I. THE HEART. COR.
The heart lies in the mediastinum, enclosed in the pericardial sac, and projects rather more toward the left than toward the right. It is an ovoid or pear-shaped organ, with its long axis directed approximately craniocaudad. Its caudal end or apex is, however, directed slightly ventrad and to the left, while the larger cranial end or base faces slightly dorsad as well as craniad.
Laterally and dorsally the heart is largely covered by the lungs. The ventral side and a considerable portion of the lateral surface are, however, not thus covered, so that they lie against the thoracic wall. The heart extends from about the fourth or the fifth to the eighth rib, and its apex touches the diaphragm.
The cavity of the heart is divided by a longitudinal dorsoventral septum into lateral halves—a right and a left side. Each side is again divided by a transverse dorsoventral septum (auriculoventricular) into two chambers, one of which, the auricle, lies at the base; the other, the ventricle, lies at the apex of the heart. There are thus right and left auricles and right and left ventricles. An external groove, partly filled with fat, separates the auricular portion of the heart from the ventricular part; this groove is known as the sulcus coronarius.
Each auricle or ventricle has a single set of blood-vessels either leaving it or entering it. Thus the blood enters the right auricle by the inferior and superior venæ cavæ ([Fig. 116], d and e) (præcava and postcava). It passes thence into the right ventricle, and from the right ventricle to the lungs by a single pulmonary artery ([Fig. 115], f). It returns to the left auricle by the numerous pulmonary veins ([Fig. 116], g, h, i), and passes thence to the left ventricle. From the left ventricle it passes to the body by a single aorta ([Fig. 115], g).
When the heart is viewed from the ventral surface ([Fig. 115]), a considerable blood-vessel, the coronary artery (q), running from the base to a point a little to the right of the apex, indicates the position of the septum (ventricular septum), between the right (a) and left (b) ventricles. The apex thus belongs to the left ventricle. From the cranial end of the right ventricle the pulmonary artery (f) is seen passing obliquely craniad and toward the left. Beneath the pulmonary artery the aorta (g) appears rising from the middle of the base of the heart and passing directly craniad. At the base appears a part of the auricular appendage of the left auricle (d), and at the right a part of the right auricular appendage (c). The former (d) is larger and bent at right angles, so that its free end is directed toward the apex.
In the dorsal view ([Fig. 116]) the position of the ventricular septum is not indicated. On the surface of the left ventricle a short distance from the ventricular septum and nearly parallel to it are seen branches of the coronary artery and the coronary vein passing toward the apex. Craniad of the left ventricle is seen the left auricle (b) with the pulmonary veins (g, h, i) opening into it. Craniad of the right ventricle (a) is the right auricle (c) with the venæ cavæ (d and e) opening into it near the middle. Coming from beneath the auricles are seen the pulmonary artery (j) and the aorta (f).
The chambers of the heart.
1. The right auricle (atrium dextrum) ([Figs. 115] and [116], c). Externally the right end of the right auricle projects so as to form its auricular appendage ([Fig. 115], c) which lies at the right of the base of the aorta ([Fig. 115], g). The superior vena cava ([Fig. 116], e) is seen entering near the cranial end of the auricular septum opposite the base of the aorta. The inferior vena cava ([Fig. 116], d) enters the auricle near the coronary sinus and close to the auricular septum. The netted appearance which the wall, especially that of the appendage, presents externally is due to the muscular thickenings of the wall (musculi pectinati).
[Fig. 115].—Heart, Ventral View.
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[Fig. 116].—Heart, Dorsal View.
![[Fig. 116]](#3418956513767812439_illo276blg.jpg.id-3166487640412470636.wrap-0.html.html){kind=link}
Fig. 115.—a, right ventricle; b, left ventricle; c, right auricular appendage; d, left auricular appendage; e, conus arteriosus; f, pulmonary artery; g, aortic arch; h, thoracic aorta; i, vena cava superior; j, innominate artery; k, left subclavian artery; l, left common carotid; m, right common carotid; n, right subclavian; o, azygos vein; p, the two innominate veins; q, coronary artery.
Fig. 116.—a, right ventricle; b, left auricle; c, right auricle; d, vena cava inferior; e, vena cava superior; f, aorta; g, h, i, groups of pulmonary veins (g, dorsal group; h, sinistral group; i, dextral group); j, pulmonary artery (division into two); k, innominate artery; l, left subclavian; m, left common carotid; n, right common carotid; o, right subclavian; p, azygos vein.
The cavity (including that of the auricular appendage) is somewhat egg-shaped, with its long axis transverse. The musculi pectinati, which branch and unite into a network, are most abundant on its dorsal wall at the right. The axes of the two venæ cavæ if produced into the cavity of the auricle would meet one another at its centre and nearly at right angles. Caudad of the opening of the inferior vena cava is seen the slit-like opening of the coronary sinus, guarded craniad by the semilunar valve of the coronary sinus (or valve of Thebesius). The coronary sinus receives blood from coronary veins, which collect it from the walls of the heart.
In the auricular septum ventrad of the opening of the inferior vena cava is seen a faintly marked smooth oval depression, the fossa ovalis. When the auricular septum is examined by transmitted light it is seen to be thinner over the fossa ovalis. There is an opening, foramen ovale, at this point in fœtal life, so that the blood of the inferior vena cava then passes directly from the right auricle to the left auricle. Caudad the cavity of the right auricle communicates with that of the ventricle by the large oval auriculoventricular opening, which is guarded by the tricuspid valves ([Fig. 117]).
