In This Chapter
Knowing some of the details of
how your horse’s body works can help you take good care of your equine
companion. It gives you a sense of what’s happening when things are going right
— and when things are going wrong.
In this chapter, we give you a
tour of your horse’s body. You discover all the important systems and how they
work. You also explore the major organs, and other important stuff like teeth,
ears, and hooves.
The Workings of Your Horse’s Innards
Your horse’s body is made up of a
number of systems, each responsible for a different function. Put all these
systems together and you have an incredible living creature known as the horse.
For your horse to stay healthy,
it’s vital that each of these systems functions properly and in conjunction
with each other. In the following sections, we take a look at what goes on
inside your horse’s body and makes her tick. You can check out the details of
these systems in the color section.
Start your engine: The cardiovascular system
The cardiovascular system — also
known as the circulatory system — is one of the most important systems in your
horse’s body. This system is responsible for moving blood from one part of the
body to another. It’s essentially the engine that runs your horse’s body, and
it’s the foundation for all other systems.
The cardiovascular system is
responsible for moving nutrients, wastes, and gases to and from cells
throughout the body. It’s also responsible for stabilizing body temperature and
body pH. It involves three different elements: the heart, the systemic
circulation, and the pulmonary system. Arteries and veins are part of both
systemic circulation and pulmonary circulation, and blood itself runs through
everything.
Fortunately, cardiovascular
ailments are rare in horses.
The heart
The equine heart is more than
just a place where your horse’s love for you resides. It’s a four-chamber
muscle weighing about 10 pounds that pumps blood throughout the blood vessels
in her body. The equine heart is particularly powerful, as you might imagine,
because it has the task of circulating blood through the horse’s rather large
mass.
Technical Stuff
The famous racehorse Secretariat was found after his death to have an abnormally large heart even for a horse (along with very large lungs). This is believed to be the reason he was able to run with such incredible speed.
The equine heart is very much
like the human heart. It has left and right chambers, left and right
ventricles, and a series of valves, veins, and arteries. The difference is that
the horse’s heart can pump about 40 quarts of blood per minute. That’s a lot more
than the human heart, which pumps only about 4 quarts in that same amount of
time.
Pulmonary circulation
The reason for the circulation of
blood in the horse’s body is to spread oxygen to various parts. That oxygen
comes from the air your horse breathes, and gets into the horse’s blood through
the lungs, or pulmonary system. Pulmonary circulation is the part within the
cardiovascular system that sends blood to the lungs so that it can be enriched
with oxygen before it goes to the rest of the body.
It works like this: Blood without
oxygen (which is deep red in color, by the way) enters the right atrium of the
heart and then moves into the right ventricle.
From there, it’s pumped through
the pulmonary arteries to the lungs, where it picks up oxygen. The oxygenated
blood is then pumped out of the heart and into the rest of the horse’s body.
Systemic circulation
While the heart works as a pump
to move blood around your horse’s body, the systemic circulation aspect of the
cardiovascular system works to carry oxygenated blood from the heart to the
rest of the body, and then back to the heart after the oxygen has been removed.
A system of arteries, veins, and blood vessels moves blood throughout the body.
Essentially, the systemic
circulation keeps the blood flowing, spreading much-needed oxygen to cells in
different parts of your horse’s body.
Arteries and veins
In order for oxygenated blood to
be carried away from the heart, it needs a passageway. This comes in the form
of arteries, which are flexible tubes. Some arteries are large, while others
are smaller. They can be found in various areas of the horse’s body (as you can
see in the color section in the center of this book).
Veins are made up of blood vessels
that carry blood with very little oxygen toward the horse’s heart. From there,
the blood is sent through special pulmonary arteries, where it gathers oxygen
before returning to the rest of the body through the arteries.
Blood
You know what blood is, but do
you know what it’s made of? Whether you’re talking about horse blood or human
blood, the essence is the same. Both are made up of red blood cells, white
blood cells, and platelets. These elements are contained in a protein-rich
fluid called blood plasma. Here’s more info on each:
- Red blood cells provide transportation of oxygen to different parts of the body, via the arteries.
- White blood cells help stave off infection from invading organisms.
- Platelets govern the consistency of the blood.
