Angiology is the description of the organs of circulation of the blood and lymph—the heart and vessels. The heart is the central hollow muscular organ which functions as a suction and force pump; the differences in pressure caused by its contraction and relaxation determine the circulation of the blood and lymph. It is situated in the middle mediastinal space of the thorax, between the two lungs, and is inclosed in a fibro-serous sac—the pericardium. The vessels are tubular and run through almost all parts of the body. They are designated according to their contents as blood- and lymph vessels.
The blood-vascular system consists of: (1) the arteries, which convey blood from the heart to the tissues; (2) the capillaries, microscopic tubes in the tissues which permit of the necessary interchange between the blood and the tissues; (3) the veins, which convey the blood back to the heart.
The blood-vessels are divided into the pulmonary and the systemic. The pulmonary artery conveys the blood from the right ventricle of the heart to the lungs, where it is arterialized, and is returned by the pulmonary veins to the left atrium of the heart, and passes into the left ventricle. The systemic arteries convey the blood from the left ventricle all over the body, whence it is returned by the venæ cavæ to the right atrium, and passes into the right ventricle.
It should be noted, however, that the lungs receive arterial blood through the systemic bronchial arteries. This blood is returned in part by the bronchial veins to the right atrium (indirectly), in part by the pulmonary veins to the left atrium.
The term portal system is often applied to the portal vein and its tributaries which come from the stomach, intestine, pancreas, and spleen. The vein enters the liver, where it branches like an artery, so that the blood in this subsidiary system passes through a second set of capillaries before being conveyed to the heart by the hepatic veins and the posterior vena cava.
The arteries (Arteriæ), as a rule, divide at an acute angle, giving off finer and finer branches. In some cases branches come off at a right angle, and others are recurrent, i. e., run in a direction opposite to that of the parent stem. The intercommunication of branches of adjacent arteries is termed anastomosis. Most commonly the connections are made by a network of numerous fine branches (Plexus vasculosus). Relatively large communicating branches (Rami communicantes) occur in certain places; they may be transverse or in the form of arches. Wide-meshed networks of vessels are termed Retia vasculosa. Terminal or end arteries are such as form isolated networks, i. e., do not anastomose with adjacent arteries. A rete mirabile is a network intercalated in the course of an artery. A collateral vessel (Vas collaterale) is one which pursues a course near and similar to that of a larger vessel.
The veins (Venæ) are in general arranged like the arteries, but are usually of greater caliber. When a vein accompanies an artery, it is termed a vena comitans or satellite vein; in many places two veins accompany a single artery. The primitive venous trunks do not run with the arteries, and most of the superficial veins (Venæ cutaneæ) pursue independent courses. They anastomose even more freely than the arteries, and large communicating branches are very common. The veins form very rich plexuses (Plexus venosi) in many places. Some veins which are without independent walls and are inclosed by dense membranes and run usually in bony grooves are termed (venous) sinuses; examples of this are the sinuses of the dura mater of the brain.
Structure of Arteries.—The wall consists of three coats. The external coat (Tunica externa) consists chiefly of fibrous connective tissue. In the deeper part are some elastic fibers, and in some arteries there are also longitudinal unstriped muscle-fibers. The middle coat (Tunica media) is composed of unstriped muscle and elastic tissue in medium-sized arteries. In small vessels there is only the muscular tissue, and in the largest trunks only elastic tissue. The internal coat or intima (Tunica intima) consists of a layer of endothelial cells, resting on an elastic membrane. The sheath (Vagina vasis) is a condensation of the surrounding connective tissue, and is attached more or less closely to the external coat.
Structure of Veins.—The walls of veins are similar in structure to those of the arteries, but are very much thinner, so that veins collapse more or less completely when empty, while arteries do not. The middle coat is very thin and consists to a large extent of ordinary connective tissue. The intima is also less elastic than in the arteries. In many veins this coat forms semilunar valves, the free edges of which are directed toward the heart. They are most numerous in the veins of the skin and the deep veins of the extremities (except the foot), while in most veins of the body cavities and viscera they are absent or occur only where the veins open into larger ones or where two veins join.
The walls of the vessels are supplied with blood by numerous small arteries, called vasa vasorum. These arise from branches of the artery which they supply or from adjacent arteries, ramify in the external coat, and enter the middle coat also.
