ANGIOLOGY.
Blood-Vascular System of the Horse.
Under this heading we describe the organs of circulation, by the action
of which certain fluids are propelled through the body. It is customary
to divide this branch of the subject into two sections, considering
respectively the blood-vascular and lymphatic systems.
BLOOD-VASCULAR SYSTEM.--This involves the consideration of the
blood, a fluid which supplies nutriment to the tissues and receives
effete material from them; the heart, a muscular organ which, by its
contraction, initiates the motion of the blood; the arteries, a series
of tubes which convey the blood from the heart to all parts of the
body; the veins, tubes which return that fluid to the heart; and the
capillaries, minute tubes joining the small arteries and veins.
BLOOD.--Blood is a fluid tissue, which nourishes all living structures,
being the medium by which nutritive material is conveyed to, and effete
or waste material conveyed away from the solid tissues. It is an
opaque, thickish, clammy liquid, with a peculiar odor, sickly saline
taste, and alkaline reaction. Its color varies in different parts of
the same animal, that in the arteries being bright red or scarlet,
while the blood in the veins is of a dark purplish hue.
When examined microscopically, the blood is found to consist of
minute corpuscles, and a clear, transparent, yellow fluid, the liquor
sanguinis, or plasma, in which the corpuscles float. The corpuscles are
of two kinds, the red and the white or colorless; the former, by far
more numerous, vary in proportion. Red corpuscles vary in shape, but in
all mammals (animals that suckle their young) are more or less flat,
the families excepted where they are oval, as in birds, reptiles, and
fish, which are also nucleated. Their average diameter in the horse,
ox or sheep is about 1/4000th part of an inch, their average thickness
being about one-fourth of this. Each surface is depressed towards its
center, hence the corpuscle is appropriately described as a bi-concaved
disc.
The white corpuscles are larger than the red, round in shape, and
nucleated.
The liquor sanguinis is pale and clear, and consists of water, fibrin,
albumen, fatty compounds, extracts, odoriferous and saline matters.
The serum is a thin, transparent liquid, of a pale-straw or yellow
color, consisting of the liquor sanguinis deprived of fibrin. It
contains nearly 90 per cent of water, is always slightly alkaline,
and coagulates when heated, owing to the large quantity of albumen it
contains. Fibrin is a white, stringy elastic substance, which, when the
blood is in circulation, is in solution, and cannot be distinguished
from the other constituents of the plasma.
HEART.--The heart is the principal organ of circulation; it weighs
about six and one-half pounds in the average horse and acts as a force
pump to force the blood through the arteries. It is composed of strong
muscular tissue, which acts involuntarily, and is situated between the
lungs, which are divided by what is known as the mediastinum. This is
a division between the lungs made up of two folds, the heart being
between them. The bottom or apex, of the heart is downward and rests
just above the breast-bone; the upper part, or base is directed upward
and to the left side, the left lung having a depression on its inner
surface for the heart to work in. There is a covering or sack around
the heart which helps to protect and support it in its place. It is
attached above to the back-bone, and below to the bones of the breast.
This sack is made up of fibrous tissue and is of a whitish appearance;
inner surface is smooth, and supplied with numerous small glands which
secrete an oily substance called serous fluid. This lubricates the
outer surface of the heart and the inner surface of the sack so that
in action it does not irritate the walls or surfaces. The cavity of
the heart is divided into two parts, the right and left sides; each of
these parts is again subdivided. The upper cavity is called auricle and
the lower cavity ventricle; thus there are the right and left ventricle
and right and left auricle. The right auricle communicates with the
right ventricle by an opening in the septum or partition on the right
side of the heart. This opening is guarded by a valve to keep the blood
from flowing back into the auricle. The left auricle communicates with
the left ventricle, same as on the right side. The right side of the
heart is sometimes called the venous side and contains only venous or
impure blood. The left side is sometimes called the arterial side. It
contains pure blood only. This side of the heart is very much stronger
and thicker than the right side.
[Illustration: PLATE V.
THE HEART AND THE CHIEF BLOOD VESSELS OF THE HORSE--AFTER MEGNIN.]
EXPLANATION OF PLATE V
BLOOD-VASCULAR SYSTEM OF THE HORSE
1. Heart, right ventricle.
2. Heart, left ventricle.
3. Heart, left auricle.
4. Pulmonary artery.
5. Pulmonary veins.
6. Anterior aorta.
7. Carotid artery.
8. Glosso-facial artery.
9. Left brachial artery.
10. Dorsal artery.
11. Superior cervical artery.
12. Vertebral artery.
12´. Internal thoracic artery.
