Читать книгу Special Report on Diseases of Cattle - Lowe - Страница 102

In cattle, as in human beings, the heart, blood vessels, and lymphatics may be described as the circulatory apparatus.

The heart is in the thoracic cavity (chest). It is conical in form, with the base or large part uppermost, while the apex, or point, rests just above the sternum (breastbone). It is situated between the right and left lungs, the apex inclining to the left, and owing to this the heart beats are best felt on the left side of the chest, behind the elbow. The heart may be considered as a hollow muscle, containing four compartments, two on each side. The upper compartments are called auricles and the lower ones ventricles. The right auricle and ventricle are completely separated from the left auricle and ventricle by a thick septum or wall, so that there is no communication between the right and left sides of the organ.

At the bottom of each auricle is the auriculo-ventricular opening, each provided with a valve to close it when the heart contracts to force the blood into the arteries. In the interval between the contractions these valves hang down into the ventricles.

The muscular tissue of the heart belongs to that class known as involuntary, because its action is not controlled by the will.

The cavities of the heart are lined by a serous membrane, called the endocardium, which may be considered as a continuation of the veins and the arteries, forming their internal lining. The walls of the ventricles are thicker than those of the auricles, and the walls of the left ventricle are much thicker than those of the right.

The heart is enveloped by a fibrous sac (or bag), called the pericardium, which assumes much of the general shape of the outer surface of the heart.

The action of the heart is similar to that of a pump and its function is to keep the blood in circulation. The auricles may be considered as the reservoirs or receivers of the blood and the ventricles as the pump chambers. During the interval between contractions, the heart being in momentary repose, the blood pours into the auricles from the veins; the auriculo-ventricular orifices being widely open, the ventricles also receive blood; the auricles contract and the ventricles are filled; contraction of the ventricles follows; the auriculo-ventricular valves are forced up by the pressure of the blood and close the auriculo-ventricular openings and prevent the return of blood into the auricles; the contraction of the ventricles forces the blood from the right ventricle into the lungs through the pulmonary artery and its branches, and from the left ventricle into the aorta, thence through the arteries to all parts of the body. After the contraction of the ventricles the heart is again in momentary repose and is being filled with blood, while the valves in the aorta and pulmonary artery close to prevent the return of blood into the ventricles. (See Pl. VII.)

The average weight of the heart of an ox is said to be from 3½ to 5 pounds; but, of course, owing to the many breeds and sizes of cattle, it must vary in different animals.

The vessels that convey the blood from the heart to all parts of the body are called arteries; those which return the blood to the heart are called veins. The arteries divide and subdivide (like the branches of a tree), become smaller and smaller, and ultimately ramify into every part of the body. Between the ultimate ramifications of the arteries and the beginning of the veins there is an intermediate system of very minute vessels called capillaries, which connect the arterial with the venous system of the circulation. The walls of the arteries are possessed of a certain amount of rigidity, sufficient to keep the tubes open when they are empty.

The blood leaves the left ventricle through a single vessel, the common aorta, consisting of the anterior and posterior aortas, which give off the large arteries.

The veins take the blood from the capillaries in all parts of the body. They begin in very small tubes, which unite to become larger in size and less in number as they approach the heart.

In its course an artery is usually accompanied with a vein and in many situations with a nerve. The more important arteries are placed deep within the body; when they are superficial, however, they are generally found where least exposed to injury, as, for example, on the inner side of the legs. Arteries are less numerous than veins, and their total capacity is much less than that of the veins. A great number of veins are in the tissue immediately beneath the skin and do not generally accompany arteries.

The blood, throughout its course in the heart, arteries, capillaries, and veins, is inclosed within these vessels. Except where the large lymphatics empty into the venous blood, there is no opening into the course of the blood.

All the arteries except the pulmonary and its branches carry bright-red blood, and all the veins, except the pulmonary veins, carry dark-red blood. The impure dark-red blood is collected from the capillary vessels and carried to the right auricle by the veins; it passes down into the right ventricle, and thence into the pulmonary artery and through its branches to the capillaries of the lungs, where the carbonic-acid gas and other impurities are given up to the air in the air cells of the lungs (through the thin walls between the capillaries and the air cells), and where it also absorbs from the air the oxygen gas necessary to sustain life. This gas changes it to the bright-red, pure blood. It passes from the capillaries to the branches of the pulmonary veins, which convey it to the left auricle of the heart; it then passes through the auriculo-ventricular opening into the left ventricle, the contraction of which forces it through the common aorta into the posterior and anterior aortas, and through all the arteries of the body into the capillaries, where it parts with its oxygen and nutritive elements and where it absorbs carbonic-acid gas and becomes dark colored. (See theoretical diagram of the circulation, Pl. VII.)

The branches of certain arteries in different parts unite again after subdividing. This reuniting is called anastomosing, and assures a quota of blood to a part if one of the anastomosing arteries should be tied in case of hemorrhage, or should be destroyed by accident or operation.