The nerves of the vessels consist of both medullated and non-medullated fibers. They form plexuses around the vessels, from which fibers pass mainly to the muscular tissue of the middle coat.
The pericardium is the fibro-serous sac which surrounds the heart, and also the great vessels in connection with it to a greater or less extent. Its form is in general similar to that of the heart. The fibrous layer is relatively thin, but strong and inelastic. It is attached dorsally to the large vessels at the base of the heart, and is continued in part up to the longus colli muscle. Ventrally it is firmly attached to the middle of the posterior half of the thoracic surface of the sternum. The serous layer is a closed sac, surrounded by the fibrous pericardium, and invaginated by the heart. It is smooth and glistening, and contains a small amount of clear serous fluid, the liquor pericardii. Like other serous membranes, it may be regarded as consisting of two parts, parietal and visceral. The parietal part lines the fibrous layer, to which it is closely attached. The visceral layer covers the heart and parts of the great vessels, and is therefore also termed the epicardium. The serous pericardium is composed of a connective-tissue membrane, rich in elastic fibers, and covered on its free surface by a layer of flat endothelial cells.
The pericardium is covered by the mediastinal pleura (Pleura pericardica) and is crossed laterally by the phrenic nerves. Its lateral surfaces are related chiefly to the lungs, but the lower part is in partial contact with the chest-wall. On the left side the area of contact is from the third to the fifth intercostal space inclusive. On the right side the contact is smaller and is at the third and fourth ribs. The anterior border of the base is opposite to the second intercostal space, and the posterior border is opposite to the sixth rib or space. The base is related to the great vessels, the trachea and its bifurcation, the bronchial lymph glands, the œsophagus, and the vagus, left recurrent, and cardiac nerves.
The two parts of the serous pericardium are, of course, continuous with each other at the line of reflection on to the great vessels. The latter are covered in varying degree by the visceral layer. The aorta and pulmonary artery are inclosed in a complete common sheath as far as the bifurcation of the latter. A pouch passes inward between the pulmonary artery and the left auricle, and is connected with another pouch which passes backward between the right auricle and the aorta, thus forming the great transverse sinus of the pericardium. The posterior vena cava is covered on the right and below for a distance of an inch or a little more (ca. 3 cm.). The pulmonary veins receive practically no serous covering. The epicardium is closely adherent to the muscular tissue of the heart, but is attached to the vessels by loose tissue and fat, and hence is easily dissected off them.
The heart (Cor) occupies the greater part of the middle mediastinal space. Its shape is that of an irregular and somewhat flattened cone. It is attached at its base by the great vessels, but is otherwise entirely free in the pericardium. It presents two surfaces, two borders, a base, and an apex for description.
Fig. 419.—Heart of Horse, Left View. Hardened in situ.
The dotted line indicates the line of reflection of the serous pericardium. The epicardium and subepicardial fat have not been removed from the heart.
In position the heart is decidedly asymmetrical, about two-fifths being to the right of the median plane and three-fifths to the left. The long axis (from the middle of the base to the apex) is directed downward, backward, and somewhat to the left. The base (Basis cordis) is directed dorsally and lies a little above the middle of the dorso-ventral diameter of the thorax. It is opposite to the ribs from the third to the sixth inclusive. The apex (Apex cordis) lies above the last segment of the sternum, and is separated by a very short interval from the sternal part of the diaphragm. The anterior border (Margo cranialis) is strongly convex and is directed obliquely downward and backward; its lower part is parallel with the sternum. The posterior border (Margo caudalis) is much shorter, nearly vertical, and is opposite to the sixth rib or intercostal space. The surfaces, right and left (Facies dextra, sinistra), are convex and are marked by grooves which indicate the division of the heart into four chambers, the two atria above and two ventricles below. The left surface (covered by the pericardium) is in contact with the lower third of the chest-wall from the third to the fifth intercostal space. On the right side the cardiac notch of the lung is smaller and further forward, so that the area of contact is at the third and fourth ribs.
Fig. 420.—Heart of Horse, Right View. Hardened in situ.
Line of reflection of serous pericardium dotted. Epicardium and subepicardial fat have not been removed from heart. Left ventricle is considerably contracted.
