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In all but the smallest arteries numerous elastic fibres are also present. The elastic element is specially strong near the middle coat in small and medium sized vessels, and is sometimes described as an external elastic membrane. In some arteries longitudinally arranged unstriped muscular fibres are also found in the external coat.
Vagina Vasis. In addition to the three tunics above described, arteries are enclosed in a sheath of the surrounding connective tissue, and are more or less connected with it by fine strands of fibrillated connective tissue.
Structure of Veins.-The walls of veins are similar in structure to those of arteries; they are, however, thinner, so much so, that, although veins are cylindrical tubes when full of blood, they collapse when empty and their lumina almost disappear. The structural details of the three tunics vary somewhat in different veins; in most the innermost tunic is marked by folds which constitute valves. Like the arteries, the veins are enclosed in connective tissue sheaths.
Tunica Intima.-In the majority of the veins the tunica intima includes an internal endothelial layer, a middle layer of subendothelial connective tissue, and an outer layer of elastic tissue. The innermost tunic of a vein is less brittle than the inner coat of an artery, and is more easily peeled off from the middle coat. The subendothelial tissue is a fine fibrillated connective tissue, less abundant than in the arteries, and in many cases it is absent. The elastic layer consists of lamella of elastic fibres which are arranged longitudinally; it rarely has the appearance of a fenestrated membrane.
One of the chief peculiarities of the tunica intima is the presence of folds of its substance which constitute valves. of the valves are of semilunar shape, and they are usually arranged in pairs. Their convex borders are continuous with the vessel wall, and their free borders are turned towards the heart; whilst, therefore, they do not interfere with the free flow of blood from the periphery, they prevent any backward flow towards it, and they help to sustain the column of blood in all vessels in which there is an upward flow. Each valve cusp consists of a fold of the endothelial layer, strengthened by a little connective tissue. As a general rule, the wall of the vein is dilated on the central side of each valve into a shallow pouch or sinus; consequently, when the veins are distended they assume a nodulated appearance. The valves are more numerous in the deep than in the superficial veins, and in the veins of children than in the veins of adults.
Tunica Media. The middle tunic is much thinner than the corresponding tunic of an artery, and it contains a smaller amount of muscular and a larger amount of ordinary connective tissue; indeed, so much does the latter preponderate that it separates the muscular fibres into a number of bands, which are isolated from one another by strands of connective tissue; therefore the muscle fibres do not form a continuous layer. In some of the veins the more internal muscular fibres do not retain the transverse direction which is usually met with both in arteries and veins; on the contrary, they run longitudinally. This condition is met with in the branches of the mesenteric veins, in the femoral and iliac veins, and in the umbilical veins. The middle tunic is absent in the thoracic part of the inferior vena cava; it is but slightly developed in many of the larger veins, whilst in the jugular veins its muscular tissue is very small in amount.
Tunica Externa.-This tunic consists of white fibrous and elastic tissue. In many of the larger veins a considerable amount of muscular tissue is also present; this is the case in the iliac and axillary veins, the abdominal part of the inferior vena cava, the azygos and hemiazygos veins, and in the renal, spermatic, splenic, superior mesenteric, portal, and hepatic veins. The striped muscle fibres of
the heart are prolonged into it at the terminations of the vena cava. The external tunic is frequently thicker than the middle tunic, and the two are not easily separable from one another.
Vascular and Nervous Supply of Arteries and Veins.-Blood-vessels.-The walls of the blood-vessels are supplied by numerous small arteries, called rasa vasorum, which are distributed to the outer and middle tunics. They arise either from the vessels they supply or from adjacent arteries, and after a short course enter the walls of the vessels in which they end. The blood is returned by small
Lymphatics. Although the cell spaces in the middle and inner tunics may be regarded as the commencement of lymphatics, definite lymphatic vessels are limited to the outer tunic.
Nerves. Arteries and veins are well supplied with nerves, which form a coarse network in the outer tunic. Branches from this network enter the tunica media, where they form a finer network which supplies twigs to the muscle fibres and sends fine filaments into the inner coat.
Divisions of the Blood-Vascular System.-Blood-vessels convey blood to or from the tissues of the body generally, or to and from the lungs. The former constitute the systemic vessels or general system; the latter form the pulmonary system. The two systems are connected together by the heart.
The venous trunks passing to the liver, and their tributaries, form a subsidiary part of the general systemic group of vessels, which is known as the portal system.