2. The right ventricle (ventriculus dexter) ([Figs. 115] and [116], a) does not reach quite to the apex of the heart, so that it makes up less than one-half of its ventricular portion. It makes a half-spiral turn about the left ventricle, from its lateral side at the apex toward its ventral side at the base, where it ends in the pulmonary artery ([Fig. 115], e). Its wall is very thin as compared with that of the left ventricle. Its cavity ([Fig. 117]) is nearly flat on its medial side, convex on its lateral side. It communicates with the auricle craniodorsad. Cranioventrad the cavity is narrower and turns around toward the ventral side of the heart and opens into the pulmonary artery. That portion of the ventricle between the auriculoventricular opening and the pulmonary artery is the conus arteriosus ([Fig. 115], e; [Fig. 117], f). Internally the wall presents many muscular trabeculæ ([Fig. 117], a) of various sizes. These are more numerous over the ventricular wall than on the septum. They do not occur on the conus (f). Surrounding the auriculoventricular opening is the tricuspid valve ([Fig. 117], d, d′, d″). It consists of three flaps. One of these (d′) is septal (i.e., it lies against the septum), while of the two others which do not lie against the septum, one is dorsal (d″) and the other ventral. Each flap is thin, semicircular, and membranous, and is attached to the border of the auriculoventricular opening by the diameter of the semicircle. The free border of each is attached to the wall of the heart by numerous delicate tendinous bands, the chordæ tendineæ (c), some of which are attached to the lower face of each valve. The chordæ tendineæ of the septal valve (or most of them) are attached at their opposite ends to the septum directly, while those of the dorsal and ventral flaps are attached to the ends of three or more band-like muscles, columnæ (or trabeculæ) carneæ (b), which are fixed by their opposite ends to the ventricular wall.
[Fig. 117].—Heart, with Right Ventricle laid open to show the Tricuspid Valve.
![[Fig. 117]](#3418956513767812439_illo278lg.jpg.id-7130166691681320027.wrap-0.html.html){kind=link}
a, trabeculæ; b, columnæ carneæ; c, chordæ tendineæ; d, d′, d″, the three flaps of the tricuspid valve; e, aorta; f, conus arteriosus, laid open; g, semilunar valves of the pulmonary artery.
Between the conus arteriosus (f) and the pulmonary artery are three pocket-like semilunar valves ([Fig. 117], g), one ventral, one dextral, and one sinistral. Between each valve and the wall of the pulmonary artery there is an enlargement of the cavity of the artery, one of the pulmonary sinuses (or sinuses of Valsalva). Beyond the sinuses the pulmonary artery divides into right and left branches ([Fig. 116], j).
3. The left auricle (atrium sinistrum) ([Fig. 115], d; [Fig. 116], b). The auricular appendage ([Fig. 115], d) is bent at right angles and its apex turned caudad. The cavity is thus irregular. Muscular trabeculæ occur only in the auricular appendage. When the auricular septum is examined by transmitted light the position of the fossa ovalis is indicated at about the middle of the septum as seen from the left side. Ventrad of it is a fold of the septal wall.
The pulmonary veins enter the dorsal wall of the auricle in three groups ([Fig. 116], g, h, i). Each group opens into a more or less pronounced sinus or extension of the auricular cavity. The sinuses may be called, on account of their position, dorsal (g), sinistral (i), and dextral (h).
4. The left ventricle ([Fig. 115], b) occupies rather more than the left half of the base of the heart. Its walls are two or three times as thick as those of the right ventricle. Internally its walls present a few muscular bands, comparable to the trabeculæ of the left ventricle. There are two very large columnæ carneæ, one dorsad and one ventrad. At the cranial end it communicates near the lateral wall with the left auricle by the auriculoventricular opening, and near the septum with the aorta. The bicuspid valve (or mitral valve), which guards the auriculoventricular opening, consists of two flaps, one septal and one lateral. Their chordæ tendineæ, which come from their free borders as well as from their outer surfaces, are attached to the columnæ carneæ, chordæ passing from both valves to each columna carnea.
The opening into the aorta is guarded by three aortic semilunar valves, each of which partly conceals an aortic sinus (or sinus of Valsalva). One valve is dorsal, one sinistral, and one dextral. In the dextral sinus is the opening of one of the coronary arteries, which carry blood to the walls of the heart.
Pericardium.
The pericardium is a sac enclosing the heart. It lies in the middle mediastinum, and the two halves of the mediastinal septum with their fat may be dissected away from it. Its wall is composed of two layers, an external firm fibrous layer and an internal layer of flattened epithelial cells similar to the peritoneal epithelium (serous layer). The fibrous layer forms a sac which repeats roughly the form of the heart. This sac is attached to the aorta at the point of origin of the subclavian artery, to the pulmonary artery at its bifurcation, and to the venæ cavæ and pulmonary veins near their entrance into the heart. At these points it is continuous with the fibrous coats of the vessels named, and from them it is reflected over the heart, forming a complete sac enclosing it but not attached to it anywhere. The heart lies within this sac. The serous layer lines the fibrous sac and gives to the surface of the heart and fibrous layer a smooth glistening appearance. It is reflected over the heart. The relation of the heart to it is much the same as the relation of the intestine to the peritoneal sac. The serous layer consists therefore of two portions, parietal, lining the sac, and visceral, covering the heart. The parietal and visceral portions are continuous along a line which runs approximately parallel to the auriculoventricular groove and encloses all the great blood-vessels. Within this line the heart lies against the fibrous layer of pericardium and is not covered by the serous layer. The serous layer is easily dissected free from the heart-wall, but its parietal portion is closely adherent to the fibrous layer.