13. Humeral artery.
14. Radial artery.
14´. Cubital artery.
15. Great metacarpal artery.
16. Ungual branches.
17. Posterior aorta.
18. Coeliac axis.
19. Mesenteric arteries.
20. Renal artery (left).
21. Small testicular artery.
22. Posterior vena cava.
23. Portal vein.
23´. Hepatic circulation.
24. External iliac artery.
25. Internal iliac artery.
26. Lateral sacral artery.
27. Femoral artery.
28. Posterior tibial artery.
28´. Anterior tibial artery.
28´´. Femoro-popliteal artery.
29. Metatarsal vessels.
30. Venous plexus of the foot.
31. Internal saphenic vein.
32. Cephalic vein.
33. Jugular vein.
ARTERIES.--Arteries are tubes the purpose of which is to convey the
blood from the heart. For this reason it is apparent that all arteries
carry pure arterial blood with but one exception. The pulmonary
artery carries the blood from the right ventricle to the lungs, and
consequently carries impure or venous blood. Each time the left
ventricle contracts it causes a wave, as it were, to pass all through
the arteries. This contraction takes place when in a healthy condition
about 36 to 42 times every minute and gives rise to what is known as
the pulse. This wave, or beating, may be detected at any point where
the artery is situated so closely to the surface as to affect the
outside of the body sufficient to be felt by placing the finger on the
point; consequently the pulse may be counted at any of these points.
Place your forefinger on the lower edge of your own lower jaw directly
under the corner of your mouth. At this point an artery passes out over
the jaw bone and therefore runs very close to the surface, making it
quite possible to feel the wave caused by the contraction of your own
heart, quite distinctly. Near this point on the jaw of the horse the
pulse is most conveniently felt and counted.
The walls of the arteries are composed of elastic tissue and after
death are always lying open. Blood is never found in them after death
because they continue to contract sufficiently long enough to force all
the blood through them.
VEINS.--Veins are tubes in construction not so strong as the
arteries--the purpose of which is to convey the blood from all parts
of the body to the heart. The heart wave does not affect the veins,
and consequently the pulse cannot be detected by placing the finger
on an exposed portion of one of them. It is also apparent that all
veins carry impure or venous blood with but one exception, viz., the
pulmonary vein, the purpose of which is to conduct the purified blood
from the lungs to the heart.
CAPILLARIES.--The small arteries terminate in a system of minute
vessels--the capillaries--which are interposed between the termination
of the arteries and the commencement of the veins, forming plexuses
(network) which vary much in arrangement. Their average diameter is
about 2/1000ths of an inch, varying in different construction of the
organs, smallest in the brain and mucous membrane of the intestines,
larger in the skin, in glands, and the interior of bones. All arteries
do not terminate in capillaries, an exception being in erectile
tissue of the penis, where arteries end in cells or cavities placed
at the origin of the veins. As the blood passes slowly through these
capillaries, the nourishment is absorbed from it through their very
thin walls to supply the tissues of the body. When the blood passes
through this capillary network it again enters into large vessels
called the veins, which carry it on its way back to the heart.
COURSE OF THE BLOOD.--We have seen that the heart is divided into a
right or venous, and a left or arterial portion. The blood is pumped
by the heart to all parts of the body, through the arteries, passing
through the capillary system, where it parts with its nourishment, is
collected and returned to the heart by the veins, is again pumped by
the heart to the lungs, where it is purified and returned to the heart
to again commence the circuit as before.
LYMPHATIC SYSTEM.--The lymphatic or absorbent system is closely
connected with the blood-vascular system, and is made up of very fine
minute tubes and glands. These convey from the tissues of the body a
clear fluid known as lymph, and pours it into the blood of the veins as
it is on its way back to the heart. These glands are found all through
the body; for instance, there is a large group inside the thigh or
stifle joint of the horse, and another large group inside the shoulder.
It is important to note these, as they sometimes become inflamed and
the leg is swollen. They are then the seat of the disease called weed
in the leg, or lymphangitis.
[Illustration: PLATE VI.
_1. Brain._
_2. Spinal cord._
_3. Brachial plexus._
_4. Sacrolumbar plexus._
_5. Pneumogastric._
_6. Sciatic._
_7. Sympathetic System._
_8. Solar plexus._
NERVOUS SYSTEM OF A HORSE--AFTER MEGNIN.]