervous System- supplies the body with information about its internal and external environment through electrical impulses between the brain, spinal cord, and other parts of the body.

8. Reproductive System- contains the organs necessary for the creation of new organisms.

9. Endocrine System- produces hormones that influence vital functions.

Two Divisions of Skeletal System and Articulations

1. The axial skeleton consists of the bones in the trunk area which includes the skull, spine (vertebral column), ribs and breastbone (chest cavity), pelvis and tail

a. Bones of the skull are flat or irregular in shape and form the framework for the brain, mouth, eyes and nasal cavities.

b. The vertebral column consists of a flexible column of small bones (vertebrae) that form the basis of the skeleton

c. Hip bones are two large flat bones attached to the spine and sacrum that form the pelvis or pelvic girdle and cavity

d. The ribs and breast bone (sternum) along with the thoracic vertebrae form the chest cavity.

2. Appendicular skeleton consists of the forelegs and hind legs that are used for locomotion, grooming and to some extent for defense and feeding

a. The forelimbs have no skeletal attachments to the axial skeleton or trunk, but are connected by muscles.

b. The hind limbs are attached to the pelvis at the hip joint

3. Articulations or joints are the union of two or more bones or cartilages held together by ligaments, tendons, or a tough fibrous capsule.

a. Joints are classified by their structure and movability.

b. Freely movable joints have a joint cavity between the two surfaces, the bones are covered with a smooth cartilage and the bones are held by ligaments

Muscular System red, lean meat compose about 50% of the

equine’s total body weight stimulated to contract or

change shape by nerve impulses from the brain

send nerve impulses back to the brain indicating the degree of contraction so that movement is smooth

Three Basic Muscle Types1. Smooth muscles are

involuntary muscles.a. Sometimes called visceral

muscle

b. Found in the digestive system and in the uterus of females

c. Capable of prolonged activity without fatigue

2. Cardiac muscles in involuntary striated muscle

a. Found only in the heart

b. Contractions of the cardiac muscle require no nerve stimulus

c. Contractions are rhythmic and require no conscious control

3. Striated or skeletal muscles are usually attached to bones of the skeletal system either directly or by tendons and act voluntarily.

a. Bones serve as levers and the muscles move the body voluntarily under the direct control of the will

b. Skeletal muscles are usually arranged in opposite sets so that one set of muscles bends the limb (flexor muscle) and the other set straightens it (extensor muscle)

c. Voluntary muscles become fatigued and need rest after a short period of time.

3. Tendons eliminate undue friction to allow muscles to act more freely

a. The tendon sheath is a synovial sac through which a tendon passes and which provides synovia to lubricate the surrounded tendon.

b. The tendon bursa does the same thing as the sheath except the tendon does not pass through it. The tendon bursa is a synovial sac interposed between the tendon and the surface over which it comes in contact which lubricates and cushions the tendon.

c. Both are found mainly near joints

Digestive System1. The parts of the

digestive system work together to convert feed into a form that can be used by the body for maintenance, growth and reproduction.

Parts include:– Mouth– Pharynx– Esophagus– Stomach– Intestines– Anus

Associated Organs– Liver, teeth pancreas

and salivary glands

A. Rate of feed passage through the stomach (30 minutes to 2 hours) and small intestine is very rapid so that any feed not digested and absorbed in the small intestine passes on to the cecum and colon within 24 hours.

B. Grinding or chopping decreased feed size which increases the rate of passage but decreases the absorption of nutrients by the equine

C. It is easy to overwhelm the digestive capacity of the equine’s stomach and small intestine with large amounts of concentrates, which if pass through to the cecum, become fermented and produce gas or lactic acid causing colic or founder

2. The mouth extends from the lips to the pharynx. It is bounded on the sides by the cheeks, above by the hard palate and below by the tongue.

3. The pharynx is the muscular somewhat funnel shaped tube from the back part of the mouth to the esophagus. It directs food and serves as an air passage.

4. The esophagus extends from the pharynx down the left side of the neck through the thoracic cavity and diaphragm to the stomach at an angle which makes regurgitation impossible.

5. The stomach is a U-shaped muscular sac at the front of the abdominal cavity close to the diaphragm and makes up less than 10% of the total digestive capacity for an adult equine.

6. The small intestine is a tube about 2” in diameter and is the site of most nutrient absorption but makes only 30% of the total digestive capacity.

7. The large intestine consist of the cecum, large colon, small colon, rectum and anus.

a. The cecum and colon make up 65% of the digestive system capacity and are greatly enlarged to allow bacteria time to break down the large quantities of cellulose from plant material.

b. The small colon extends from the large colon to the rectum and is the place where the balls of dung are formed. The contents are solid as most of the moisture in the food is reabsorbed in this part of the large intestine.

