The Auricles are the receivers, they are the upper cavities, have smooth internal walls, and have thinner muscular walls than do the ventricles. The large posterior part of each auricle is called the atrium, receiving chamber, while projecting forward from the anterior and upper part of each atrium there is an auricular appendix, a small, conical, muscular pouch, the margins presenting a dentated edge;
these right and left auricular appendices are ear-shaped; the internal surfaces of their walls are marked by a number of muscular elevations, arranged as closely set vertical parallel bands, which look like the teeth of a comb, from which resemblance they receive the name mus-culi pectinati.
The Ventricles are the propellers, that is, they send the blood from the heart; they are the lower cavities, have thicker walls than the auricles, are triangular in shape, and have the chordae tendineae, and the columnae carneae.
The right auricle is the larger of the two auricles, its walls are thinner and the right auricular appendix is shorter, broader, and less curved, than the left appendix, but is more deeply notched on its margin. Into the right auricle opens the pre-cava, or superior vena
cava; it opens at the upper and back part of the auricle and has no valve. It returns the blood from the upper half of the body. The post-cava, or inferior vena post-cava, is larger than the pre-cava and opens into the lowest part of the auricle, the opening being guarded by the
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Eustachian valve. This vein returns the blood from the lower half of the body.
The coronary sinus opens into the auricle between the inferior vena cava and the auriculo-ventricular orifice, and is protected by an incomplete valve, the coronary or the valve of Thebesius; the coronary vein returns the blood from the substance of the heart. The opening between the right auricle and the right ventricle is the auriculo-ventricular or the tricuspid orifice, an oval aperture, situated in the lower and the anterior part of the atrium, in front of the orifice of the inferior vena cava with the intervention of the opening of the coronary sinus; three fingers, the index, middle and the ring fingers, held close together and side by side, could be passed thru the tricuspid orifice without difficulty. Thru this opening the blood passes from the right auricle into the right ventricle; the opening is guarded by a valve—tricuspid—which is composed of three cusps or segments covered with endocardium, they project into the cavity of the ventricle and are triangular in form. The auricular cusps are smooth but their ventricular surfaces are roughened by the chordae tendineae, which are attached to the margins.
These are numerous foramina, the foramina of Thebesius, which are found in the walls of both the right and the left auricles, some of them are merely depressions, while others are the orifices of minute veins which return the blood directly from the heart muscle substance.
At the lower part of the right auricle, in the auricular septum is an oval depression called the fossa ovalis, which corresponds to the situation of the foramen ovale in the fetus, an opening by which the auricles communicated before birth, and which closes almost immediately after birth.
Into the left auricle, open the three, or four, pulmonary veins which bring the arterial blood from the lungs; they open into the upper part of the posterior surface of the left auricle, two on either side of its middle line, and they have no valves.
The opening between the left auricle and the left ventricle is the mitral, or bicuspid, orifice, situated in the anterior part of the floor of the atrium, below and to the left of the aortic orifice. Thru the mitral valve the blood passes from the left auricle into the left ventricle. It is oval in form and could admit two fingers held side by side and in close contact. It is smaller than the tricuspid orifice and is guarded by the mitral or bicuspid valve, which consists of two cusps, in the same way that the tricuspid valve guards the opposite side, it also is formed by the infoldings of the lining membrane, and to the cusps are attached the chordae tendineae.
The ventricles differ, in that the right ventricle is triangular in shape, forms the greater part of the anterior external surface, while the left ventricle is conical in form, is longer, forms most of the posterior surface, and its walls are three times as thick as are those of the right ventricle; this can be easily accounted for, by the fact that the left ventricle is concerned with the systemic circulation, while the right ventricle has to do with the pulmonary circulation and the latter involves a much shorter circuit, therefore less powerful muscular contractions are sufficient to propel the blood thru it.
The openings in the right ventricle are the auriculo-ventricular, between the right ventricle and the right auricle; and the opening for the pulmonary artery, which is circular in form, is situated at the base, the upper part, of the cavity, and is guarded by the right semilunar or
pulmonary valve. The pulmonary artery, altho it conveys venous blood from the heart to the lungs for oxygenation, is called an artery because it carries blood away from the heart. The openings in the left ventricle are two in number; one opens into the aorta, and the other, the auriculo-ventricular orifice, lies between the left auricle and the
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left ventricle. The aorta is the large artery that, thru its branches, carries the blood from the heart to all parts of the body and it is guarded by a valve, the aortic or left semilunar valve.
The heart is composed of muscular tissue, which is known as the myocardium and it is enclosed in a conical sero-membranous sac, the pericardium.
