Digestive system
The digestive
system provides nutrients the horse’s cells need to produce energy, necessary
for living. That is why it has a double function: breaking down fats, proteins
and carbohydrates into smaller pieces and absorption of these pieces into the
bloodstream.
Equine Digestive System
From: http://www.netplaces.com/horse-care/systems-of-the-horses-body/the-digestive-system.htm
The horse’s
digestive system begins with the mouth,
which consist of by two lips, lower and upper. They surround an oral gap which
leads food to oral cavity. From the above oral
cavity is separated from the nasal cavity by hard palate, which, together
with tongue, lips and cheeks creates underpressure, which allows to suck and
intake fluids. The upper and frontal parts of the oral cavity are limited by
maxilla and the bottom by mandible. These bones inside the oral cavity are covered
with mucosa called gums, where the teeth are placed. Teeth are very strong
structures, covered with hard enamel under which are dentine and pulp which
fills the tooth cavity together with nerves and blood vessels. As all mammals
horses lose and regrow teeth while they mature. Foals have 24 (stallions may
have 28) teeth and grown up horses have 36 (mares) and 40 (stallions and
geldings). Teeth function is to break down the food.
MARE
|
STALLION/GELDING
|
|
INCISOR TEETH
|
12
|
12
|
CANINE TEETH
|
0
|
4
|
PREMOLARS
|
12
|
12
|
MOLARS
|
12
|
12
|
While the food is
inside the oral cavity, work begin the three salivary glands: sublingual,
submandibular and parotid. Their function is to secrete saliva, which not only
moisturize a bolus, but also contains enzymes digesting sugars, so at the level
of oral cavity, digestion already begins. Another important organ placed in the
oral cavity is the tongue. It is long and narrow, matched with tight space of
mandible. The tongue is a very strong muscle with huge mobility that helps in
forming bolus. From the oral cavity bolus is moved through the throat isthmus
to pharynx. It is a structure about 20 cm long, which is common
for the respiratory and digestive system. So how is it possible the bolus does
not fall into the larynx? While the horse swallows a fold called epiglottis
closes the larynx, so the food can safely pass further. The oral cavity with
all its organs is made for the intake of food, taste control, grinding and
mastication of food with teeth and
finally producing saliva and forming bolus.
After the bolus is
swallowed it travels along a very long tube called the esophagus, which is 125-150 cm long and ends in the stomach. At the
beginning it runs between the spine and trachea and later moves a little to the
left and at the level of thoracic cavity it goes again at the dorsal part of the
trachea. After it goes through the diaphragm it connects with the stomach by
gastric cardia.
A horse’s stomach is built of one chamber and
comparing to the animal’s size it is really small, only 8-15 l of volume. It rests on
the left side of the abdominal cavity attached to the diaphragm. The stomach
has a shape of a horseshoe and is situated at the level of 9th-15th intercostal
space. The structure of the stomach, esophagus and very strong cardia sphincter
make it impossible for a horse to vomit. Also gases collecting in the stomach
can not expel themselves (a horse can not belch), that is why gastric problems
are very common with horses, and good and healthy diet is so important. In the
stomach the gastric juice is produced. It contains hydrochloric acid and pepsin,
which break down proteins. After all the processes the food goes through, it
becomes a chyme, liquid substance which passes through the pyloric valve to the
duodenum.
Scheme of the Digestive System
From: http://www.vetmed.wsu.edu/cliented/anatomy/cat_digest.aspx
Duodenum is the first part of the small intestine and
it is 1m long. Here the majority of digestion takes place. Bile, from the liver,
and pancreatic enzymes ( trypsinogen, chymotripsynogen, pancreatic amylase and
lipase) reaches the duodenum by the duodenal papilla major. Their function is
to break down fats, carbohydrates and proteins into micelles, simple
carbohydrates and amino acids. As in the duodenum, digestion takes place,
absorption occurs in the next part of small intestine- jejunum, which is 17-25
m long. This intestine is looped many times, easily
movable and situated mostly on the left side of abdomen. Ileum, the last part of the small intestine is 70 cm long.
Any liquids and roughage
pass further to the large intestine. The first part of the large intestine is a
cecum. It is a huge sack, 1 m long and 33 l in volume. It contains
bacteria that digest cellulose through fermentation. These bacteria feed upon
digestive chyme, and also produce certain fat-soluble vitamins which are
absorbed by the horse. The reason horses must have their diets changed slowly
is so all bacteria in the cecum have to modify and adapt to the different chemical
structure of the food. Ingesta moves from cecum to the colon. The large colon, the first part is 3-4 m long. It creates a two-level
loop in the shape of a horseshoe. Its function is to absorb simple
carbohydrates that were made in cecum. The small colon is 2,5-4 m long. In this area the
majority of water is absorbed, and fecal balls are formed. Then by the
peristaltic movements feces are moved to the rectum. The rectum is about 20-30 cm long, and acts as a
holding chamber for waste matter, which is then expelled from the body through
the anus.