The heart is a hollow muscular organ, which is enclosed in a fibro-serous sac known as the pericardium. It receives blood from the veins, and propels it into and along the arteries. The cavity of the fully developed heart is completely separated into right and left halves by an obliquely placed longitudinal septum, and each half is divided into a posterior receiving chamber, the atrium, and an anterior ejecting chamber, the ventricle. The separation of the atria from the ventricles, however, is not complete. Externally a comparatively shallow constriction, running transversely to the long axis of the organ, indicates the distinction between the atria and ventricles; internally a wide aperture is left between the atrium and ventricle of each side. Each atrioventricular aperture is provided with a valve which allows the free passage of blood from the atrium to the ventricle, but effectually prevents its return.
The delicate walls of the blood capillaries allow the fluid part of the blood-blood plasmato pass outwards to the tissues. In the tissues the plasma enters spaces, or intercellular channels, in which the tissue elements lie; thus the latter are directly bathed in blood plasma which contains nutritive materials and oxygen. The intercellular spaces may be looked upon as the commencement of the lymph-vascular system. They communicate together, and lymph plasma passes from them into lymph vessels which carry it to the blood-vascular system. It must be remembered, in addition, that materials also pass from the tissues into the blood capillaries. Lymph vessels, in other words, convey material from the tissues. Blood-vessels convey material both to and from the tissues.
The removal of waste products which have passed from the tissues to the blood is provided for by special organs, some of which are simply interposed in the course of the general circulation -e.g. the liver, the kidneys, and the skin. The lungs, however, where the impure or venous blood receives its main supply of oxygen and gives up most of its carbon dioxide, etc., do not lie in the course of the general or systemic circulation; for them a secondary or pulmonary circulation is established, by which venous blood is conveyed from the heart to the lungs by the pulmonary artery and its branches, and, after passing through the pulmonary capillaries, is returned again to the heart, as oxygenated or arterial blood, by the pulmonary veins.
The heart, anatomically a single organ, is correspondingly modified, and, as described above, it is divided by a septum into a right and a left part. The right side receives the blood from the systemic veins, and ejects it into the pulmonary artery; whilst the left side receives blood from the pulmonary veins, and ejects it into the main systemic artery—the aorta.
The shape of the heart is that of an irregular and somewhat flattened cone; and a base, an apex, two surfaces (inferior or diaphragmatic and antero-superior or sterno-costal), and three borders (right, left, and inferior) are distinguishable.
An oblique groove, the sulcus coronarius, runs transversely to the long axis of the organ; it separates the postero-superior or atrial portion from the anteroinferior or ventricular part. The separation of the atrial portion into right and left chambers is marked, externally, at the base of the heart only, where an indistinct interatrial groove exists. The division of the ventricular part into right and left ventricles is more definitely marked on the surface by anterior and an inferior longitudinal sulcus which meet at the inferior border to the right of the apex.
The heart lies in the middle mediastinum. It rests below on the diaphragm, and is enclosed in the pericardium, which intervenes between it and the neighbouring structures. Its long axis, from base to apex, runs obliquely from behind forwards, downwards, and to the left.
Basis Cordis. The base, which is formed by the atria, and almost entirely by
FIG. 750. THE BASE AND DIAPHRAGMATIC SURFACE OF THE HEART, showing the openings of the great vessels and the line of reflection of the serous pericardium in a formalin hardened preparation.
the left atrium, is directed upwards, posteriorly, and to the right. It lies anterior to the descending thoracic aorta, the oesophagus, and the lower right pulmonary vein, which separate it from the bodies of the sixth, seventh, and eighth thoracic vertebræ.
On the whole the base is somewhat flattened. It is irregularly quadrilateral in outline, and the terminations of the superior and inferior vena cava and the four pulmonary veins pass through it. The opening of the superior vena cava is situated at the upper right angle, that of the inferior cava occupies the lower angle on the right side; between and a little to the left of those openings are the orifices of the two right pulmonary veins, and immediately to the right of the latter is the indistinct posterior interatrial sulcus, which descends to the left of the orifice of the inferior vena cava. The openings of the two left pulmonary veins are situated near the left border of the base. The portion of the surface which lies between the right and left pulmonary veins forms the anterior boundary of a section of the pericardial cavity called the great oblique sinus.
