HEART, Anatomy and Functions of the. The continuity of life depends upon nutrition supplied to the individual organs that together constitute the human body. The nutritive ma terial is the blood; its equable distribution is accomplished by a central pumping organ, known as the heart.
Situation and Anatomical Relation.— This organ is situated in the central and lower part of the thoracic or chest cavity and rests upon the upper convex surface of the diaphragm. It is pear-shaped, with its base directed upwards, backwards and toward the right side; while its apex points downward and forward to the left side and strikes the chest wall in the space be tween the fifth and sixth ribs, one to one and a half inches to the right of an imaginary line drawn vertically through the left nipple. The heart is enclosed in a sac called the pericardium. The anatomical relation between the pericar dium and the heart is such as to allow the lat ter, within its envelope, relatively free motion. This is necessary in order to enable the heart to perform the physiological function of con trolling the circulation. But to maintain this liberal mobility, and at the same time to keep the heart within its proper boundaries, the peri cardium is secured to its adjacent structures; namely, beneath, to the central tendon of the diaphragm; in front, to the breastbone (ster num) • laterally, to the sides of the coverings of the lungs facing the pericardium (medias tinal pleura) ; and behind, to the anterior sur face of the food-conducting-tube (esophagus) and the large wind pipes (trachea and large bronchi).
Dimensions and Structure.— The size of the heart gradually increases until middle life, from which time it remains practically un changed until old age; then it gradually di minishes. The average weight of the heart has been calculated to be one one-hundred-and-fif tieth (1/150) of the weight of the body in the male, and one one-hundred-and-sixtieth (1/160) in the female. The dimensions of the ,heart in adults are about five inches in length, three and a half inches in its greatest width and two and one-half inches in its extreme thickness. Con sidering the topography of its normal apex and its dimensions and position, it is evident that the major portion of the heart lies to the left of the median line. By virtue of the structure of its component parts the heart is able to di rect and control two currents of blood which differ from each other in their chemical com position, and hence in their functions. One of the currents leaves the heart by one channel and, after traversing a designated distance and performing its function, returns to the heart by another avenue, to leave it again through an other exit; and ultimately returns to the place where it first left the heart. Because of this uninterruped circuitous course, Harvey in 1628 named it Circulation. This circulation contin ues throughout the life of the individual.
The heart is a hollow, muscular pouch, whose interior is divided by a longitudinal par tition into two cavities, which have no direct communication with each other. From their anatomical position in the chest cavity, these two heart cavities are called, respectively, the right and the left heart. Each of these hearts is in turn subdivided transversely into two compartments; the upper is called auricle and the lower ventricle. The two chambers of each heart communicate freely with each other; that is to say, the right auricle with the right ventricle, and the left auricle with the left ven tricle. this is accomplished through openings between them called the auriculo-ventricular openings. These apertures of communication are guarded by valves which are so constructed that they automatically open and close within a given period of time, thereby allowing a stream of blood to pass freely from the auricle to the ventricle, but not in the opposite direction.
The capacity of each chamber of the heart in adults is about three to three and a half ounces. The wall of the heart is composed of three coats. The outer coat is called the epicardium and constitutes one of the component layers of the pericardial sac. The inner surface of the two pericardial layers are required to glide over each other during the movements of the heart. To prevent their undue friction, the sac is provided with a lubricating substance, called lymph or pericardial fluid. The middle coat is actually the muscle of the heart, and is called the myocardium. It is made up of bands and layers of muscular tissue, very in tricately arranged. The accounts of their in terlacement, given by different investigators, vary. It is certain, however, that three dis tinct units can be identified, namely, the fibres of the auricles, those of the ventricles and the auriculo-ventricular bundle of His, so named after its discoverer. The work performed by muscles in general depends upon two properties that they possess, extensibilty and perfect elastic ity. The rhythmic exercise of these two properties by the myocardium creates sufficient kinetic energy for the propelling of the blood currents. The inner coat, called the endocardium, lines the whole interior of the heart. By its redupli cation, the endocardium forms part of the valves at the auriculo-ventricular and arterial orifices, namely, between the auricles and their respective ventricles and the ventricles and the vessels that emerge from them. Each orifice is encompassed by a strong fibrous ring which serves for the attachment of the, corre sponding auriculo-ventricular muscle and its valve. 'The rings at the arterial orifices bind the lower margin of the ventricular muscle and its valve. The boundary between the auricles and ventricles is marked by a perceptible groove in which lie the nutrient vessels (coronary ar teries) of the heart substance.