The coronary or atrio-ventricular groove (Sulcus coronarius) indicates the division between the atria and the ventricles. It almost completely encircles the heart, but is interrupted by the origin of the pulmonary artery. The longitudinal or interventricular grooves, right and left (Sulci longitudinales sinister et dexter), correspond to the septum between the ventricles. The left groove is left-anterior in position. It begins at the coronary groove behind the origin of the pulmonary artery, and passes downward almost parallel to the posterior border, but does not reach the apex. The right groove is right posterior in position. It begins at the coronary groove below the termination of the posterior vena cava and passes downward and a little backward, ending about an inch and a half (ca. 3 to 4 cm.) above the apex. The grooves are occupied by the coronary vessel and a variable quantity of fat.
Fig. 421.—Base of Heart of Horse with Large Vessels, Dorsal View. Specimen Hardened in situ.
Size and Weight.—In horses of medium size the heart usually weighs about seven or eight pounds (ca. 3.5 kg.), or about 0.7 per cent. of the body-weight. There is, however, great range of variation in apparently normal specimens.
The following measurements were obtained in medium-sized hearts in diastole:
| Sagittal diameter of base | 25 cm. |
| Greatest width of base | 18 to 20 cm. |
| Circumference at coronary groove | 65 to 70 cm. |
| Distance between origin of pulmonary artery and apex | 25 cm. |
| Distance between termination of posterior vena cava and apex | 18 to 20 cm. |
The right atrium (Atrium dextrum) or auricle forms the right-anterior part of the base of the heart, and lies above the right ventricle. It consists of a sinus venosus, into which the veins open, and an auricle or auricular appendix. The latter is a conical diverticulum which curves around the right and anterior surfaces of the aorta, its blind end appearing on the left side in front of the origin of the pulmonary artery.
Fig. 422.—Right Side of Heart of Horse Opened up by Removal of Greater Part of Right Wall. Organ Hardened in situ.
The right ventricle was in diastole. r, Intervenous crest; v, v, tricuspid valve; p, p, papillary muscles; m, m, moderator bands. Arrow points into origin of pulmonary artery.
There are five chief openings in the right atrium. The opening of the anterior vena cava (Ostium venæ cavæ cranialis) is in the upper and fore part. The opening of the posterior vena cava (Ostium venæ cavæ caudalis) is at the lower posterior part. Between the two the wall pouches upward somewhat, forming what is sometimes termed the sinus venosus or sac of Lower. In the anterior part of this the vena azygos opens. The coronary sinus opens immediately below the posterior vena cava; the orifice is provided with a small semilunar valve or valve of Thebesius (Valvula sinus coronarii). The middle coronary vein has a separate opening in some cases close to that of the coronary sinus. The atrio-ventricular opening is in the lower part, and leads into the right ventricle. In addition to the foregoing there are several small orifices of the venæ cordis parvæ; these are concealed in the depressions between the musculi pectinati.
The atrium is lined with a glistening membrane, the endocardium. Its walls are smooth except on the right and in the auricle (or appendix), where it is crossed in various directions by muscular ridges, the musculi pectinati. Small bands extend across some of the spaces inclosed by the musculi pectinati. The latter terminate above on a curved crest, the crista terminales, which indicates the junction of the primitive sinus venosus of the embryo with the atrium proper, and corresponds with the sulcus terminalis externally. The openings of the venæ cavæ are without valves. A ridge, the intervenous crest,[156] projects downward and forward from the roof just in front of the opening of the posterior vena cava; it tends to direct the flow of blood from the anterior vena cava to the atrio-ventricular opening. The fossa ovalis is an oval depression in the septal wall at the point of entrance of the posterior vena cava, bounded internally by a thick margin (Limbus fossæ ovalis). The fossa is the remains of an opening, the foramen ovale, through which the two atria communicate in the fœtus.
The left atrium (Atrium sinistrum) or auricle forms the posterior part of the base of the heart. It lies behind the pulmonary artery and the aorta and above the left ventricle. The auricle (or appendix) extends outward and forward on the left side, and its blind end is behind the origin of the pulmonary artery. The pulmonary veins, usually seven or eight in number, open into the atrium behind and on the right side. The cavity of the atrium is smooth, with the exception of the auricle (or appendix), in which the musculi pectinati are present. In some cases there is a depression on the septal wall opposite the fossa ovalis, bounded above by a fold which is the remnant of the valve of the foramen ovale of the fœtus. The atrio-ventricular opening is situated below and in front; it usually appears smaller than the right one on account of the contraction of the left ventricle in the dead subject. The apertures of small veins of the heart are found in the spaces inclosed by the musculi pectinati.
