EXPLANATION OF PLATE V

Blood-Vascular System of the Horse

1.Heart, right ventricle.18.Coeliac axis.
2.Heart, left ventricle.19.Mesenteric arteries.
3.Heart, left auricle.20.Renal artery (left).
4.Pulmonary artery.21.Small testicular artery.
5.Pulmonary veins.22.Posterior vena cava.
6.Anterior aorta.23.Portal vein.
7.Carotid artery.23´.Hepatic circulation.
8.Glosso-facial artery.24.External iliac artery.
9.Left brachial artery.25.Internal iliac artery.
10.Dorsal artery.26.Lateral sacral artery.
11.Superior cervical artery.27.Femoral artery.
12.Vertebral artery.28.Posterior tibial artery.
12′.Internal thoracic artery.28′.Anterior tibial artery.
13.Humeral artery.28″.Femoro-popliteal artery.
14.Radial artery.29.Metatarsal vessels.
14′.Cubital artery.30.Venous plexus of the foot.
15.Great metacarpal artery.31.Internal saphenic vein.
16.Ungual branches.32.Cephalic vein.
17.Posterior aorta.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.