The digestive
system is connected with two other organs through which food does not pass, the
liver and pancreas, but they are of great importance for proper digestion.
The liver is a big organ(its weight is
about 2,5-3,5 kg)
situated in the epigastric area mostly on the right. Its function, with regards
to digestion, is to produce bile- an enzyme, which takes part in breaking down
fats. What makes the horse’s liver different from other animals is that, it
does not have a gall bladder to store bile. It flows all the time through a hepatic
duct straight to a duodenum, which is why horses must feed constantly- are
“grazers”.
The Liver
From: http://loudoun.nvcc.edu/vetonline/vet111/digestive%20sys/digestive%20lesson.htm
The pancreas is an organ with a double
function: secreting hormones and producing enzymes. It lies at the level of
XVII-XVIII thoracic vertebrae. It is built of three parts: stem, left lobe and
right lobe. The pancreatic duct leads enzymes to the duodenum accompanying
hepatic duct.
Lymphatic system
The lymphatic
system is built of lymphatic tissue, creating lymphatic organs and lymphatic
vessels, in which lymph flows. The importance of this system is multilateral
and very essential for the functioning of whole organism. Except hematopoietic
function (lymphocytes multiplication), it also resorbes proteins and fats and
supports the immune system. When the organism is injured, a swelling occurs as
a result of an increase of lymphatic fluid at the site of injury.
Lymph is
light-coloured fluid which consists of plasma, salts, proteins and blood
cells-lymphocytes. Depending on which part of the body the lymph comes from may
differ in the composition-number of proteins, fat and cells. The lymph
circulation is one-way, from the periphery of the body towards the heart. Lymph’s
flow is usually slow and supported by the horse’s movements, breathing,
peristaltic activity. Lack of exercises can provoke lymphedema, which results
in a swelling of the limbs.
Lymphatic vessels
begin as small capillaries and as they become wider, inside appear valves
(usually double), so the lymph doesn’t retreat. Among these vessels we can
distinguish interorgan vessels, externorgan vessels, lymph trunks and lymph
wires. A horse has six lymph trunks, two tracheal, two lumbar, one visceral and
one intestinal. Tracheal trunks, left and right, run aside the trachea and may
be multipartite. The lumbar trunks, left and right, run aside the abdominal
aorta under the lumbar part of spine and usually are single. Lymph wires, thoracic
wire and right lymphatic wire, are the final lymphatic vessels, which lead the
majority of the lymph to the venous system. Thoracic wire collects lymph from
nearly the whole body, while the right lymphatic wire which is only four cm
long collects lymph from the right part of head, neck and right front leg. The
whole system of lymphatic vessels is connected with a venous part of the
circulatory system and, as veins, lymphatic vessels have various diameters,
which affect structure of their walls. Their function is to collect excess
fluid from the tissues and lead it to lymph nodes where lymphocytes eliminate
waste products. The flow through lymphatic vessels is usually slow and needs to
be supported by the muscle contractions (the movement of the horse).
Lymphatic organs
are scattered over the whole body. Their number can change depending on age,
defensive activity and the health of the horse. Lymphatic organs are:
1
lymphatic papules and platelets and tonsils
2
lymphatic nodes-superficial and deep
3
thymus
4
spleen.
Lymphatic
papules are most primitive in respect of structure among all the lymphatic
organs. They are clusters of lymphatic tissue placed in mucous membrane of the
digestive system. Platelets are accumulation of papules surrounded by a thin
bag of connective tissue. Tonsils are a modification of platelets placed in the
oral cavity.
Lymphatic
nodes are similar in shape to bean
seeds. Their size may vary from a few mm to many cm. Their function is to
filter the lymph, so internal structure slows the flow of lymph. Unlike the
lymphatic papules and platelets, nodes have both vessels-afferent and efferent.
Lymphatic
node
From: http://canceragain.wordpress.com/2010/10/08/what-is-a-lymph-node/
Lymphatic nodes of head and
neck:
1
Parotid- two to eight
nodes creating a mass two to six cm long
2
Mandibular- is situated under
the mandible, creating a V-shaped structure
3
Retropharyngeal- is situated
dorsal and lateral to walls of throat
5
Deep cervical- consists of three
groups: cranial, medial and caudal. All are situated in lateral from the larynx
Lymphatic nodes of forelimb:
1
Axillary- placed near the
shoulder girdle, consists of three groups: axillar appropriate, axillar of
first rib and cubital.