The number and the arrangement of the pulmonary veins are variable. They may be five to nine in number. The largest orifice is posterior. Usually three veins of considerable size enter close together on the right above the posterior vena cava, and three or four open close to the ridge which projects from the roof at the base of the auricle (appendix).
The right ventricle (Ventriculus dexter) constitutes the right-anterior part of the ventricular mass. It forms almost all of the anterior border of the heart, but does not reach the apex, which is formed entirely by the left ventricle. It extends from the third rib to the fourth intercostal space on the left side, to the fifth rib and space on the right side. It is somewhat triangular in outline, and is semilunar in cross-section. Its base faces upward and a little to the right and is connected largely with the right atrium, with which it communicates through the atrio-ventricular orifice; but its left part projects higher and forms the conus arteriosus, from which the pulmonary artery arises. Its apex is two inches or more (ca. 5 to 6 cm.) above the apex of the heart. On opening the cavity it is seen that the two openings are separated by a thick rounded ridge (Crista supraventricularis). The axis of the cavity, taken from this ridge to the apex forms a spiral curve downward and to the right. The septal wall is convex and faces obliquely forward and to the right.
Fig. 423.—Section of Heart of Horse.
Specimen hardened in situ and cut nearly at right angles to the ventricular septum. The left ventricle is contracted, but not ad maximum. V. a., Segment of aortic valve.
The right atrio-ventricular orifice (Ostium venosum dextrum) is oval and is situated opposite to the lower parts of the third and fourth ribs. It is guarded by a tricuspid valve (Valvula tricuspidalis); of the three large cusps of this valve, one is between the atrio-ventricular opening and the conus arteriosus, one is septal, and the third is on the right margin. Small intermediate cusps intervene between the large ones. The peripheral edges of the cusps are attached to the fibrous ring at the atrio-ventricular opening. The central edges are irregular and hang down into the ventricle; they give attachment to chordæ tendineæ. The auricular surfaces are smooth. The ventricular surfaces are rough and furnish attachment to interlacing branches of the chordæ tendineæ. The valves are folds of the endocardium, strengthened by fibrous tissue and at the periphery by muscular fibers also. The chordæ tendineæ are attached below to the three musculi papillares, which project from the ventricular wall; superiorly they divide into branches which are inserted into the ventricular surfaces and the free edges of the valves. Each segment of the valve receives chordæ tendineæ from two papillary muscles. Of the latter, two are on the septum and the third and largest springs from the anterior wall.
The pulmonary orifice (Ostium arteriæ pulmonalis) is circular and is situated at the summit of the conus arteriosus, opposite to the lower part of the third intercostal space. It is guarded by the pulmonary valve, composed of three semilunar cusps (Valvulæ semilunares arteriæ pulmonalis); of these, one is internal, one external, and the third posterior. The convex peripheral border of each cusp is attached to the fibrous ring at the junction of the pulmonary artery and the conus arteriosus. The central border is free and slightly concave. Each cusp consists of a layer of endocardium on its ventricular surface, a continuation of the inner coat of the artery on its arterial surface, and an intermediate layer of fibrous tissue. The upper edge of the conus arteriosus forms three arches with intermediate projecting angles or horns, to all of which the cusps are attached; and the artery forms opposite each cusp a pouch, the sinus of the pulmonary artery (or of Valsalva).
Fig. 424.—Section of Heart of Horse. Specimen Hardened in situ.
The section is cut nearly at right angles to the ventricular septum, and is viewed from the right and posteriorly.
The walls of the ventricle (except in the conus arteriosus) bear muscular ridges and bands, termed trabeculæ carneæ. These are of three kinds, viz., (1) ridges or columns in relief; (2) musculi papillares, somewhat conical flattened projections, continuous at the base with the wall and giving off the chordæ tendineæ to the tricuspid valve; (3) moderator bands (Musculi transversi cordis) which extend from the septum to the opposite wall. The latter are partly muscular, partly tendinous, and vary in different subjects. The strongest one is usually about midway between the base and apex and extends from the septum to the base of the anterior musculus papillaris. It is considered that they tend to prevent overdistention.