Urinary System The urinary system is made up of the

kidneys, ureters, bladder and urethra which filter the blood and dispose of waste products– All the blood in the body passes though

the two kidneys more than 400 times per day and is filtered of nitrogenous wastes each time

– The kidneys are from 6-7 inches long, 4-6 inches wide about 2 inches thick and are located on each side of the backbone at about the 18th rib

– Millions of tiny nephrons in the outer cortex of the kidneys filter about 200 gallons of liquid per day, rejecting blood cell and proteins but permitting fluid, salts and other chemical waste to pass through them

– The kidneys return about 198 gallons of the 200 gallons to the bloodstream, including salts, sugars and most of the fluids

– The two gallons of waste fluid, urine is collected in the inner portion of the kidney, the renal pelvis and drained drop by drop through the ureters to the bladder.

Respiratory System

1. The respiratory system takes in oxygen and delivers it to tissue and cells and removes carbon dioxide from those same tissues and cells.

– The lungs are the essential organ of respiration

– Air is taken into the lungs where oxygen is removed by diffusion into the blood

2. Supporting parts of the respiratory systems carry air to and from the lungs and include the nasal cavity, pharynx, larynx, trachea and bronchi

– The pharynx is common to both the digestive and respiratory systems

– The larynx serves as the voice box, allowing equine to make sounds such as neighing, whinnying, nickering and regulates the amount of air passing into or out of the lungs.

– The trachea is a long tube connecting the larynx with the lungs and branches into bronchi in each lung containing minute air sacs called alveoli where the gaseous exchange of carbon dioxide takes place between the circulating blood and air.

Circulatory System1. The circulatory

system distributes blood throughout the body to nourish each cell with food substance and oxygen while removing waste products

2. The parts of the circulatory system include the:

• Heart- A muscular organ of about 7-8 pounds in ordinary sized equine that is the main pump of the circulatory system that creates blood flow

• Arteries- Vessels with rather thick elastic walls that carry blood from the heart to the tissues of the body.

• Veins Vessels that carry blood from the tissues back to the heart

3. Capillaries connect the arteries carrying blood cells and the veins carrying blood away from the cells

• It is through the walls of the capillaries that the exchange of food and oxygen for waste products takes place

• Capillaries are microscopic in size

4. Blood is the red alkaline fluid tissue that is the regulator of the body. It is composed of blood plasma and red and white blood cells

• White blood cells are the active agents in fighting disease germs in the body

• Red blood cells originate in the bone marrow, liver and spleen and carry oxygen from the lungs and carbon dioxide from the tissues

5. Lymph is the fluid that assists in carrying food from the digestive tract to the tissues and waste products back to the bloodstream

• Lymph vessels are ducts that transport lymph and lymph nodes or lymphatics, are gland-like bodies found in the lymphatic vessels that act as filters.

• The vessels converge to form on large ducts that lies parallel to the aorta, the main artery from the heart and empties into one of the large veins near the heart.

Nervous System1. The nervous system is a complex system

that uses electrical-chemical changes to send impulses to and from the brain or spinal cord, nerve fibers and sensory receptors.

2. The nervous system is the communication system of the body made up of the brain, spinal cord, ganglia (secondary nerve center along the spinal cord) and nerves.

3. The nervous system has two main portions

• The autonomic or automatic nervous system is directed by the brain stem to control respiration, digestion and major organs like the heart automatically.

• The central nervous system is directed by the brain to control the conscious or voluntary actions of the body like movement.

Endocrine System

1. Consists of ductless glands producing internal secretions called hormones that control and promote homeostasis (balance) between physiologic functions

2. Hormones regulate bodily reactions by slowing or speeding the rate of organ functions

3. The pituitary and hypothalamus work together to coordinate the endocrine and nervous system

• The hypothalamus is the center of the autonomic nervous system

• The hypothalamus releases hormones that regulate the pituitary

4. Some hormones and glands:• Hypothalamus: ADH (vasopressin)

is the trauma hormone, oxytocin controls lactation and reproductive phases of the mare.

• Pituitary: • FSH stimulates the ovaries of the mare and

sperm production in the male, • LH stimulates secretion of estrogen in the

mare and testosterone in the male, • LTH (prolactin) promotes lactation once the

mammary system has been primed by estrogen and progesterone,

• TSH stimulates the thyroid gland to release thyroxin that regulates body metabolism

• Pancreas: insulin regulates glucose metabolism.

Reproductive System

Stallion1. Testicles- Primary sex organ

used to produce sperm and the male hormone testosterone

2. The seminal vesicles, prostrate gland and bulbo uretral gland are accessory sex glands that furnish fluid secretions for the semen.

3. The epididymis carries sperm from the testes to the Vas deferens and provides a place for storage and maturation of sperm

4. The Vas deferens transports mature sperm from the epididymis to the urethra

5. The urethra takes sperm from the Vas deferens to the end of the male genitalia

6. The penis deposits the sperm into the female reproductive tract.

• Stallions can ejaculate from 60-300 milliliters of sperm with up to about 10 billion live spermatozoa per ejaculate

• A sperm cell may live in a mare’s reproductive tract for 24-48 hours.

