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muscle in several ways: they are shorter, many of them being oblong cells, with forked extremities which are closely cemented to similar processes of adjacent cells; they form a reticulum, and the nuclei lie in the centres of the cells. Moreover, still more peculiar fibres, the fibres of Purkinje, are found immediately beneath the subendocardial tissue. The fibres of Purkinje are large cells which unite with one another at their extremities; their central portions consist of granular protoplasm, in which sometimes one but more frequently two nuclei are embedded, and the peripheral portion of each cell is transversely striated. These cells, in short, present, in a permanent form, à condition which is transitory in all other striped muscle cells.
The reticulating cardiac muscle cells are grouped in sheets and strands which have a more or less characteristic and definite arrangement in different parts of the heart; by careful dissection, and after special methods of preparation, it is possible to recognise many layers and bundles, some of which are, however, probably artificially produced.
In the atria the muscular fasciculi fall naturally into two groups: (a) superficial fibres common to both atria; (b) deep fibres special to each atrium.
The superficial fibres are most numerous on the sterno-costal aspect and in the neighbourhood of the coronary sulcus. They run transversely across the atria and a few of them dip into the interatrial septum.
The deep fibres are―(1) looped fibres. The extremities of the looped fibres are attached to the fibrous rings around the atrio-ventricular orifices and the fibres pass antero-posteriorly over the atria. (2) Annular fibres which surround (a) the extremities of the large vessels which open into the atria; (b) the auricles; (c) the fossa ovalis.
In the ventricles the muscular fasciculi form more or less definite V-shaped loops which commence from and end at the fibrous rings which surround the large orifices at the bases of the ventricles. In their courses the loops embrace the cavities of either one or both ventricles, one stem of each loop lying on the outer surface of the heart and the other in the interior, and some of the loops possessing very acute whilst others have very open bends.
The superficial fibres on the sterno-costal surface pass towards the left, those on the inferior surface towards the right. At the apex all are coiled into a whorl or vortex through which they pass into the interior of the ventricular walls and run towards the base, some in the septum and others in the papillary muscles. The various bundles which have been described can, according to Mall, be resolved into two main systems. One system arises from the conus arteriosus and the root of the aorta, that is from the remains of the primitive aortic trunk: it is called the "bulbo-spiral" system. The other springs from the region of the primitive venous sinus and is termed the "sino-spiral." Both systems are separable into superficial and deep portions, and the general plan of more or less spirally curved V-shaped loops is retained in each, but the details of the arrangement are too complicated for consideration within the limits of an ordinary text-book (see Amer. Journ. of Anat. vol. ii. 1910-1911).
The Atrio-Ventricular Bundle. It would appear from the preceding description that the muscle-fibres of the atria and the ventricles are entirely separated from one another by the fibrous rings which surround the atrio-ventricular orifices; that, however, is not the case, for the two groups are connected together by a bundle of muscle fibres of pale colour and rudimentary structure, which lies immediately adjacent to the endocardium and constitutes the atrio-ventricular bundle.
The bundle commences in a nodular enlargement which lies in the lower part of the wall of the right atrium, close to the base of the medial cusp of the tricuspid valve. From that point the bundle runs along the posterior and lower borders of the membranous part of the ventricular septum to the upper and posterior part of the muscular portion of the septum, where it divides into right and left branches. The right branch runs along the right side of the ventricular septum to the moderator band, along which it passes into the anterior papillary muscle of the right ventricle. The left branch runs along the left side of the septum and both branches give off numerous ramifications, by means of which the main bundle is associated with all parts of the walls of the two ventricles.
Both the function and the origin of the atrio-ventricular bundle are uncertain. After the discovery of the bundle it was asserted that impulses were conveyed from the atria to the ventricles by the muscle fibres of the atrio-ventricular bundle and by them alone; more recently it has been shown that minute nerve fibrils are intimately intermingled with the muscle fibres of the bundle, and it has yet to be decided whether the impulses which pass from the atria to the ventricles, for the purpose of maintaining the proper sequence of the movements of the chambers, travel by the nerve fibrils or the muscle fibres or by both.
