Part 64

Hunter and Compar, who have made extensive researches, consider the function of the air-sacs as threefold.

First, the air-sacs are subsidiary respiratory organs, which aid in ridding the blood of waste products and in taking in oxygen.

Second, they aid mechanically the actions of respiration in birds. During the act of inspiration the sternum is depressed, the angle between the vertebral and the sternal ribs is made less acute, and the thoracic cavity proportionately enlarged; the air then rushes into the lungs and into the thoracic receptacles, while those of the abdomen become flaccid. When the sternum is raised, or approximated toward the spine, part of the air is expelled from the lungs and the thoracic air-sac through the trachea, and part is driven into the abdominal receptacles, which are thus alternately enlarged and diminished with the expansion and the contraction of the thorax. Hence the lungs, notwithstanding their fixed condition, are subject to due compression through the medium of the contiguous air receptacles, and are affected equally and regularly by every motion of the sternum and of the ribs.

Third, they reduce decidedly the specific gravity of the whole body. This must necessarily follow from the large spaces filled with air as well as from the absence in the bones of marrow and other fluids. The air-sacs by their position also render equilibrium more stable.

ANGIOLOGY

=The Circulatory Apparatus.=—The circulatory apparatus consists of two tubular systems: the blood vascular system and the lymphatic system.

The blood vascular system consists of the heart, the arteries, the veins, and the capillaries.

The heart is the central, propelling organ. The arteries form a series of efferent tubules, which, by branching, constantly increase in number and decrease in caliber, and which serve to carry the blood from the heart to the tissues. The capillaries are extensions from these latter tubules into which the arteries empty, and through the walls of which the interchange of elements between the blood and the other tissues takes place. The veins form a system of converging tubules which receive the blood from the capillaries, decrease in number and increase in size as they approach the heart, and return the blood to that organ.

The lymphatic system consists of capillaries and veins alone. As in the blood system, the lymph capillaries collect the effete material and pour it into the lymph veins, and these in turn, carry it to the large blood veins adjacent to the heart.

Both these systems have one and the same continuous lining, which consists of a single layer of endothelial cells. In the heart this lining is called the endocardium, and in the vessels, the endothelium. It forms a perfectly smooth surface.

THE HEART (Fig. 21, No. 7)

The heart of the domestic fowl is located in the median line of the thoracic cavity. It is more anterior and mesial than in mammals. Its axis is parallel with the axis of the trunk. The lungs being confined to the dorsal part of the trunk, the lower part of the heart is not surrounded by them, but extends backward, the apex resting in the anterior part of the anterior median fissure of the liver.

The heart has the form of an acute cone (Fig. 50, No. 1), the apex of which is bluntly rounded.

The heart is surrounded by a sero-fibrous sac, the pericardium. This sac adheres to the cervical air reservoirs anteriorly and to the diaphragmatic septum posteriorly. It is composed of two membranous layers: the parietal, external, dense, and fibrous; and the visceral, internal, and serous. The pericardial sac has no direct attachment to the heart, except at the upper extremity where it surrounds the large vessels emerging from it. The serous layer is reflected over the outer portion of the heart, where it is called the epicardium. The function of the pericardium is to prevent friction during the beating of the heart. It contains a small amount of serous fluid for perfect lubrication. This fluid is called the liquor pericardii.

Internally the heart has four cavities: two auricles and two ventricles. The right ventricle is more crescent-shaped than in solipedes, and in a manner envelops the left ventricle in front and to the right, though it does not reach the point of the heart. The right auricle is larger than the left. The auriculo-ventricular valve is not tricuspid as in mammals. This valve instead of being formed as usual by a membranous curtain, with margins retained by cords fixed to the walls of the ventricles, is composed of a wide muscular leaf, which appears to be a portion of the inner wall of the ventricle detached from the interventricular septum. This septum is convex; and the auriculo-ventricular orifice is an oblique slit situated between it and the muscular valve in question; so that, when the heart wall contracts at the systole, the valve is applied against this septum and closes the passage. The bicuspid, or auriculo-ventricular valve of the left side usually has two segments, though occasionally there may be three. The fossa ovalis is a depression behind the posterior semi-lunar valve in the septum of the heart. The membranous septum closing the foramen ovale is complete and strong but thin and transparent. The right auricle receives the blood from the two venæ cavæ coming from the anterior extremity, and from the posterior vena cava. These empty into a sinus. The left auricle has two vessels, the pulmonary veins which bring blood to it from the lungs.

=Structure of the Heart.=—The heart is lined by a serous membrane, the endocardium, which is a continuation of the endothelium of the blood-vessels. There are a few muscular pillars in the inner wall, called the columnæ carnæ. To give the heart its pumping power, it is made up of contractile tissue, a specialized kind of muscle called heart muscle. It is involuntary-striated and occupies an intermediate position, both morphologically and embryologically, between smooth involuntary muscle and striated voluntary muscle (Fig. 74, No. 4). It, like striated voluntary muscle, is both transversely and longitudinally striated. Heart muscle cells are short, thick cylinders, which are joined end to end to form long fibers. By means of lateral branches the cells of one fiber anastomoses with cells of adjacent fibers. Each cell of heart muscle contains one centrally located nucleus. There is no distinct sarcolemma, but the sarcoplasm is more dense near the surface of the cell, which gives it the appearance of an enveloping cell wall. There is a zone free from fibrillæ around the nucleus. The longitudinal fibrillæ, which make up the cell, are held together by a cement-like substance.