Lymphatic nodes of thorax:
1
Dorsal thoracic- are created by
two groups: intercostal (situated between ribs) and thoracic aortic (situated
along aorta)
2
Ventral thoracic- consists of
sternal lymphatic knots (cranial and caudal) situated on internal surface of
sternum
3
Mediastinal- is localized in
mediastinum, divided into three groups: cranial, medial, caudal.
4
Brachial- is situated in the
final part of trachea and lungs, divided into two groups: tracheobronchial and
pulmonary.
Lymphatic nodes of abdominal
and pelvic cavity:
1
Visceral- mid sized, situated surround the visceral artery, they
collect lymph from stomach, pancreas, spleen, diaphragm and peritoneum
2
Splenic- they surround
single blood vessels
3
Gastric- placed near
gastric groove, hard to distinguish from visceral nodes
4
Hepatic- they create a
mass up to nine cm long
5
Pancreatico-duodenal- situated along the
right gastric artery, they collect lymph from the duodenum, pancreas and
stomach
Lymphatic nodes of hind limb:
1
Iliofemoral- create an elongated
mass made of sixteen to thirty five nodes, they are situated in a femoral
triangle
2
Popliteal- in number three
to twelve are situated in popliteal fossa
Thymus is an organ, which
is fully developed during fetal life and the time before puberty. Thymus is
built of two lobes, left and right, which are connected by connective tissue.
Its function is to provide appropriate development of whole Immune System and
T-cells (T-lymphocytes), that is why the thymus is very important. In foals a
few months old the thymus is situated in
mediastinum, then it begins to degenerate. We can find traces of thymus
in horses aged two and a half-years.
Spleen lies on the left
side of the abdominal cavity between its wall and stomach. This organ is flat,
elongated and narrow. Blood flows through the spleen slowly to collect
lymphocytes and monocytes. At the same time old and used erythrocytes are
dissolved and freed iron is stored for further transformations. Other functions
of the spleen are neutralization of venoms, production of antybodies and
storage of blood.
Circulatory system
The circulatory
system of all mammals is a closed-circuit system, so the blood stays inside the
vessels, organs, tissues and does not leak into body cavities. The circulatory
system consists of two bloodstreams: small and large, which are strictly
connected with its function.
The main function
of the circulatory system is to supply blood throughout and around the whole
body. Blood is a specific tissue,
which is liquid. It consists of plasma, water, fats, proteins, carbohydrates,
erythrocytes, lymphocytes, platelets, hormones, antibodies and other compounds
created by the horse’s cells. Thanks to
so many components, blood has a lot of functions, for example: distribution of
oxygen (red blood cells) and nutrients, removal of metabolic wastes and carbon
dioxide from cells, carrying antibodies and lymphocytes to fight many types of
infections. All blood cells are formed in a red bone marrow. There most of them
mature and penetrate into the blood vessels.
Blood composition
From: http://www.english-online.at/biology/blood/blood-supply-and-blood-diseases.htm
The heart
with its pericardial sac is placed in mediastinum in the lower part of the
thoracic cavity at the level of 2nd and 6th intercostal
space. 3/5 of the heart is on the left from the median line of the body. The
muscle tissue that builds heart is crosswise
striated and while contracting it acts like a pump. The shape of a heart
is similar to an irregular cone. On its base, which is directed upward and
forward, are situated incoming and outgoing vessels. The tip of the heart is
directed to the sternum, which makes the heart lie in a horizontal position.
The interior of the heart consists four chambers:
1.
Right atrium- this is the
place, where vena cava inferior and superior lead de-oxygenated blood, it is
separated from the left atrium by interatrial septum, by the atrio-ventricular
outlet blood passes to the right ventricle;
2.
Right ventricle- to secure blood
from going back to atrium, there is a special tricuspid valve, which closes
while the heart contracts, right ventricle is separated from left ventricle by
interventrical septum, the pulmonary trunk leads blood from right ventricle to the
lungs and in its outlet we can find three semilunar valves;
3.
Left atrium- here pulmonary
veins in number from 7 to 12 lead oxygenated blood from the lungs;
4.