- Plasma serves to hold all the other components of blood together and provides immunoglobulins and clotting factors. If you were to separate the elements of blood in a test tube, a majority of what you’d see would be plasma.
Eat up! The digestive system
A very important system in the
equine body is the digestive system. Horses are virtual eating machines,
designed to consume food for nearly 18 hours a day. So, as you can imagine,
their digestive systems are designed to work overtime.
The horse’s digestive system
consists of the mouth, pharynx, esophagus, stomach, pancreas, intestines,
liver, rectum, and anus. Each of these body parts serves a distinct function in
helping your horse’s body obtain nutrients from her food.
Warning!
When a horse’s digestive system stops functioning properly due to illnesses or a particular condition, colic is often the first symptom. In Chapter Tackling Common Ailments, you find details on a variety of problems that affect the function of the equine digestive system.
The mouth
The process of digestion begins
with your horse’s mouth. The mouth not only provides entry to the digestive
system, but also begins the digestive process.
When your horse chews her food,
her teeth crush it into tiny particles. The food mixes with saliva and becomes
moist, and then slips down into the esophagus through the pharynx.
The pharynx
Shaped like a funnel, the pharynx
is a six-inch muscular sac in the area where the trachea (the tube that
leads to the lungs) and the esophagus (the tube that leads to the
stomach) meet. When your horse eats, her food passes quickly through the
pharynx. After the food gets past this point, it can’t go back into the mouth.
The esophagus
Next in the process is the
esophagus, a tubular muscle that’s nearly five feet long. It begins in the
pharynx and feeds into the stomach. A wave-like process called peristalsis moves
food that has been moistened with saliva down through the esophagus and into
the stomach.
Technical Stuff
In the horse, the esophagus is a one-way street. The peristalsis can go only from the top down to the stomach. This is one reason why horses can’t vomit.
The stomach
The next destination for the swallowed food is your horse’s stomach, which is shaped like a J and pretty small, considering how big the horse is. (It can hold only from 5 to 15 liters at a time.) The entrance to the stomach is governed by the cardiac sphincter, a muscle
that opens to allow food to come in, but it doesn’t allow food to come out.
After the food reaches the
horse’s stomach, it’s enveloped by digestive juices, which begin the process of
breaking it down. The stomach then sends the food on to the intestines, where
the majority of the nutrients are absorbed.
The pancreas
Some of the digestive juices that
the stomach uses to begin to break down the cellulose material that your horse
ingests come from the pancreas. Called pancreatic juices, these
substances consist of enzymes that are alkaline in nature.
The pancreas also produces
hormones, including insulin, which helps regulate blood sugar.
The intestines
The intestines are the most
impressive part of your horse’s digestive system. Your horse has 70 (or more)
feet of intestine inside her abdominal cavity. The intestines absorb most of
the nutrition from your horse’s food.
The intestinal tract consists of
the small intestine and the large intestine:
- The stomach empties into the small intestine, which contains the duodenum and the jejunum. Both the duodenum and the jejunum have a part in digestion; the duodenum breaks down food in the small intestine, and the jejunum absorbs nutrients from the food.
- The small intestine is connected to the large intestine (also called the colon) via the ileum. Microbes in the large intestine are responsible for breaking down the plant fiber ingested by the horse. Many of these critters are in part of the intestines called the cecum. The cecum joins the ileum on one side and the large intestine on the other.
Without the amazing cecum, the horse wouldn’t be able to digest its food. The comma-shaped bit of intestine can hold as much as 30 liters of fluid, and is considered part of the large intestine.
The liver
Your horse’s liver is part of her
digestive system; it converts amino acids into fat and glucose or glycogen,
metabolizes ammonia, and keeps the horse healthy by purifying her blood like a
filter. It serves several other functions: It stores glycogen, which is
important in energy, and it synthesizes proteins found in blood plasma.
The rectum and the anus
After your horse has finished
digesting her food, waste moves to the rectum, which is about one foot in
length. From there, it goes to the anus, where it’s projected out of the
horse’s body. That’s when you come in, manure fork in hand, and clean it up.
Raging hormones: The endocrine system
If you’ve ever wondered where
your hormones come from, wonder no more. The endocrine system — a series of
small organs — produces the hormones in your body. Oh, and in your horse’s body
too.