The left ventricle (Ventriculus sinister) forms the left posterior part of the ventricular mass. It is more regularly conical than the right ventricle and its wall is much thicker except at the apex. It forms all of the posterior border of the ventricular part and the apex of the heart. Its base is largely continuous with the left atrium, with which it communicates through the left atrio-ventricular opening, but in front it opens into the aorta. The cavity usually appears smaller than that of the right ventricle in the dead subject, on account of the greater contraction of its wall. It is almost circular in cross-section.
Fig. 425.—The Bases of the Ventricles of the Heart of the Horse.
The atria have been removed and the aorta and pulmonary artery cut off short. The left ventricle is contracted. p.v., Pulmonary valve; a.v., aortic valve.
The left atrio-ventricular orifice (Ostium venosum sinistrum) is opposite to the fifth rib and intercostal space. It is almost circular and is guarded by the bicuspid or mitral valve (Valvula bicuspidalis). The two segments of this valve are larger and thicker than those of the right side of the heart. One is placed in front and separates the atrio-ventricular and aortic openings. The other is placed behind and laterally and is usually divided into two or three flaps.
The aortic orifice (Ostium aorticum) is directed upward and slightly forward. It is situated opposite to the fourth rib. It is guarded by the aortic valve, composed of three semilunar cusps (Valvulæ semilunares aortæ); one cusp is anterior, the others right and left posterior. They are similar to those of the pulmonary valve, but are much stronger. The free edge of each contains a central nodule of fibrous tissue, the corpus Arantii (Nodulus valvulæ semilunaris).
The chordæ tendineæ are fewer but larger than those of the right ventricle.
There are two large musculi papillares, one on each side; they are usually compound. The moderator bands are variable. Commonly two or three larger ones (which are often branched) extend from the musculi papillares to the septum. Smaller ones may be found in various places, especially at the apex. The other trabeculæ are fewer and less prominent than in the right ventricle.
The interventricular septum (Septum ventriculorum) is the musculo-membranous partition which separates the cavities of the two ventricles. It is placed obliquely. One surface is convex, faces forward and to the right, and bulges into the right ventricle. The other surface, which faces into the left ventricle, is concave and looks backward and to the left. The greater part of the septum is thick and muscular (Septum musculare), but the upper part is thin and membranous (Septum membranaceum). The latter intervenes between the left ventricle, on the one hand, and the right ventricle and atrium on the other.
The heart-wall consists mainly of peculiar striped muscle, the myocardium, which is covered externally by the visceral part of the serous pericardium or epicardium, and is lined by the endocardium.
The epicardium is in general closely attached to the muscular wall, but is loosely attached over the coronary vessels and the associated subepicardial fat. It consists of a layer of flat polygonal cells, resting on a membrane of white and elastic fibers.
The myocardium consists of planes of fibers arranged in a somewhat complicated manner. The muscular tissue of the atria is almost completely separated from that of the ventricles by the fibrous rings around the atrio-ventricular orifices.
In the atria the muscle bands fall naturally into two groups—superficial and deep. The former are common to both atria, the latter special to each. The superficial or common fibers for the most part begin and end at the atrio-ventricular rings, but some enter the interatrial septum. The deep or special bundles also form two sets. Looped fibers pass over the atria from ring to ring, while annular or spiral fibers surround the ends of the veins which open into the atria, the auricles, and the fossa ovalis.
The muscular wall of the ventricles is much stronger than that of the atria. That of the left ventricle is in general about three times as thick as that of the right one, but is thin at the apex. The superficial fibers are attached above to the atrio-ventricular fibrous rings and pass in a spiral toward the apex. Here they bend upon themselves and pass deeply upward to terminate in a papillary muscle of the ventricle opposite to that in which they arose. The loops so formed at the apex constitute a whorl, the vortex cordis. The deep fibers, although they appear to be proper to each ventricle, have been shown by MacCallum to be in reality almost all common to both. Their arrangement is scroll-like. They begin on one side, curve around in the wall of that ventricle, then pass in the septum to the opposite side, and curve around the other ventricle. There is a layer of deep fibers which is confined to the basal part of the left ventricle; it is attached to the left atrio-ventricular ring.
Four fibrous rings (Annuli fibrosi) surround the orifices at the bases of the ventricles. The atrio-ventricular rings separate the musculature of the atria from that of the ventricles. Those which surround the origins of the pulmonary artery and aorta are festooned in conformity with the attached borders of the valves. The aortic ring contains on the right side a plate of cartilage (Cartilago cordis), which frequently becomes more or less calcified in old animals. Sometimes a smaller plate is present on the left side.
