*Valves of the Heart.*—Located at suitable places in the heart are four gate-like contrivances, called valves. The purpose of these is _to give the blood a definite direction_ in its movements. They consist of tough, inelastic sheets of connective tissue, and are so placed that pressure on one side causes them to come together and shut up the passageway, while pressure on the opposite side causes them to open. A valve is found at the opening of each auricle into the ventricle, and at the opening of each ventricle into the artery with which it is connected. The valve between the right auricle and the right ventricle is called the _tricuspid_ valve. It is suspended from a thin ring of connective tissue which surrounds the opening, and its free margins extend into the ventricle (Fig. 16). It consists of three parts, as its name implies, which are thrown together in closing the opening. Joined to the free edges of this valve are many small, tendinous cords which connect at their lower ends with muscular pillars in the walls of the ventricle. These are known as the _chordæ tendineæ_, or heart tendons. Their purpose is to serve as _valve stops_, to prevent the valve from being thrown, by the force of the blood stream, back into the auricle. The _mitral_, or bicuspid, valve is suspended around the opening between the left auricle and the left ventricle, with the free margins extending into the ventricle. It is exactly similar in structure and arrangement to the tricuspid valve, except that it is stronger and is composed of two parts instead of three. [Fig. 16] Fig. 16—*Right side of heart* dissected to show cavities and valves. _B._ Right semilunar valve. The tricuspid valve and the chordæ tendineæ shown in the ventricle. The _right semilunar_ valve is situated around the opening of the right ventricle into the pulmonary artery. It consists of three pocket-shaped strips of connective tissue which hang loosely from the walls when there is no pressure from above; but upon receiving pressure, the pockets fill and project into the opening, closing it completely (Fig. 16). The _left semilunar_ valve is around the opening of the left ventricle into the aorta, and is similar in all respects to the right semilunar valve. *Differences in the Parts of the Heart.*—Marked differences are found in the walls surrounding the different cavities of the heart. The walls of the ventricles are much thicker and stronger than those of the auricles, while the walls of the left ventricle are two or three times thicker than those of the right. A less marked but similar difference exists between the auricles and also between the valves on the two sides of the heart. These differences in structure are all accounted for by the work done by the different portions of the heart. The greater the work, the heavier the structures that perform the work. [Fig. 17]