The organs that make up the
endocrine system are the hypothalamus, the pineal gland, the thyroid, the
parathyroid, the thymus, the pituitary gland, the adrenal gland, the pancreas,
and the ovaries or testes, depending on the gender of your horse. Each of these
organs produces a different type of hormone. Hormones regulate all kinds of
stuff in your horse’s body, including adrenaline, thyroid, insulin, estrogen,
testosterone, and progesterone, to name just a few.
As you probably know, hormones
affect not only the way your body functions, but also your moods. This is true
of your horse too. For example:
- Adrenaline kicks in when your horse spooks at something.
- If you have a mare, varying levels of estrogen and progesterone cause her to develop the equine version of PMS when she goes into season.
- If you have a stallion, you’ve no doubt seen his assertive attitude toward mares and sometimes life in general; this attitude is the result of testosterone.
Fighting off germs: The immune system
The immune system is a
complicated series of protein types, organs, cells, and tissue, all interacting
in a complex and elaborate way. The function of your horse’s immune system is
to protect her from viruses, bacteria, parasites, and any other foreign
organism or material that may compromise her health.
Of all the elements that make up
the immune system, white blood cells, also known as leukocytes, serve as one of
the body’s defensive linemen. When an outside invader enters the body, white
blood cells attack the offending party and render it helpless.
The lymphatic system is another
important part of your horse’s immune system. A network of lymph nodes in your
horse’s body produces phagocytes, Pac Manlike cells that may remove bacteria
and cancerous cells in the bloodstream.
Warning!
When a horse’s immune system becomes weakened, she becomes more susceptible to illness. The bone marrow doesn’t produce enough white blood cells to combat invading organisms, and the lymph nodes don’t produce enough phagocytes. The result is that the horse easily becomes infected by viruses or bacteria that exist in the environment. (Check out Chapter Fighting Infectious Diseases for an introduction of common infectious diseases that may affect your horse.)
Sometimes the immune system
becomes hyperactive, overreacting to foreign bodies and causing a problem as a
result. Allergies are a good example of this problem. Horses who suffer from
allergic reactions are victims of a hyperactive immune system that’s
overreacting to the pollen, dust, or other allergens in the horse’s
environment. (For more information on allergies, see Chapter Tackling
Common Ailments.)
Strong stuff: The muscular system
Your horse is a pretty muscular
beast, as you have no doubt noticed. In fact, her muscular system is one of the
most incredible things about her. It’s what gives her the ability to run like
the wind, spin on a dime, and buck like a fiend.
More than 60 percent of your
horse’s body is made up of skeletal muscle. These muscles come in bundles,
which are in turn made up of muscle fibers. These fibers contract to make the
muscles move. Fascia, a fibrous tissue, covers the muscles. Muscles attach to
one another through the fascia, and attach to bone through the tendons.
Tendons are important because
they’re vulnerable to injury and can render a horse lame if they’re damaged.
This is most true of the tendons found in your horse’s legs. These include the
common digital extensor, the lateral digital extensor, the deep digital flexor,
and the superficial digital flexor. A tendon sheath protects tendons that
extend over joints, and contains synovial fluid, which works as a lubricant.
Disorders of the muscles and
tendons are one of the biggest issues for horses because they’re athletes.
Horses who are worked hard can damage muscles and tendons. See Chapter Tackling
Common Ailments for
more information on muscle and tendon injuries.
Feeling sensitive: The nervous system
The most complicated system in
your horse’s body is easily the nervous system. You could say that the nervous
system is a big part of what makes your horse who she is. So much of her
personality and functionality are the result of workings in her nervous system.
Your horse’s nervous system is
made up of two subsystems: the central nervous system and the peripheral
nervous system.
- The central nervous system consists of your horse’s brain and spinal cord.
- The peripheral nervous system is made up of the cranial and spinal nerves, nerve ganglia, and the autonomic nervous system.
The central nervous system is
largely responsible for your horse’s behavior. It also controls your horse’s
senses, in conjunction with the peripheral nervous system. What your horse
sees, feels, tastes, and hears is all deciphered by the central nervous system.
The way she reacts to these stimuli is also a function of her nervous system.