The endocardium lines the cavities of the heart and is continuous with the internal coat of the vessels which enter and leave the organ. Its free surface is smooth and glistening and is formed by a layer of endothelial cells. The latter rest on a thin layer of connective tissue, which is connected with the myocardium by a subendocardial elastic tissue containing vessels and nerves.
Vessels and Nerves.—The heart receives its blood-supply through the two coronary arteries which arise from the aorta opposite to the anterior and left cusps of the aortic valve. Most of the blood is returned by the coronary veins, which open into the right atrium by the coronary sinus.[157] A few small veins open directly into the right atrium, and others are said to open into the left atrium and the ventricles. The lymph vessels form a subepicardial network which communicates through stomata with the cavity of the pericardium. There is a less distinct subendocardial network. The vessels converge usually to two trunks which accompany the blood-vessels in the atrio-ventricular grooves and enter the glands at the bifurcation of the trachea. The nerves are derived from the vagus and sympathetic through the cardiac plexus.
The pulmonary artery (A. pulmonalis) springs from the conus arteriosus at the left side of the base of the right ventricle. It curves upward, backward, and inward, and divides behind the arch of the aorta into right and left branches. It is related in front to the right auricle (appendix), behind to the left auricle (appendix), and internally to the aorta. It is enveloped with the latter in a common sheath of the visceral layer of the serous pericardium. Near the bifurcation it is connected with the arch of the aorta by a fibrous band about half an inch (ca. 1.2 cm.) in width; this is the ligamentum arteriosum, a remnant of the large ductus arteriosus, which conducts most of the blood from the pulmonary artery to the aorta in the fœtus. The artery is bulbous at its origin, and forms three sinuses or pouches, which correspond to the cusps of the semilunar valve. Beyond this it gradually diminishes in caliber.
In a horse of medium size the artery is about seven inches (ca. 17 to 18 cm.) long. At the origin it is about two and a half inches (ca. 6 to 6.5 cm.) in width; at the bifurcation its caliber is about one and a half inches (ca. 3.5 to 4 cm.). The wall is relatively thin, especially at the origin.
The right branch (Ramus dexter) of the pulmonary artery is a little longer and wider than the left one. It passes over the fore part of the left atrium and below the bifurcation of the trachea to the hilus of the right lung, and enters the latter below the right bronchus. In the lung it passes to the outer and lower side of the stem bronchus and accompanies it to the base of the organ. The branches correspond to the ramification of the bronchi. The left branch (Ramus sinister) is very short. It passes backward and enters the lung below the left bronchus. Its branches within the lung are arranged like that of the right one.
The aorta is the main systemic arterial trunk. It begins at the base of the left ventricle and is practically median at its origin. It passes upward and slightly forward between the pulmonary artery on the left and right atrium on the right. It then curves sharply backward and upward and inclines somewhat to the left, forming the arch of the aorta (Arcus aortæ), and reaches the ventral surface of the spine at the eighth or ninth thoracic vertebra. After passing backward along the ventral aspect of the bodies of the vertebræ between the lungs it traverses the hiatus aorticus and enters the abdominal cavity, where it lies below the vertebral bodies and the psoas minor, just to the left of the median plane. It divides under the fifth lumbar vertebra into the two internal iliac or hypogastric arteries.
From the bifurcation a small vessel, the middle sacral artery (A. sacralis media), sometimes passes backward on the pelvic surface of the sacrum. It becomes lost in the periosteum or joins the coccygeal artery, or in exceptional cases is traceable to the sphincter ani externus.
The caliber of the aorta is greatest at its origin, which is termed the bulbus aortæ. Here it forms three pouch-like dilatations, the sinuses of the aorta (or of Valsalva). These correspond to the cusps of the aortic valve, and the coronary arteries arise from the left posterior and anterior sinuses. At the arch the diameter is about two inches (ca. 5 cm.), and beyond this it diminishes gradually in width.
Fig. 426.—Cardiac Vessels of Horse, Right Side.
Veins are black, arteries white.