Left ventricle- its wall is much
thicker than right’s ventricle because of its function- pumping blood with high
pressure to the aorta and then to the whole body, as the right
atrio-ventricular outlet the left one has a valve as well- mitral valve and
three semilunar valves creating a barrier between left ventricle and the aorta;
From: http://www.thehorse.com/articles/17321/anatomy-and-physiology-part-11-of-blood-and-breath
The cardiac conduction
system of heart is responsible for the work of the heart. It is made of
modified muscle cells called Purkinje fibers. These fibers create three
aggregations- sinoatrial node, atrioventricular node and atrioventricular bundle.
This system lets the heart work independently because of creating its own
impulses. Size of the heart is connected with what kind of work the horse does
and the intensity of training. Average weight of horse’s heart is about 3,5-4,5 kg.
The large
bloodstream begins in the left ventricle of a horse’s heart. From there blood
is taken by the aorta. Aorta is the biggest artery and runs from the left
ventricle along the ventral surface of the spine to sacrum. There it divides
into smaller arteries, for example, the left subclavian artery, cervical
superficial artery, axillar artery, left gastric artery and many others. All these
arteries have a special structure.
Their walls are thick, built mainly of muscles (smooth), which contract and
relax to pump the blood forward. Cross-section of arteries is round. Then
arteries become smaller and smaller to finally change into capillaries in the
body tissues. The blood provided by arteries gives oxygen, nutrients and other
essentials cells need, takes carbon dioxide, wastes and travel along capillaries, which are going to create
veins. Veins lead “used” blood
towards the heart. Examples of veins are: external jugular vein, brachial vein,
vena cava inferior and superior. Walls of veins are thinner than arterial,
because blood does not have such pressure as in arteries. The flow of blood in
veins is provided by contraction of body muscles, for example during walking or
trotting. To prevent blood from going backwards veins have valves, which stops
this. Through the vena cava inferior and superior blood goes to the right atrium
where the large bloodstream ends.
From: http://www.irishhorsesociety.com/horsedata/circulartorysystem.htm
The small
bloodstream begins in the right ventricle. Blood from there is taken by the pulmonary
trunk, which divides into the left and right pulmonary arteries going to the
lungs. There they accompany bronchi and again divide into smaller vessels to become
capillaries surrounding alveoli where gas exchange takes place. Then, as in the
large bloodstream, capillaries link to finally form venules, and then pulmonary
veins to bring blood back to the left atrium. This is an unique procedure
when arteries lead de-oxygenated blood,
while veins bring to the heart oxygenated blood. The reason of such a situation
is that arteries are discharging vessels while veins are vessels leading to
heart.
The normal heart
rate at rest is 28 to 40 beats per minute. During exercises it can increase up
to 210-280 per minute. Such differences are caused by larger request for oxygen
and nutrients by cells (the metabolism increases when there is more physical
effort). To provide them everything they need and to collect carbon dioxide and
wastes, blood must flow faster and that is a task of the heart, which has to
speed up Its rate. Different situations takes place while sleeping or
hypothermia, the blood flow slows down, so the heart slows as well. During
hypothermia we can also observe contraction of vessels further from the heart.
Blood accumulates in the central part of body to keep the most important organs
warm- brain, lungs and heart. On the other hand, while overheating blood
vessels dilate to move blood closer to skin, where it can give more warmth and
keep organs safe from high temperature. That is how the circulatory system
controls a horse’s body temperature.
Respiratory system
The respiratory
system provides organism life-giving oxygen, which is necessary in metabolic
processes taking place in every cell of a horse’s body. While cells intake
oxygen and use it to grow, divide, secrete, burn carbohydrates fats or proteins,
they expel toxic carbon dioxide, which is a product of these processes. That is
why proper breathing is essential for good organism metabolism. Other functions
of this system is to regulate the horse’s body heat and maintain the acid-base
pH balance. To provide best gas exchange, horses should breathe deeply, the
rate of breathing for horses at rest should be 12-16 breaths per minute. During
heavy training the rate can increase up to 120-180 breaths, so it is important
to let a horse cool-down, release toxic carbon dioxide and take in fresh air,
full of oxygen.
Breathing is
possible thanks to respiratory muscles situated between ribs and the diaphragm,
which is very strong and pulls the lungs backward. Respiratory muscles work to
provide space for air and also to provide underpressure, which is necessary to
lead the air to the final parts of airways. That is why it is very important
not to restrict the expansion of the ribcage.
Before the gas
exchange takes place, air has to pass through airways: nose, mouth, pharynx, larynx,
trachea and bronchi to finally reach the alveoli-tiny air sacs creating lungs. During
this process the air is cleaned and warmed by tiny hair and capillaries that
line the nasal passages, trachea and bronchi. Airways are opened the whole
time, which differs them from ingestion.
