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,[pg 044] 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—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—Diagram of the circulation, showing in general the work done by each part of the heart. The right ventricle forces the blood through the lungs and into the left auricle. The left ventricle forces blood through all parts of the body and back to the auricle. The auricles force blood into the ventricles.
[pg 045]Connection with Arteries and Veins.—Though the heart is in communication with all parts of the circulatory system, it makes actual connection with only a few of the blood tubes. These enter the heart at its upper portion (Fig. 15), but connect with its different cavities as follows:
1. With the right auricle, the superior and the inferior venæ cavæ and the coronary veins. The superior vena cava receives blood from the head and the upper extremities; the inferior vena cava, from the trunk and the lower extremities; and the coronary veins, from the heart itself.