The base is limited below by the inferior part of the coronary sulcus, in which the coronary sinus lies; its upper border is in relation with the pulmonary arteries. A fold of pericardium, the vestigial fold, descends, near the left border of the base, from the left branch of the pulmonary artery, above, to the left superior pulmonary vein below. It contains the ligamentum v. cava sinistræ, and from its lower end a small vein, the oblique vein of the left atrium, passes below the orifice of the lower left pulmonary vein, and descends to the coronary sinus. Further, it is from the base that the visceral layer of the pericardium, which elsewhere completely invests the heart, is reflected to the fibrous layer, the lines of reflection corresponding with the orifices of the great vessels.1
FIG. 751.-THE STERNO-COSTAL SURFACE OF FORMALIN-FIXED HEART.
The apex, bluntly rounded, is formed entirely by the left ventricle. It is directed downwards, anteriorly, and to the left, and is situated, under cover of the anterior borders of the left lung and pleura, behind the fifth left intercostal space, three and a half inches from the anterior median line.
The diaphragmatic surface is formed by the ventricular part of the heart. It rests upon the diaphragın, chiefly on the central tendon, but, upon the left side, on a small portion of the muscular substance also, and it is divided into two areas -a smaller to the right side and a larger to the left side by an oblique anteroposterior groove, the inferior interventricular sulcus. It is separated from the base by the posterior or inferior portion of the coronary sulcus.
The sterno-costal surface is directed upwards, anteriorly, and to the left. It lies posterior to the body of the sternum and the medial extremities of the cartilages of the third, fourth, fifth, and sixth ribs of the right side, and a greater extent of the corresponding cartilages of the left side.
is separated into upper and lower sections by the anterior portion of the 1 In the fœtus and young child the atrial portion of the heart forms not only the base, but also the posterior part of the inferior or diaphragmatic surface.
coronary sulcus, which runs obliquely from above downwards, and from left to right, from the level of the third left to that of the sixth right costal cartilage. The upper section of the surface, which is concave anteriorly, is formed by the atria; it is separated from the sternum by the ascending aorta and the pulmonary artery, and is continuous laterally with the auricles of the atria which, projecting forwards, embrace the great vessels.
The lower section of the sterno-costal surface is convex; it is formed by the ventricular part of the heart, and is divided, by an anterior interventricular sulcus, into a smaller left and a larger right part. At the junction of the atrial and ventricular parts of this surface are the orifices of the pulmonary artery and the aorta, the former lying anterior to the latter.
The right margin of the heart is formed by the right atrium. It lies posterior to the cartilages of the third, fourth, fifth, and sixth ribs on the right side, about half an inch from the margin of the sternum; it is in relation with the right pleura and lung, the phrenic nerve with its accompanying vessels intervening, and it is marked by a shallow groove-the sulcus terminalis-which passes from the front of the superior vena cava to the front of the inferior vena cava. The inferior margin of the sterno-costal surface is sharp, thin, and usually concave corresponding with the curvature of the anterior part of the diaphragm; it is formed mainly by the right ventricle and only near the apex by the left ventricle It lies, almost horizontally, in the angle between the diaphragm and the anterior wall of the thorax, passing from the sixth right costal cartilage, posterior to the lower part of the body of the sternum, or the xiphoid process, and the cartilages of the sixth and seventh ribs on the left side, to the apex of the heart.
FIG. 752.-THE RELATION OF THE HEART TO THE
I, II, III, IV, V, VI, the upper six costal cartilages.
The left margin is formed mainly by the left ventricle, and only to a small extent by the left atrium; it is thick and rounded. It lies in relation with the left pleura and lung, the phrenic nerve and its accompanying vessels intervening, and it passes from just above the third left costal cartilage, about an inch. from the sternum, to the apex of the heart, descending obliquely and with a convexity to the left.
THE CHAMBERS OF THE HEART.
Atria.—The atrial or basal portion of the heart is cuboidal in form. axis, which lies transversely, is curved, with the concavity of the curve forwards. Its cavity is divided into two chambers-the right and left atria-by a septum which runs from the anterior wall backwards and to the right, so obliquely that the right atrium lies anterior and to the right, and the left atrium posterior and to the left.
Each atrium is also somewhat cuboidal in form, the long axes of both being vertical, and each possesses a well-marked ear-shaped, forward prolongation, known as the auricle, which projects from its anterior and upper angle.
Atrium Dextrum.-The right atrium receives, posteriorly, the superior vena cava above and the inferior vena cava below. Between them, and a little above its middle, it is crossed posteriorly by the lower right pulmonary vein. It is continuous below and anteriorly with the right ventricle, at the atrio-ventricular