An
alveolus
From: http://bodysystems-rlj00.blogspot.com/2012/07/gas-exchange-in-lungs-breathing-in.html
From: http://bodysystems-rlj00.blogspot.com/2012/07/gas-exchange-in-lungs-breathing-in.html
Equine Respiratory System
From: http://www.localriding.com/horses-respiratory-system.html
The apical part of
horse’s nose and its upper lip are
very independent and have increased movability. On the both sides of nose top
are two symmetric holes of frontal nostrils (left and right). Their shape is a
of characteristic species, at rest they are falcated with a narrow
cross-section, but during increased breathing they enlarge and become round. Between
the frontal nostrils is a septum nostril, which goes backward creating nasal
septum dividing the nasal cavity into two parts. The nasal cavity ends with
rear nostrils, opening to the pharyngeal cavity.
The air is inhaled
through nostrils and cleaned, warmed and moisturized in the nasal cavity. Then
through the rear nostrils air passes to the pharynx, which is common for both, respiratory and digestive
system. From there air goes to the larynx, where lower airways begin.
The larynx is a short pipe, made of
cartilage, membrane and muscles, placed between the pharynx and trachea- in
transition of head into neck (at the level of the skull base). Except for
building airways, the larynx is also an organ of voice formation.
Finally air comes
to the trachea, which is an oval
tube consisting of 48 to 60 tracheal cartilages. In the wall of the trachea we
can find three layers: mucosa, fibro-cartilage membrane and adventitia
(connective tissue containing vessels, nerves, fat cells and springy fibers),
which surrounds a trachea from the outside. On the dorsal side of the trachea we
can find the tracheal muscle. Cervical part of trachea lies on the spine, then
it is pushed a little to the right by esophagus, and finally at the level of
4-6th intercostal space trachea divides into two main bronchi, left and right, which divide
into cranial bronchus and stronger caudal bronchus. Cranial bronchus gives
segment bronchi dorsal and ventral. Caudal bronchus gives 4 strong segment
ventral bronchi and 5-7 short segment dorsal bronchi. The bronchus for the
accessory lobe of lungs starts at the right caudal bronchus and divides
similarly. Both, the trachea and bronchi are lined with epithelium, which
contains cilia and mucus to clean and moisturize the air once again.
As the branches of
bronchi become smaller and smaller, they lead the air further and further to
its destination-alveoli, which create lungs.
A horse has two lungs, which are not exactly the same. They lie in the thoracic
cavity surrounded by separate pleural sacs. Their shape is similar half a cone
with its base directed backwards and connected with a diaphragm and top
directed cranial. A horse’s left lung has two lobes- cranial and caudal. The
right lung is built of three lobes- cranial, caudal and accessory. Very
important place in the lung is its hylus, where bronchi, nerves and vessels
enter the lung. Arteries of the lungs are situated very close to the bronchial
tree, while veins run between segments and discharge blood from them.
While the horse
inhales, air has to travel a very long way, as we said before, to get to the
actual place of gas exchange. While the bronchi divide into smaller pieces,
their epithelium changes to become one layer of cells. At the end of each
branch are alveoli. These are tiny air sacs surrounded by capillaries. Their
function is to take from blood carbon dioxide and replace it with oxygen. Carbon
dioxide is exhaled as “used” air. It is brought to capillaries around the alveoli
by pulmonary arteries left and right, which come from the pulmonary trunk, that
begins in the right ventricle. The blood, full of oxygen travels through
vessels, which become larger and larger and finally create pulmonary veins.
They lead blood to the left heart atrium.
Skeletal system
The generality of horse’s bones which allows Its
movability is called the skeleton. It is built in organism and creates internal
skeleton. Its function is to protect the horse’s internal organs (the skull
protects the brain, the rib cage protects the lungs and the heart and the
vertebral column protects spinal cord), give the horse certain size and shape,
support the whole body, produce blood cells, store minerals and, together with
muscles, allows the body to be mobile.