The main mass of the heart wall, called myocardium, consists of the specialized muscular tissue just described. The myocardium differs in thickness in different parts of the heart wall. It is thickest in the left ventricle and thinnest in the auricles. The left ventricle forces the blood through the systemic circulation and hence must be thicker to give it more power than is needed for the right ventricle, which forces the blood only through the lungs. The auricles are thinnest of all; for they receive the blood and pass it only to the chambers below. The auricular appendages at the base of the heart in fowls are not so well marked as in mammals. The auricular muscles consist of an outer coat common to both auricles, the fibers of which are transverse and of an inner coat, independent for each auricle, the fibers of which are longitudinal. Between the two coats, occur bundles of muscle the fibers of which run in various directions. The disposition of the muscle tissue of the ventricles is much more complicated. It is composed of several layers of fibers intricately interwoven.

The endocardium, covering the inner surface of the myocardium, forms a serous lining of all the chambers of the heart. At the arterial and venous openings it is continuous with and similar in structure to the intima of the vessels. The endocardium consists of two layers, an external layer closely attached to the myocardium and consisting of mixed fibers, including those of elastic tissue and smooth muscle cells; and an inner, single layer of endothelial cells, spoken of above.

The heart is supplied with nutrient blood by the two coronary arteries, which are given off from the common aorta just above the semi-lunar valves.

A right, or anterior, and a left, or posterior, coronary from their point of origin, turn in a ventral direction between the root of the aorta and the pulmonary artery, the right going to the right coronary groove and the left to the left coronary groove in the crown furrow. From here they send branches into the heart. The anterior, right, coronary, or coronaria dextra, the larger, is given off from the inferior wall of the aorta. It divides into a ramus superficialis and a ramus profundus. The ramus superficialis enters the crown furrow and divides into two or three branches on the right heart wall. These branches extend to the apex of the heart. Twigs from this artery along its course extend into the muscular wall reaching the posterior of the coronary groove where they anastomose with those of the left coronary, the ramus profundus, and with other branches from the same artery. The ramus profundus, larger than the preceding, gives off fine branches into the walls of the aorta and of the pulmonary artery, then enters from behind into the right wall of the septum ventriculorum, extends into the apex of the heart, and supplies the septum, or right inner chamber wall with the last branch, this breaking through the posterior wall of the auricular appendage.

The posterior, left coronary, or coronaria sinistra, originates from the dorsal wall of the aorta, proceeds as one branch on the upper surface of the left auricular appendix, and then extends between the left appendix and the pulmonary artery to the ventral surface of the heart. On the left side it supplies the wall of the pulmonary artery and gives off a ramus profundus. It sometimes divides into two parts and supplies the ventral wall of the right chamber and then extends to the left wall of the septum medium. The rest of the coronaria sinistra enters into the left crown furrow as the ramus superficialis, which provides the left and dorsal upper surface of the left chamber to the apex. From this furrow it extends into the left chamber and the left appendage, and finally fuses with the ramus superficialis of the coronaria dextra.

THE BLOOD-VESSELS

The blood-vessels consist of arteries, veins and capillaries.

=The Structure of the Capillaries and Arteries.=—The capillaries are minute vessels which connect the arterioles, or terminal arteries, with the venules, or terminal veins. They are only from 6 to 14 microns in diameter. Their walls consist of a single layer of endothelial cells, which are somewhat elongated in the long axis of the vessels. Their edges are serrated, and are united by a small amount of intercellular cement-like substance. Capillaries branch without diminution in caliber, and these branches anastomose to form capillary networks, the meshes of which differ in size and shape in different tissues and organs. The largest meshed networks occur in the serous membranes and in the muscles; and the smallest occur in the glands, such as the liver.

The walls of the arteries are thick and stand open when empty, owing to the elastic tissue contained in their walls, while the walls of the veins collapse when empty, owing to their containing a smaller amount of elastic tissue. The arterial wall is provided with three coats: tunica intima, or inner coat; tunica media, or median coat; and tunica adventitia, or outer coat.

The tunica intima consists of a single layer of endothelial cells, continuous with and similar to that forming the walls of the capillaries. In passing from the capillaries to the arterioles, there is first a thin coat, or sheath-like layer, of connective tissue around the outside of the endothelial tubes. Further along, isolated smooth muscle cells arranged in a circular manner occur between the endothelial layer and the layer of connective tissue, this structure forming vessels called precapillary arteries. Further along still, the muscle cells form a complete layer; in this section the vessels are called arterioles and are made up of three coats: the inner endothelial, the middle muscular, and the outer fibrous.

In arteries of medium size the intima consists of the endothelial layer, a layer of delicate white and elastic fibers, connective-tissue cells, and the membrana elastica interna, or an outer layer, the elastic layer, of the intima.

The media consists of a thick coat of circularly arranged smooth muscle cells, its thickness depending largely upon the size of the vessels. There is also a small amount of fibrillary connective tissue, which supports the muscle cells. Elastic tissue is present in the media, the amount depending on the size of the vessel, the larger the vessel the more elastic tissue there is present. In the large arteries coarse elastic fibers intermingle with the finer ones. When much elastic tissue is present the muscle cells are separated into more or less well-defined groups.