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 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.