• Sperm travel to the mare’s Fallopian tubes in as short a time as 15-18 minutes but normally requite 2-6 hours

Mare1. The ovaries produce eggs and secrete the

hormone estrogen that causes heat (estrus) and stimulates the production of other hormones to condition the reproductive tract

2. The Fallopian tubes connect the ovaries to the uterus and are the normal site of fertilization of the egg by the sperm

3. The uterus consists of the body, cervix and two horns. It is in one of the two horns that the egg normally develops after fertilization

4. The vagina receives the sperm during mating and serves as the birth canal

5. The uterus is prone to infections due to its elongated shape

6. The mare’s reproductive system has an unusual placenta arrangement and inefficient cervical closure that may hinder full-term pregnancy.

Functional Muscle Groups1. Flexors: Bend the limb

• Decrease the angle of a joint• Examples: Teres major in the front leg

and Iliacus in the hind leg

2. Extensors: Straighten the limb• Increase the angle of a joint• Examples: Brachiocephalicus of the

front leg and Gluteus medius of the hind leg

3. Abductors• Move a limb away from the center plane of

the equine• The abductor of the front leg is the deltoid

4. Adductors• Pull a limb toward the center plane of the

equine• Adductors of the front legs are the pectoral

muscles

Terminology of Movement

1. A gait is an equine’s way of going or moving its legs during movement

2. A beat refers to the time a foot or two feet simultaneously hit the ground.

3. A step is the distance between the imprints of the two front legs or the two hind legs

4. A stride is the distance between successive imprints of the same foot

5. Directness, also called trueness is the line in which the foot is carried forward during the stride.

6. Spring is the manner in which weight settles back on the supporting leg at the completion of the stride.

7. Balance refers to the ability of the equine to coordinate action, go composed and in form

Components of a Stride1. A stride has two phases

• Stride stance is the weight bearing phase

• Stride suspension or swing is the non-weight bearing phase

2. The speed of a horse is affected by:

• Length of stride, rapidity or frequency of stride and overlap time

• Overlap time is the time on the ground versus the time off the ground

Natural Gaits(no training)

1. Walk is a slow, even, four-beat gait• The sequence of hoof-beats is first – left

hind, next, left fore, then- right hind, and finally – right fore

• Both feet on one side strike the ground before the feet on the opposite side strike the ground

Walk

2. Trot is a two-beat gait where the diagonal fore and hind legs act together

• A period of suspension when all four feet are off the ground occurs between each beat

• There are different styles of trotting: road horse trot is fast-stepping, hackney trot and heavy harness trot are high-stepping forms of trotting

Trot

3. Canter or lope is a three beat collected gait where the equine carries more weight on its haunches or rear quarter

• Sequence of hoof-beats for canter is first – right rear hoof, next- left rear and right front striking the ground at the same time and last- left front hoof.

• Canter is performed in slow, animated, rhythm.

Canter

4. Gallop or run is a fast, four-beat gait• Sequence of hoof-beats is first- one

hind foot, next- the other hind foot, then- the diagonal forefoot followed by the remaining forefoot

• A period of suspension follows the four beats.

• Racehorses use the gallop or run gait

Other Gaits

1. Pace is a two-beat, lateral gait used for speed with the fore and hind legs on the same side moving together. The gait has a rolling motion.

2. Slow gait or stepping pace is a show gait that uses a lateral, four beat gait where the front foot on the right is followed by the hind foot on the right

3. Rack (sometimes called the single foot) is an even, fast four-beat lateral gait that is hard on the equine’s forelegs due to increased amount of concussion from the excessive leg movement. Rack is easy on the rider

4. Running walk is the fast walk of a Tennessee walking horse where the horse travels with a gliding motions as a result of extending its hind leg forward to overstep the forefoot print.

5. Back is actually trotting in reverse

Gait Defects1. May interfere or cause injury to the equine

2. Forging is the striking of the toe of the hind foot against the sole of the forefoot.

3. Interfering is when the foot of the striding leg strikes the supporting leg and usually occurs between the supporting front leg a striding front leg or between a supporting hind leg and a striding hind leg

4. Winding is twisting the front leg around in front of the supporting leg as each stride is taken and is common with wide-chested horses

5. Other gait defects include scalping, speedy-cutting, cross-firing, pointing, swelling, trappy, pounding and rolling

Center of Gravity, Conformation and Gaits

1. Most commonly located in the middle of the rib cage just caudal to the line separating the cranial and middle thirds of the body

2. Because the center of gravity is located more cranially, the forelimbs bear 60-65% of the equine’s weight resulting in more stress and lameness in the forelimbs

3. Conformation for structure of the horse can be generalized to say that basically an equine that stands straight is likely to move straight and true.