It is convenient to divide the aorta into thoracic and abdominal parts. The thoracic aorta (Aorta thoracica) lies within the pericardium to the point of attachment of the ligamentum arteriosum, and is inclosed with the pulmonary artery in a prolongation of the epicardium. Beyond this it is between the two pleural sacs. It is crossed on the right by the œsophagus and trachea, on the left by the left vagus nerve. The left recurrent nerve winds around the concavity of the arch from left to right, and the vena azygos and thoracic duct lie along the dorsal part of its right face. The trachea causes it to deviate to the left; but beyond this it becomes median. The abdominal aorta (Aorta abdominalis) (Fig. 450) is related above to the lumbar vertebræ, the inferior common ligament, and the left psoas minor muscle; in the hiatus aorticus it is related to the cisterna chyli. On its right is the posterior vena cava, and on its left the left kidney and ureter.
The two coronary arteries, right and left, are distributed almost entirely to the heart, but send some small twigs to the origins of the great vessels.
The right coronary artery (A. coronaria dextra) arises from the anterior sinus of the aorta. It passes forward between the conus arteriosus and the right auricle (appendix) to the coronary groove, in which it curves around to the right and backward. It then descends in the right ventricular groove almost to the apex of the heart.
Fig. 427.—Cardiac Vessels of Horse, Left Side.
The dotted lines indicate part of the left coronary artery which is concealed by the left auricle.
The left coronary artery (A. coronaria sinistra) arises from the left posterior sinus of the aorta, passes to the left behind the origin of the pulmonary artery, and divides into two branches. The descending branch (Ramus descendens) passes down the left ventricular groove toward the apex. The circumflex branch (Ramus circumflexus) runs backward in the coronary groove, in which it winds around to the right side.
The common brachiocephalic trunk or anterior aorta (Truncus brachiocephalicus communis) is a very large vessel which arises from the convexity of the arch of the aorta within the pericardium. It is directed forward and upward. Its length in horses of medium size is usually about two inches (ca. 5 to 6 cm.), but it is sometimes only half an inch or less (ca. 1 cm.). It is crossed on the left by the left vagus and cardiac nerves, and the left recurrent nerve runs between it and the trachea. It divides opposite to the second intercostal space or third rib into the brachiocephalic and left brachial arteries.
The brachiocephalic artery (A. brachiocephalica) is directed forward and a little upward in the anterior mediastinum, beneath the trachea. Opposite the first rib it gives off the bicarotid trunk and is continued as the right brachial artery. The latter (A. subclavia dextra) turns downward and bends around the anterior border of the first rib and the insertion of the scalenus muscle above the brachial vein. Its course and branches beyond this point will be described with the vessels of the thoracic limb.
Fig. 428.—Topography of Thoracic Cavity of Horse, Left Side, after Removal of Left Lung.
The ninth, fifteenth, and eighteenth ribs are retained. a, Pericardium; b, thoracic aorta; c, brachiocephalic trunk; d, d″, dorsal artery; d′, subcostal artery; e, e′, superior or deep cervical artery; f, vertebral artery; g, g′, left brachial artery; h, inferior cervical artery; i, internal thoracic artery; k, common carotid artery; l, jugular vein; m, brachial vein; m′, inferior cervical vein; n, anterior vena cava; o, thoracic duct; p, œsophagus; q, costal part of diaphragm; q′, tendinous center of same; q″, left crus of same; r, anterior mediastinal lymph glands; s, trachea; t, bronchial lymph glands; u, root of left lung; v, anterior mediastinum; v′, posterior mediastinum; w, longus colli; x, sterno-cephalicus; y, scalenus; z, transversalis costarum; 1, longissimus; 2, splenius; 3, spinalis et semispinalis; 4, 4′, rhomboideus; 5, left phrenic nerve; 6, left vagus; 6′, 6″, œsophageal continuations of vagi; 7, dorsal branch of right vagus; 8, cardiac nerve; 9, left recurrent nerve; 10, cervical trunk of sympathetic; 11, first thoracic ganglion of sympathetic; 12, thoracic trunk of sympathetic; 13, brachial plexus. (After Ellenberger-Baum, Top. Anat. d. Pferdes.)
The left brachial artery (A. subclavia sinistra) is longer than the right one and rises to a higher level. It forms an almost semicircular curve, the concavity being ventral. It is related internally to the œsophagus, trachea, and thoracic duct, and the left vagus, phrenic, and cardiac nerves cross under its origin. It emerges from the thorax like the artery of the right side. There is thus a difference at first between the trunks of opposite sides, but beyond this their course and distribution are similar.