(Check out Chapter Connecting
Your Horse’s Behavior to Health for details on the connection between your horse’s health
and behavior.)
Tip
If your horse is nervous, does that mean that she has more nerves in her body than a naturally calm horse? No, but her nervous system may be more sensitive than that of a quieter horse.
Breathe in, breathe out: The respiratory system
If you ride your horse in any
kind of performance event — or even if you just enjoy watching her gallop
across the pasture — you can appreciate her glorious respiratory system.
Horses have pretty much the same
respiratory organs and processes that humans have. The biggest difference is
the size. Your horse’s nasal passages are very long compared to yours. Her
lungs are tremendous too, compared to a human being’s lungs. This isn’t
surprising, of course, because such a large animal needs to be able to take in
a lot of air.
When your horse breathes, she
inhales air into her nasal passages with the help of her diaphragm. The air
passes through her trachea, into her lungs via the bronchial tubes. Here,
oxygen is taken from the air and transferred to the blood (see the earlier
section “Start your engine: The cardiovascular system” for more information).
The remaining carbon dioxide is exhaled back out through the nostrils.
Horses are susceptible to many of
the same respiratory problems as humans, including allergies, chronic
obstructive pulmonary disease, and pneumonia. For more information on
respiratory problems, see Chapter Tackling
Common Ailments.
Boning up on the skeletal system
The horse has a big skeleton,
which serves several purposes. It provides a frame for the body, protects the
organs inside it, and supports soft tissue.
Your horse has around 205
individual bones in her body. These include the following:
- Long bones, which help the horse move
- Short bones, which absorb concussion that’s the result of movement
- Flat bones, which enclose the organs
- Irregular bones, which encase the central nervous system (which we discuss earlier in this chapter)
Your horse’s skeleton is held
together by tendons and ligaments. Tendons attach muscle to bone, and ligaments
attach bone to bone. The joints are essentially the flexible parts of your
horse’s skeleton, which allow the individual bones to move independently of
each other.
Cartilage is also part of your
horse’s skeletal system, and is located at the ends of your horse’s bones.
Cartilage is nourished by synovial fluid (joint fluid). Because it’s pliable,
it’s often the first place to show wear and tear, even in your horses.
Warning!
Horses are prone to skeletal injuries, which can be very serious because of the weight of the horse’s body. Broken legs are difficult to repair, and fractured skulls are usually fatal. For details on conditions and injuries of the lower skeleton, see Chapter Tackling Common Ailments.
Wasting away: The urinary system
Ever heard the expression “pee
like a racehorse”? Well, part of that term comes from the fact that racehorses
are sometimes given diuretic drugs, which make them urinate a lot. The other
reason for this expression is that horses have pretty big bladders!
The horse’s urinary system is
responsible for producing, storing, and eliminating urine. Urine is the liquid
waste that the kidneys produce. The kidneys filter waste from the blood, as
well as excrete and reabsorb water and electrolytes to help keep the body’s
blood electrolytes balanced. Horses have two kidneys, just like humans do,
although the equine kidneys are much bigger. Each weighs almost 2 pounds.
The urine produced by the kidneys
is mostly made up of water, and contains only a small portion of solid waste.
After production in the kidneys, urine moves to the bladder and is excreted
through the horse’s urethra.
Tip
You can tell a lot about a horse by her urine. Horses with abundant, clear or light-colored urine are well-hydrated. Horses who have darker, shorter streams of urine may not be drinking enough water. A horse’s urine may also change depending on what she’s eating. And very copious, clear urine can signal a kidney or endocrine problem.
Remember
The good news is that kidney disease and urinary issues in general are pretty uncommon in horses. As long as your horse has plenty of fresh water to drink, she’s unlikely to ever develop kidney or urinary troubles. Flip to Chapter The Building Blocks of Good Nutrition for more about the importance of water in a horse’s diet.
Important Parts on the Outside of Your Horse
Your horse’s internal organs and
systems are pretty darn important, but what about all the stuff that you can
actually see on the outside? How important is that? Well, very, of course.
Without those external body parts, your horse couldn’t function. Or at least
not very well! In the following sections, we introduce you to some of the most
important outside parts of your horse.