The atrio-ventricular bundle is probably a remnant of the muscular continuity which existed in the early stages of development between the atrial and ventricular chambers of the heart, but it may be, wholly or in part, a new formation.
The epicardium, or visceral portion of the pericardium, consists of white connective and of elastic tissue, the latter forming a distinct reticulum in the deeper part. The surface which looks towards the pericardial cavity is covered with flat polygonal endothelial plates, which are partially separated, here and there, by stomata. It has been asserted that the pericardial cavity communicates with the lymphatics of the epicardium through the stomata.
The endocardium lines the cardiac cavities and is continuous with the inner coats of the vessels which enter and leave the heart. It consists, like the epicardium, of white connective tissue and elastic fibres, but it is much thinner than the epicardium, and its elastic fibres are in some places blended into a fenestrated membrane. Its inner surface is covered with endothelial cells, and it rests externally upon the subendocardial tissue, in which there are blood-vessels and the endocardium itself is entirely devoid of vessels.
Size of the Heart. The heart is about 125 mm. (five inches) long, 87 mm. (three and a half inches) broad; its greatest depth from its sterno-costal to its diaphragmatic surface is 62 mm. (two and a half inches), and it is roughly estimated as being about the same size as the closed fist. The size, however, is variable, the volume increasing at first rapidly, and then gradually, with increasing age, from 22 cc. at birth to 155 cc. at the fifteenth year, and to 250 cc. by the twentieth year From that period to the fiftieth year, when the maximum volume (280 cc.) is attained, the increase is much more gradual, and after fifty a slight decrease sets in. The volume is the same in both sexes up to the period of puberty, but thereafter it preponderates in the male.
Weight. The average weight of the heart in the male adult is 310 grms. (11 ounces), and in the female adult 255 grms. (9 ounces); but the weight varies greatly, always, however, in definite relation to the weight of the body, the relative proportions changing at different periods of life. Thus at birth the heart weighs 24 grms. (13 drachms), and its relation to the body weight is as 1 to 130, whilst in the adult the relative proportion is as 1 to 205. The heart is said to increase rapidly in weight up to the seventh year, then more slowly up to the age of puberty, when a second acceleration sets in; but after the attainment of adult life the increase, which continues till the seventieth year, is very gradual.
The above changes affect the whole heart, but the several parts also vary in their relation to one another at different periods of life. During foetal life the right atrium is heavier than the left; in the first month after birth the two become equal; at the second year the right again begins to preponderate, and it is heavier than the left during the remainder of life. In the latter part of foetal life the two ventricles are equal; after birth the left grows more rapidly than the right, until, at the end of the second year, a position of stability is gained, when the right is to the left as 1 to 2, and this proportion is maintained until death.
Capacity. During life the capacity of the ventricles is probably the same, and each is capable of containing about four ounces of blood, whilst the atria are a little less capacious. After death the cavity of the right ventricle appears larger than that of the left.
Vascular Supply of the Heart.-The walls of the heart are supplied by the coronary arteries (p. 887), the branches of which pass through the interstitial tissue to all parts of the muscular substance and to the subendocardial and subepicardial tissues; the endocardium and the valves are devoid of vessels. The capillaries, which are numerous, form a close-meshed network around the muscular fibres. Sometimes the valves contain a few muscular fibres, and in those cases they also receive some minute vessels. The majority of the veins of the heart end in the coronary sinus, which opens into the lower part of the right atrium; some few very small veins, however, open directly into the right atrium, and others are said to end in the left atrium, and in the cavities of the ventricles.
Lymphatics of the Heart.-Lymphatic vessels are freely distributed throughout the whole substance of the heart. They all communicate with the superficial network which lies beneath the epicardium. The efferent vessels from the subepicardial network accompany the coronary arteries to the coronary sulcus and pass thence to the anterior mediastinal glands (p. 1011).
