The two auricles arc placed at the ha. sis or broulest part of the heart ; and the two ventricles, composing the chief bulk of the organ, are found in front of the former cavities.
In the following description of the structure of the heart, we shall trace the parts in the same order in which the blood passes through them. This fluid, then, after circulating through the blood vessels of the body, after serving the various purposes of nutrition, secretion, &c. is retiumed into the right auricle of the heart by three large veins, 1.-/.7. the superior and inferior vena cat-a, and the g-reat coronary vein. The properties of this blood have been so altered in its course, that it is necessary for it to be subjected to the action of the atmosphere in the lungs, before it is again fit to be sent into the arteries of the body. The right auricle derives its name of auricle from a small fringed process, which is found at its anterior part ; the rest of the cavity is called the sinus of the vena cavx. The lining of this bag, as indeed that of all the other parts of the heart, consists of a smooth and polished surface. The muscular fibres of the auricle are not nu merous nor large ; they- are arranged in parallel fasciculi, which have been com pared to the teeth of a comb ; and hence the epithet of musculs pectinati has been given to thein.
The right auricle transmits thc blood into the right, anterior, or pulmonary ventricle, through a large circular orifice, called the annulus enosus, or the cular orifice of the ventricle. When this the blood would latter cavity contracts, bc driven back towards the auricle, were n.ot this prevented by a valve, called the tricuspidal or triglochine. This valve is formed by a production of the lining of the heart, divided into three point ed portions. These are tied by temlinous strings to certain projecting packets of the muscular fibres, called the fleshy co lumns of the ventricle. The structure of the ventricle is very different from that of the auricle. It is a strong mus
cular cavity, adapted to the office of for cibly projecting the blood through the arterial ramifications; whereas the auri cle is a mere reservoir, holding the blood until the ventrical has emptied itself by its contraction.
The pulmonary artem which arises from the upper and anterior part of this ventricle, -conveys the blood into the lungs. The opening of this artery-, which is called the arterial orifice of the ventri cle. is furnished with three valves, called sigmoid or semilunar, which preverft any retrograde motion of the blood from the artery towards the heArt.
The venous blood, by being exposed to the atmospheric air in the lungs, is alter ed in its properties, and becomes arterial blood, in which state it is returned to the left auricle of the heart by four pulmo nary veins, two of which belong to &tell lung. This left or posterior auricle con sists of a large cavity, called the sinus or the pulmonary veins ; and of a smaller process or auricula. It is situated quite at the upper and back part of the heart, and-transmits the blood through the au ricular orifice of the left ventricle into that cavity. This opening is perfectly si milar in JI essential circumstances to the* corresponding part on the right side of the heart. But its valve, being divided into two portions only, is called mitralis, from a comparison with a bishop's mitre.
The left ventricle is much thicker and stronger than the right. It feels exter nally almost like a solid mass of flesfs ; while the right is comparativery thin and tabby. The reason of this difference is obvious. The left ventricle has to drive the blood to the most remote parts of the body, whereas the right only sends it through the lungs. The aorta arisel from the left ventricle, and its mouth is • gatarded by three semiltinar valves. This is the trunk from which the arteries of the whole body arise.