Anterior
and posterior view of horse’s skeletal system
From: http://m.pinterest.com/lilbabyblues20/equine-skeleton/
The 207-214 bones which create horse’s the skeletal
system vary in size and shape according to their function. They are made of
very hard and withstanding tissues. Though different shapes and sizes the
structure of all bones is similar. The bone is covered with periosteum-membrane
which contains nerves and blood vessels to provide nutrients for the bone and
protect it from injuries. Inside, the bone is built of two types of tissue:
compact bone (which we can find in the diaphysis (shaft), which is the middle
part of the long bones) and spongy bone with marrow cavities (which builds
base). Depending on the shape of the bones we can classify them as:
1. Long
Bones- containing bone marrow
they are responsible for production of new blood cells; examples: humerus,
femur, radius, ulna;
From: http://www.mhhe.com/biosci/ap/dynamichuman2/content/skeletal/visuals.mhtml
2. Short
Bones- they are strong and
compact; examples: carpal bones, tarsal bones;
Calcaneus
From: https://www2.aofoundation.org
3. Flat
Bones- their surfaces are
flat and broad, they protect organs and provide large area of attachment for
muscles; examples: scapula, sternum, skull;
Scapula
From: http://www.rodnikkel.com/content/index.php/saddle-tree-blog-from-shop-and-desk/the-trapezius-muscle/
Sesamoid Bones
From: http://www.3d-it.vet.ed.ac.uk/xrayhandbook/webpages/other/Foot/Fetlockpost.html
5. Irregular
Bones- examples are
vertebrae;
Thoracic and Lumbar Vertebrae
From: http://seedmagazine.com/content/article/prime_vertebrae/
Axial skeleton consists of:
a) Skull- is very complicated in its structure, it is a kind
of a box consisting of 34 irregular bones protecting the brain and organs of
senses- sight, balance and hearing.
b)
Spinal column- is divided into 5 parts: cervical (7 vertebrae),
thoracic (18 vertebrae), lumbar (6 vertebrae), sacral ( 5 fused vertebrae at
the age of 5 years) and coccygeal ( 18-22 vertebrae). Two first cervical
vertebrae, atlas and axis provide the connection between the spinal column and
skull. Function of the spinal column is to protect the spinal cord and hold
horse’s stance.
c)
Ribs- a horse has 18 pairs of ribs
creating the ribcage. Its function is to protect internal organs (heart,
lungs). First 8 pairs are called true ribs, because they connect with the sternum
and next 10 pairs are called false ribs, which means they connect with the sternum
by the costal arch, created by the costal cartilage of posterior ribs. The 11th
rib is the longest one of all the ribs.
d)
Sternum- consists of
three parts: manubrium, body and xiphoid process. At the sides of manubrium and
body we can find rib indentations.
Appendicular
skeleton is made of front and hind limbs’ bones:
a)
Shoulder girdle- is
represented by a shoulder blade. It is an extensive and flat bone in the shape
of triangle, connected with the frontal part of the thorax. The shoulder blade
connects with the humerus.
b)
Humerus
c)
Radius and Ulna
d)
Knee- compiled of 7 bones
e)
Metacarpal (cannon) bone
f) Finger bones-
consists of 3 bones: long pastern
bone, short pastern bone and coffin bone and 3 sesamoid bones.
g) Pelvis- compiled of two pelvic bones (left and right)
connected by pelvic symphysis (at the ventral surface) and by iliosacral joints
connected with the sacrum at the dorsal surface. Pelvic symphysis is made up of
the by ischial and pubic bones.
h) Femur- is the only bone in the thigh.
i) Tibia
and Fibula
j) Tarsus- consisting of 6 bones: I and II tarsal bones (which
are fused), III tarsal bone, IV tarsal bone, central tarsal bone, talus and calcaneus.
k) Metatarsal
(cannon) bone
l) Finger bones-
consists of 3 bones: long pastern
bone, short pastern bone and coffin bone and 3 sesamoid bones.
From: http://en.wikipedia.org/wiki/Skeletal_system_of_the_horse
Without connection between bones it would be
impossible for horses to move, such places, where the bones meet are called
joints. Some of them are not moveable, while most are and permit a great range
of motion. Bones have a layer of a cartilage at the ends creating a joint. It
protects them from rubbing, chaffing and absorbs shocks while moving. To keep
the joint stable, it is equipped with ligaments. These structures are made of
connective tissue and they connect bones by attaching both bones at their ends
and they limit excessive range of motion in a joint . Ligaments do not
contract, so they can not move a joint, but they are very strong (because of
collagen fibers and fibrous protein found in the connective tissue). Their
blood supply is very small, so in case of any injury they regenerate very
slowly. Sometimes the ligaments become stretched too much, then their strength
decreases. All joints are surrounded by the joint capsule. It is a kind of a
sack lined with synovial membrane, which produces a synovial fluid, that lubricates
the whole joint, reduces rubbing and usage of joint cartilage.