Nerves of the Heart. The heart receives its nerves from the superficial and deep cardiac plexuses. The former lies beneath the aortic arch and the latter between the arch and the bifurcation of the trachea. Through the plexuses it is connected with the vagus, the accessory (through the vagus), and the sympathetic nerves. After leaving the cardiac plexuses many of the nerve-fibres enter the walls of the atria and anastomose together in the subepicardial tissue, forming a plexus in which many ganglion cells are embedded, especially near the terminations of the inferior vena cava and the pulmonary veins. From the subepicardial atrial plexus, nerve filaments, on which nerve ganglion cells have been found, pass into the substance of the atrial walls.
Other fibres from the cardiac plexuses accompany the coronary arteries to the ventricles, and upon those also ganglion cells are found in the region immediately below the coronary sulcus.
The nerve-fibres which issue from the ganglionated plexuses of the heart are non-medullated. They form fine plexuses round the muscle fibres, and they terminate either in fine fibrils on the surfaces of the muscle fibres, or in nodulated ends which lie in contact with the muscle cells.
The pericardium is a fibro-serous sac which surrounds the heart. It lies in the middle mediastinum, and is attached below to the diaphragm, and above and posteriorly to the roots of the great vessels. Anteriorly and posteriorly it is in relation with the structures in the corresponding mediastina and laterally it is in close apposition with the pleural sacs.
The fibrous pericardium is a strong fibrous sac of conical form; its base is attached to the central tendon and to the adjacent part of the muscular substance of the diaphragm, and it is pierced by the inferior vena cava. At its apex and posteriorly it is gradually lost upon the great vessels which enter and emerge from the heart, giving sheaths to the aorta, the two branches of the pulmonary artery, the superior vena cava, the four pulmonary veins, and the ligamentum arteriosum. Its anterior surface forms the posterior boundary of the anterior
mediastinum, and it is attached, above and below, by the superior and inferior sterno-pericardial ligaments, to the sternum. In the greater part of its extent it is separated from the anterior wall of the thorax by the anterior margins of the lungs and pleural sacs, but it is in direct relation with the left half of the lower portion of the body of the sternum and, in many cases, with the medial ends of the cartilages of the fourth, fifth, and sixth ribs of the left side and the left transversus thoracis muscle. Its posterior surface forms the anterior boundary of the posterior
FIG. 756.-POSTERIOR WALL OF THE PERICARDIUM AFTER THE REMOVAL OF THE HEART.
mediastinum; it is in relation with the oesophagus and the descending aorta, both of which it separates from the back of the left atrium. Each lateral aspect is in close contact with the mediastinal portion of the parietal pleura, the phrenic nerve and its accompanying vessels intervening. The inner surface of the fibrous. sac is lined by the serous pericardium, which is closely attached to it.
The serous pericardium is a closed sac containing a little fluid (liquor pericardii). It is surrounded by the fibrous pericardium and invaginated by the heart. It is, therefore, separable into two portions-the parietal, which lines the inner surface of the fibrous sac, and the visceral, which ensheaths, or partially ensheaths, the heart and the great vessels; but the two portions are, of course, continuous with one another where the serous layer is reflected on to the great vessels as they pierce the fibrous layer. The majority of the great vessels receive only partial coverings from the visceral layer: thus, the superior vena cava is covered anteriorly and on
each side; the pulmonary veins anteriorly, above, and below; and the inferior vena cava anteriorly and on each side. The aorta and the pulmonary artery are enclosed together in a complete sheath of the visceral layer. Therefore, when the pericardial sac is opened from the front, it is possible to pass the fingers behind them and in front of the atria, from the right to the left side, through a passage called the great transverse sinus of the pericardium (Fig. 756). The spaces or pouches which intervene between the vessels which receive partial coverings from the serous pericardium are also called sinuses; and the largest of them, which is bounded below and on the right by the inferior vena cava, and above and on the left by the left inferior pulmonary vein, is known as the great oblique sinus. It passes upwards and to the right behind the left atrium, and lies anterior to the oesophagus and the descending thoracic aorta.
