337. The VENTRICLES not only have their walls thicker than the auricles, but they differ in their internal structure. 157 From the interior of these cavities arise fleshy columns, called co-lum´næ car´ne-æ. The walls of the left ventricle are thicker and stronger than those of the right.
335. Of what is the heart composed? Give its divisions. 336. Describe the auricles. 337. Describe the ventricles.
338. The cavities in the right side of the heart are triangular in shape; those of the left, oval. Each cavity will contain about two ounces of blood. Between the auricle and ventricle in the right side of the heart, there are three folds, or doublings, of thin, triangular membrane, called the tri-cus´pid valves. Between the auricle and ventricle in the left side, there are two valves, called the mi´tral. There are seen passing from the floating edge of these valves to the columnæ carneæ, small white cords, called chor´dæ ten´di-næ, which prevent the floating edge of the valve from being carried into the auricle.
339. The right ventricle of the heart gives rise to the Pul´mo-na-ry artery; the left ventricle, to a large artery called the A-ort´a. At the commencement of each of these arteries there are three folds of membrane, and from their shape, they are called sem-i-lu´nar valves.
340. The heart is supplied with arteries and veins, which ramify between its muscular fibres, through which its nutrient blood passes. It has, likewise, a few lymphatics, and many small nervous filaments from the sympathetic system of nerves. This organ, in its natural state, exhibits but slight indications of sensibility, and although nearly destitute of the sensation of touch, it is yet, however, instantly affected by every painful bodily excitement, or strong mental emotions.
Observation. To obtain a clear idea of the heart and its valves, it is recommended to examine this part of an ox or 158 calf. In order that each ventricle be opened without mutilating the fleshy columns, tendinous cords, and valves, cut on each side of the septum parallel to it. This may be easily found between the ventricles, as they differ in thickness.
338. How do the cavities in the heart differ? What is found between the auricle and ventricle in the right side of the heart? How many valves in the left side, and their names? Where are the tendinous cords, and what is their use? 339. What vessels proceed from the ventricles? What is said of their valves? 340. With what is the heart supplied? What is said of its sensibility? How can an idea of the structure of the heart be obtained?
341. The ARTERIES are the cylindrical tubes that convey the blood from the heart to every part of the system. They are dense in structure, and preserve, for the most part, the cylindrical form, when emptied of their blood, which is their condition after death.
342. The arteries are composed of three coats. The external, or cellular coat, is firm and strong; the middle, or fibrous coat, is composed of yellowish fibres. This coat is elastic, fragile, and thicker than the external coat. Its elasticity enables the vessel to accommodate itself to the quantity of blood it may contain. The internal coat is a thin, serous membrane, which lines the interior of the artery, and gives it the smooth polish which that surface presents. It is continuous with the lining membrane of the heart.
343. Communications between arteries are free and numerous. They increase in frequency with diminution in the size of the branches, so that through the medium of the minute ramifications, the entire body may be considered as one circle of inosculation. The arteries, in their distribution through the body, are enclosed in a loose, cellular investment, called a sheath, which separates them from the surrounding tissues.