The Pericardium consists of two layers, the parietal and the vis-ceral, between which is a secretion of a thin fluid, serous in character, in sufficient amount to moisten the surface and thus lessen friction during the heart movements. The outer or parietal layer of the pericardium is a fibrous layer, and is strong and dense; inferiorly, it is attached to the central tendon of the diaphragm, superiorly it ensheaths the great vessels connected with the base of the heart, with the single exception of the inferior vena cava. This outer layer is also attached to the sternum, by two fibrous bands, known as the superior and inferior sterno-pericardial ligaments; the superior fibrous band is attached to the manubrium, and the inferior one to the ensiform cartilage. On each side it is intimately related to the pleura, and in front and on the sides to the costal fascia. Above, the fibrous layer not only blends with the external coats of the great vessels, but it is continuous with the pretracheal layer of the deep cervical fascia.
The inner layer or the visceral, also called the epicardium, is serous in structure; it lies within the walls of the pericardium and serves to line them. It is a typical serous membrane, containing endothelial cells, it thus gives to the surface of this layer a smooth polished appearance. In the course of pericarditis it is this inner layer of the pericardium which becomes dry and roughened.
The Endocardium, the lining of the heart, is a smooth delicate membrane, covered on its internal surface by endothelium. It consists of a connective-tissue basement membrane containing elastic fibres, and is continuous, thru the arterial end venous orifices, with the tunica intima of the vessels; it also enters into formation of the segments of the auriculo-ventricular and semilunar valves. The walls of the interior of the ventricles are elevated into muscular bands called the columnae carneae; these project into the cavities, and, by their reticular arrangement, render the interior walls very irregular.
According to the manner of their attachment, the columnae carnea are classified in three sets, as simple elevations, which are attached to the wall thruout their entire length, as well as at their extremities, some are attached to the wall only at their extremities, being free elsewhere; those of the third set are attached at one extremity only.
Those of this last set are called the musculi papillares. They are conical in shape, their bases are attached to the wall of the ventricle and their free extremities are connected with a number of filiform processes, called the chordae tendineae. These are delicate tendinous cords, which are connected at one extremity with the wall of the ventricle by the musculi papillares or the papillary muscle, and, at the other extremity, with the cusps of the auriculo-ventricular valve. When the ventricle contracts, the musculi papillares also contract, and, by tightening the chordae tendineae, they prevent the segments
of the auriculo-ventricular valve from being driven too far upward into the auricle.
The valves in the heart are the tricuspid and bicuspid, previously
described as being between the auricles and ventricles, the pulmonary, aortic, valve of Thebesius or the coronary valve, and the Eustachian valve. The pulmonary valve guards the opening into the pulmonary
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artery, the large vessel which carries the blood from the right ventricle to the lungs. It consists of three semilunar segments, two anterior and one posterior, which are formed by reduplications of the lining membrane of the ventricle, strengthened by interposed fibrous tissue.
The wall of the artery, opposite each segment of the valve, presents a recess, or dilation, and these recesses are called the sinuses of Valsalva.
The aortic valve guards the orifice of the large artery, which leaves the left ventricle, and consists of three semilunar segments, similar to those of the pulmonary valve in their mode of attachment, two are posterior and one anterior, but the cusps are larger and stronger than in the pulmonary valve. The interior of the wall of the aorta, also, presents three well-marked dilations or recesses; each recess is placed opposite to a segment of the valve, and they form the sinuses of Valsalva, which are larger than those at the origin of the pulmonary artery. The aortic valve serves to prevent regurgitation of the blood from the aorta into the left ventricle during the elastic recoil of the arterial wall.
The Coronary Valve, or Valve of Thebesius, is a semilunar fold of the lining membrane of the right auricle, guarding the orifice of the coronary sinus. It prevents a regurgitation of blood into the sinus during the contraction of the auricle.
The Eustachian Valve is found between the anterior margin of the inferior vena cava and the auriculo-ventricular opening. It is larger in the fetus, at which time it serves to direct the blood of the inferior vena cava thru the foramen ovale. In the adult, this valve is rather rudimentary and is a crescentic fold of endocardium, which varies in size and sometimes presents several small openings.
The heart substance is supplied by the right and left coronary arteries, branches from the ascending aorta, the veins accompany the arteries and terminate in the right auricle. The pericardium receives the blood supply from the pericardial branches from the thoracic aorta.
The nerve supply to the heart is by branches from the cardiac plexus, being derived principally from the second dorsal pair of spinal nerves, emitting thru the 9th intervertebral foramina.