The brachial and brachiocephalic arteries within the thorax give off the dorsal, superior cervical, vertebral, and internal thoracic arteries. At the first rib they give off the external thoracic and inferior cervical arteries.
1. The left dorsal or dorso-intercostal artery (A. costo-cervicalis) passes dorsally across the left face of the trachea and œsophagus toward the second intercostal space. The right artery arises usually by a common trunk with the superior cervical, crosses the right face of the trachea and has no contact with the œsophagus. Both detach small branches to the trachea, lymph glands, and pleura, and divide on reaching the longus colli into two branches. Of these the subcostal artery (A. intercostalis suprema) is the smaller. It passes backward under the costo-vertebral joints with the sympathetic trunk. It gives off the second, third, and fourth intercostal arteries, and ends at the fifth space, where it anastomoses with the first aortic intercostal artery, or constitutes the fifth intercostal artery, or dips into the longissimus muscle. It also gives off spinal branches and twigs to the longus colli and the pleura. The other branch (A. transversa colli) is the direct continuation of the trunk. It emerges through the upper end of the second intercostal space, passes across the transversalis costarum and longissimus toward the withers, and divides into several diverging branches. An anterior branch passes upward and forward between the splenius and complexus and anastomoses with branches of the superior cervical artery; the others are directed upward under the serratus magnus and rhomboideus to the withers, supplying the muscles and skin of this region.
Fig. 429.—Topography of Thoracic Cavity of Horse, Right View.
The ninth, fifteenth, and eighteenth ribs are retained. a, Pericardium; b, posterior vena cava; c, vena azygos; c′, œsophageal vein; d, e, anterior vena cava; f, brachial vein; g, inferior cervical vein; h, jugular vein; i, vertebral vein; k, superior cervical vein; l, dorsal vein; m, trachea; n, root of right lung; o, œsophagus; p, mediastinal lobe of right lung; q, mediastinum; r, bronchial lymph glands; s, mediastinal lymph glands; t, costal part of diaphragm; t′, tendinous center of same; u, intercostal muscle; v, longus colli; w, posterior deep pectoral muscle; w′, anterior deep pectoral muscle; x, superficial pectoral muscle; y, sterno-cephalicus; z, scalenus; 1, 5, right brachial artery; 2, dorso-cervical trunk; 2′, superior or deep cervical artery; 3, dorsal artery (cut); 3′, subcostal artery; 4, internal thoracic artery; 6, external thoracic artery; 7, inferior cervical artery; 8, bicarotid trunk; 10, aorta; 11, œsophageal artery; 12, thoracic duct; 13, right phrenic nerve; 14, brachial plexus; 15, right vagus; 15′, 15″, œsophageal continuations of vagi; 16, right recurrent nerve; 17, cervical trunk of sympathetic; 18, thoracic trunk of sympathetic; 20, inferior cervical ganglion; 21, first thoracic ganglion of sympathetic; 22, right cardiac nerve; 23, longissimus; 24, transversalis costarum; 25, spinalis; 26, multifidus; 27, ligamentum nuchæ. (After Ellenberger-Baum, Top. Anat. d. Pferdes.)
The left dorsal artery sometimes arises with the superior cervical by a common trunk. On the right side there may be a common stem for the dorsal, superior cervical, and vertebral arteries. Occasionally the artery arises from the anterior aorta. Sometimes it emerges through the third space. The subcostal may arise independently behind the dorsal or from the superior cervical artery.
2. The superior or deep cervical artery (A. cervicalis profunda) arises in front of the dorsal or by a common trunk with it. It crosses the œsophagus (left side), the trachea (right side), and the longus colli, and emerges from the thoracic cavity by passing through the space behind the first costo-transverse articulation. In the thorax it gives off a small branch (A. mediastini cranialis) to the mediastinum and the pericardium; also the first intercostal artery (A. intercostalis prima), a very small vessel which passes down in the first intercostal space. After leaving the thorax the artery passes upward and forward on the spinalis muscle and the lamellar part of the ligamentum nuchæ, covered by the complexus. Its terminal branches anastomose with branches of the occipital and vertebral arteries in the region of the axis. Numerous collateral branches are detached to the extensor muscles of the head and neck, the ligamentum nuchæ, and the skin, and anastomoses occur with the dorsal artery above and the vertebral artery below.
The artery sometimes emerges through the second intercostal space.