Tip
Figure 10-1 shows different parts on the outside of a horse’s body. The terms in Figure 10-1 aren’t anatomical (like those in the following sections); they’re simply meant to help horse people identify different areas of the equine form.
Figure 10-1: Non-anatomical
names of some of the horse’s outside parts.
To serve and protect: The skin
The horse’s skin is more than
just a covering for his body. It’s actually an organ — and the largest organ in
your horse’s body. Made up of different layers called the epidermis, dermis,
and subcutis, the skin holds hair follicles, sweat glands, and sebaceous glands
(see Figure 10-2).
The most important role that your
horse’s skin plays is protecting him from invasion by outside pathogens. The
skin serves as a barrier, keeping your horse’s vital organs from coming into
contact with bacteria that could cause serious damage.
Your horse’s skin also serves as insulation, keeping his internal organs from becoming too cold or too hot, in response to outside temperatures. And because his skin is sensitive to touch, it also protects him by keeping him apprised of possible dangers in his environment.
Figure 10-2: The skin is considered
an important organ in the horse’s body.
Technical Stuff
When your horse’s skin is damaged by trauma, it forms scar tissue as it heals. During the healing process, before the wound resolves into a scar, granulation tissue may be produced in significant quantities. Some areas of the horse’s body are particularly prone to producing an excessive amount of granulation tissue when a wound isn’t managed properly. Another common name for excessive granulation tissue is proud flesh.
Your horse’s skin is susceptible
to fungal and bacterial infections, as well as injuries. See Chapter Tackling
Common Ailments for
more information on these kinds of problems.
Looking at the eyes
The eyes are the windows to the
soul. This is true not only of humans, but of horses as well. The equine eye is
very expressive, and horse owners who know how to read their horse’s eyes
become good at judging their horse’s moods.
Although the eyes are a good way
to determine your horse’s state of mind, they’re also highly functional
external organs. The horse has a very large eye
that’s built very similarly to
the human eye. The exterior of the eye has a cornea, pupil, iris, and sclera,
just like the human eye, as well as upper and lower eyelids. Further in, the
eye contains a lens, retina, optic disc, and optic nerve. Horses also have a
third eyelid, which helps protect the eye from trauma. Figure 10-3 shows a side
view and an anterior view of the equine eye.
Figure 10-3: Horses have a
large eye with good capabilities for seeing in the dark.
Your horse is equipped with
better night vision than you are. He also has a very wide range of vision on
either side of his head. On the other hand, he can’t see well directly in front
of his nose or directly behind him.
Unfortunately, several ugly
conditions and diseases can affect equine eyes. Keeping a close eye on your
horse’s eyes is important because such problems must be treated immediately.
See Chapter Tackling
Common Ailments for more details.
Listen up: The ears
The equine ear does more than
just allow the horse to hear. It also provides a great way to judge how a horse
is feeling at any given moment. Horses use their ears as tools of expression,
and to send messages to other horses. (See Chapter Connecting
Your Horse’s Behavior to Health for an illustration of
equine expressions, and take note of the ears.)
Besides mood gauges, your horse’s
ears are also functional — they detect sound. Because horses evolved as prey
animals, they’re astute at picking up noises that could possibly be a predator
lurking in the bushes, or approaching in the distance. Watch a horse’s ears
swivel around to take in various sounds and you get a good sense of how these
sound receivers really work.
The inside of your horse’s ears
are similar to yours (see Figure 10-4). They contain pinna, an ear canal, an
ear drum, cochlea, vestibule, and semicircular canals. All these parts work
together to collect sound from the environment and pass it along to the horse’s
brain for filtering. They also provide a source of balance and body position
for the horse.
Horses are prone to a few
different ear problems. Ear ticks can get into the horse’s ear canals and cause
head rubbing and much discomfort. Gnats can also bite the insides of the
horse’s ears and make them sore. In cold climates, the tips of a horse’s ears
can be lost due to frostbite.
If you suspect any problems with
your horse’s ears (he tilts his head, shakes it a lot, or rubs his ears on
objects), contact your vet immediately.
Figure 10-4: The horse’s ears
provide him with the ability to hear sounds at a distance.
Taking a bite out of the teeth
You have no doubt noticed that
your horse has a lot of teeth. Anywhere from 36 to 44, to be exact. They break
down like this (see Figure 10-5 for the top and side views):
- Twelve of those are incisors, used for grasping and tearing food.