A small fold of the serous pericardium, the vestigial fold, passes from the left pulmonary artery to the left superior pulmonary vein, posterior to the left extremity of the transverse sinus. It merits special attention because it encloses a fibrous strand, the ligamentum venæ cava sinistra. This is a remnant of the left superior vena cava or duct of Cuvier, which atrophied at an early period of fœtal life.
Structure. The fibrous pericardium consists of ordinary connective tissue fibres felted together into a dense, unyielding membrane. The serous pericardium is covered on its inner aspect by a layer of flat endothelial cells. The endothelium rest upon a basis of mixed white and elastic fibres in which run numerous blood-vessels, lymphatics, and nerves.
The pulmonary artery springs from the anterior and left angle of the base of the right ventricle, at the termination of the conus arteriosus. It is slightly larger at its commencement than the aorta, and is dilated, immediately above the cusps of the valve, into three pouches, the sinuses of the pulmonary artery. It runs upwards and posteriorly, towards the concavity of the aortic arch, curving from the front round the left side of the ascending aorta to reach a plane posterior to the latter; and it terminates, by dividing into right and left branches, opposite the fifth thoracic vertebræ. Its length is a little more than two inches.
Relations. The pulmonary artery is enclosed within the fibrous pericardium, and is enveloped, along with the ascending aorta, in a common sheath of the visceral layer of the serous pericardium. It lies behind the anterior extremity of the second left intercostal space, from which it is separated by the anterior margins of the left lung and pleural sac.
Its posterior relations are the bulb of the aorta, the anterior wall of the left atrium, and the first part of the left coronary artery. To the right it is in relation with the right coronary artery, the auricle of the right atrium, and the ascending aorta, and to the left with the left coronary artery and the auricle of the left atrium. Immediately above its bifurcation, between it and the aortic arch, is the superficial cardiac plexus.
The right branch of the pulmonary artery is longer and larger than the left. It passes to the hilum of the right lung, forming one of the constituents of the root of the lung, and, after entering the lung, it descends, with the main bronchus, to the lower extremity of the organ.
Relations. Before it enters the lung the right pulmonary artery passes posterior to the ascending aorta, the superior vena cava, and the upper right pulmonary vein. At first, it lies below the arch of the aorta and the right bronchus, anterior to the oesophagus, and above the left atrium and the lower right pulmonary vein; then it crosses anterior to the right bronchus, immediately below the eparterial branch of that bronchus, and reaches the hilum of the lung. After it has passed through the hilum the artery descends, in the lung, posterior and lateral to the main bronchus and between its ventral and dorsal branches. Branches. Before entering the hilum it gives off a large branch to the upper of the right lung which accompanies the eparterial bronchus, and in the substance of
the lung it gives off numerous branches which correspond with and accompany the dorsal, ventral, and accessory branches of the right bronchus (see p. 1097).
The left branch of the pulmonary artery, shorter, smaller, and somewhat higher in position than the right, passes laterally and posteriorly from the bifurca
FIG. 757.-THE PULMONARY ARTERIES AND VEINS AND THEIR RELATIONS IN A FORMALIN
11. Internal mammary artery.
13. Transverse scapular artery.
16. Inferior thyreoid artery.
22. Sterno-hyoid muscle.
24. Sterno-hyoid muscle.
30. Inferior thyreoid artery.
32. Left subclavian artery.
33. Subclavius muscle.
35. Left common
37. Ligamentum arteriosum.
tion of the pulmonary stem, and runs, in the root, to the hilum of the left lung; it then descends, in company with the main bronchus, to the lower end of the lung.
Relations. Before it enters the lung it is crossed, anteriorly, by the upper left pulmonary vein; posterior to it, are the left bronchus and the descending aorta; above, are