Division of types of joints is based on the shape of joint surfaces. We
can distinguish:
a) Flat
joint- both surfaces are
flat, it is nearly non movable.
b) Spherical
joint- one surface is spheric
and another is concave, it is most movable type of joint.
c) Ellipsoidal
joint- both surfaces are
ellipsoidal.
d) Hinge
joint- moves are possible in
one plane, one surface is a roller and the second one is concave.
e) Rotary
joint- for example the joint
between atlas and axis.
Muscular system
Muscles are made of
specific tissue, they come in all shapes and sizes. They are made of fibers
arranged in bundles, which are surrounded by a fascia, connective tissue,
supporting and separating muscles from each other. It allows for better
flexibility in the movement of each muscle groups. Special properties of muscle
tissue enables muscles to contract. It is because of actin and myosin, which
slide on each other and make the fibers contract. Depending on the type of the
fibers we can distinguish slow twitch fibers (they have very strong endurance
qualities and need a lot of oxygen and nutrients to function properly) and fast
twitch fibers ( they can deliver quick and maximal muscle effort, but only for
short periods of time). Another division depends on the kind of control. In
horse’s body dominate voluntary muscles, which are skeletal. The other kind are
involuntary muscles- smooth and cardiac.
These play part in the digestive,
respiratory, circulatory and urogenital systems.
Skeletal muscles
are responsible for movement. They are attached to bones by tendons, and pull
them while in contraction. Horse’s muscles are arranged in two layers-
superficial and deep.
Horse’s superficial
muscles:
a)
Head- levator of upper
lip, levator of upper lip and nostril wing, lateral nasal muscle, lateral
nostril dilator, orbicular muscle of the mouth, buccinators muscle, depressor
of lower lip, zygomatic muscle, masseter muscle, transverse nasal muscle,
depressor of lower eyelid, orbicular muscle of the eye, corrugator supercilii
muscle, auricular muscles;
b)
Neck- sternothyrohyoid
muscle, omohyoid muscle, sternocephalic muscle, brachiocephalic muscle,
splenius muscle, cervical part of rhomboid muscle, cervical part of serratus
muscle, cervical part of trapezius muscle;
c)
Thorax- thoracic part of
trapezius, thoracic part of serratus muscle, cranial superficial pectoral
muscle, caudal part of dorsal serratus muscle, external intercostal muscles;
d)
Forelimb- deltoid muscle (originates
from a tendinous area that is equivalent of our collarbone and from shoulder
blade, it inserts at the upper arm, it brings the front leg forward and can
flex the shoulder joint), triceps muscle (divided into three upper sections and
originates from the posterior edge of shoulder blade and two areas of the upper
arm, it inserts at the elbow), brachialis muscle, biceps muscle(originates from
shoulder blade and inserts at the ulna, it flexes the elbow, extends the
shoulder and stabilizes the knee), radial carpal extensor muscle, common digital
extensor muscle, lateral digital extensor muscle, lateral carpal flexor muscle,
superficial digital flexor, oblique carpal extensor, medial carpal flexor,
middle carpal flexor;
e)
Abdomen- external
abdominal oblique muscle;
f)
Back- latissimus dorsi,
short tail levator muscles, long tail levator muscles, tail depressor muscles;
g)
Hind limb- tensor fascia
latae, superficial gluteal muscle, biceps femoris muscle (this two-headed
muscle originates from the lumbar vertebrae, the pelvic ligament and hipbone,
it inserts at the front of the fibula, it extends the hip joint, stifle and
hock), semitendinosus muscle( has similar origins and insertions as biceps
femoris), long digital extensor muscle, gastrocnemius muscle, soleus muscle,
popliteal muscle, cranial tibial muscle;
Horse’s muscles
From: http://www.netplaces.com/horse/horse-anatomy/the-muscles.htm
Horse deep muscles:
a)
Head- dorsal part of
lateral nasal muscle, temporal muscle, jugulomandibular muscle;
b)
Neck- sternomandibular
part of sternocephalic muscle, complexus muscle, longissimus capitis muscle,
longissimus atlantis muscle, rectus capitis ventralis muscle, scalene muscle,
intertransverse muscle;
c)
Thorax- cranial deep
pectoral muscle, internal intercostal muscles, retractor costae muscle, iliocostalis
dorsi muscle, caudal deep pectoral muscle, spinalis dorsi muscle;
d)
Forelimb- supraspinatus
muscle, infrasinatus muscle, teres minor muscle, deep digital flexor muscle;
e)
Abdomen- internal
abdominal oblique muscle, transverse abdominal muscle;
f)
Back- longissimus
dorsi;
g)
Hind limb- medial gluteal
muscle, deep gluteal muscle, gemellus muscle, quadratus femoris muscle,
adductor femoris muscle, lateral vastus muscle, rectus femoris muscle, iliac
muscle, semimembranosus muscle, lateral head of gastrocnemius muscle, peroneus
tertius muscle, coccygeus muscle;
To provide correct
movement muscles have to be in balance. It means the strength of agonists
(muscles responsible for a certain movement) and antagonists (opponents of
agonists) has to be equal. If misbalance occurs it can lead to injuries and
improper movement. There can also occur misbalance between left and right side
of the horse’s body, as most of the muscles are paired. One muscle can be
responsible for a few movements. That is because of the different directions of fiber arrangement
in certain parts of the muscle.