- Twenty-four of your horse’s teeth are cheek teeth; they grind the food down into fine particles before he swallows it.
- Horses (usually only males) also may have four canine teeth, which are located between the incisors and the molars.
- Some horses have two wolf teeth, which are found just in front of the upper molars. Occasionally, horses have four wolf teeth, two upper and two lower.
Figure 10-5: The horse’s mouth
contains anywhere from 36 to 44 teeth.
Your horse’s teeth grow
continuously, and will do so throughout most of his life. This constant growth
keeps the teeth from being worn down into useless nubs from all that grinding
of tough plant material. However, in horses who are kept in domesticity, the
fact that the horse’s upper jaw is wider than his lower jaw creates a problem.
The upper-outside teeth and lower-inside teeth have nothing to wear against as
they grow, and ridges and sharp points result. These areas restrict the horse’s
normal side-to-side rotary chewing motion.
This restriction causes pain to
the horse when he chews, and may cause him to drop his food. These sharp points
can also cause sores, or ulcers, on the lining of the cheeks or on the tongue.
Horses with this problem tend to chew their food abnormally or incompletely.
They can be prone to choking, colic, or weight loss (see Chapter Tackling
Common Ailments for more
about colic). These sharp points also hurt the horse when the noseband is
tightened and when the bit hits the horse’s teeth.
Remember
To keep ridges and points from getting to the point where they hurt your horse, you need to have his teeth filed — called floating or dressing the teeth — periodically. See Chapter Getting Up to Speed on Routine Care for more details on this procedure.
Step on it: The hooves
The expression “no hoof, no
horse” is truer than we’d like it to be. Your horse’s hooves are the foundation
for his structure and his ability to move. If they aren’t in good shape,
neither is your horse.
Your horse’s hooves are made of
what’s called cornified material, which is a tough, fibrous protein. The bottom
of the hoof is made up of the heel, bars, frog, sole, outer wall, and white
line. The hoof wall is divided into sections: the toe, quartet, and heel (see
Figure 10-6). The hard, insensitive areas of the hoof — the heel, bars, frog,
sole, heel bulbs, and the outer wall — are called the hoof capsule, and
are made of hoof horn. The hoof capsule houses and supports structures
such as the coffin bone, the very important hoof-shaped bone within the horse’s
foot. Between the coffin bone and the hoof capsule is tissue lined with blood
vessels and nerve endings, called the laminae. (See Chapter Tackling
Common Ailments for more about
the coffin bone and the laminae.)
A cool trick: Telling your horse’s age by looking at his teeth
You may already know that you can tell a horse’s approximate
age by looking at his teeth. As a horse gets older, the length, color, shape,
and markings of his teeth change. As the teeth grow, the surfaces also wear
down and change their shape. Dark marks in the surface of the teeth — called
dental cups — slowly appear and then disappear with age. The number and
condition of dental cups visible in a horse’s mouth may aid in determining
the horse’s approximate age.
A mark in the horse’s upper incisors called the Galvayne’s
groove also helps determine age. The mark appears on the outside surface of
the upper incisors after a horse hits about 10 years of age. As the horse
ages, the groove expands downward. By the time a horse reaches 20, the groove
extends all the way to the end of the tooth.
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The function of your horse’s hoof
is not only to carry your horse’s weight and protect it from excessive impact
from the ground, but also to protect the coffin bone and laminae from damage.
The frog portion of the hoof acts as a cushion that aids in the circulation of
the lower leg.
When your horse stands or moves,
weight is borne on different parts of his hoof, depending on the way that he’s
built and how he moves. Wild horses don’t need to have their feet trimmed
because they gradually wear them down as they move in their daily lives — as
much as 20 miles a day. Domestic horses who are barefoot don’t move as much as
wild horses and therefore don’t wear their feet down naturally. Horses who wear
shoes can’t wear down their feet at all. That’s why domestic horses need their
feet trimmed by a farrier. (See Chapter Getting
Up to Speed on Routine Care for more information about hoof
care.)
Figure 10-6: The hoof is the
foundation of the horse’s body.
by Audrey Pavia with Kate Gentry-Running,DVM,CVA
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