The skin
The skin is a kind of protector that covers
the whole horse’s body. It stops bacteria and other microorganisms from
entering the body, thanks to its hair it is also one of the thermoregulation
levels. In the skin we can also find receptors of pain, heat and touch, so it
also has a sensory function.
How is the skin built? The skin comprises of the epidermis and underlying dermis.
The epidermis, which is 0,053
mm thick, contains multiple layers of cells and is the
outer, nonvascular layer of the skin. The cell layers of the epidermis at the
bottom are usual and they modify as they move upwards. The epidermis imparts
pigmentation, immunologic regulation, and touch perception. The epidermis is a
stratified squamous epithelium that originates from the ectoderm and consists
of multiple layers of cells. Throughout the epidermis there are several cell
types, including Merkel cells, melanocytes, and Langerhans’ cells. Merkel cells
are located in the basal region and function as slow-adapting touch mechanoreceptors.
Melanocytes are also located in the basal region as well as in the sweat gland
ducts, sebaceous glands, and outer root sheaths of hair follicles. Melanin
pigments produced by melanocytes provide skin and hair colour in horses. The
colour is particularly determined by the number, distribution, and degree of
melanization and is controlled by genetics and a melanocyte-stimulating
hormone, which is secreted by the pituitary. Langerhans’ cells are commonly
located in the upper layer. Langerhans’ cells originate from bone marrow and
are functionally and immunologically related to the monocyte–macrophage cell
line. Langerhans’ cells which have been identified in horses, are proposed to
function as antigen-presenting cells to lymphocytes.
The dermis is much thicker than epidermis. It supports the epidermis and
provides flexibility to the skin through its composition of elastin and
collagen. The thickness of the dermal layer varies throughout a horse’s body,
depending on the region of the body, and ranges from 1 to 6 mm. The thickest areas are
located on the dorsal part of horse (head, mane, back, croup, tail), while the
thinnest regions are on the ventral (udder, medial thigh, external genitalia)
and the medial surfaces of the limbs. The dermis also supports secondary
structures, including hair follicles, sweat (apocrine) glands, sebaceous
glands, blood vessels, and nerves.The dermis originates from the mesoderm and
is composed of dense connective tissue, collagen, elastin, and reticular fibers
that lie beneath the epidermal basement membrane. The dermis is divided into a
papillary layer (superficial dermis) and reticular layer (deep dermis). In
horses, a third layer involves the skin of the dorsal thorax, croup, dorsal
surface of the back, and lateral aspect of the neck. This unique layer, located
below the reticular layer, is composed of fine collagen, elastin, and reticular
fibers. Additional structures located within the dermis include hair follicles,
blood vessels, lymph vessels, nerves, sebaceous glands, and sweat glands.
The subcutis (hypodermis) is
formed by a loose arrangement of collagen and elastic fibers and attaches the
dermis to the deeper structures of bone and muscle. Within these fibers are
variable amounts of fat cells that provide energy, protection, support, and heat
insulation to the body.
Structure of the
skin
From: http://us.arcanatura.com/horse-healthcare/causes-of-equine-scratches/
Thanks to sweat glands horses can expel excess heat by sweating. It
saves the organism from overheating during training on hot summer days. On the
other hand, to protect horses from hypothermia before winter the horse’s hair
become thicker and longer. It creates an isolating layer. The skin also prevents
horse from excessive water loss by evaporation.
The horse’s skin (exactly the epidermis) creates hooves and chestnuts.
The chestnuts are situated on the internal surface of the front and hind limbs.
They are the remains of the first finger (the thumb). The hooves are structures
surrounding the coffin bone. Its external structures are: heel bulb, heel,
quarter, toe, coronary band, frog, bars, sole, white line and walls.
Structure
of the hoof
From: http://www.localriding.com/horses-feet.html