The Zonce Tendinosce of the heart are four in number; one is situated at the narrow portion of the infundibulum of the right ventricle, and gives attachment to the origin of the pulmonary artery: the second is placed at that part of the left ventricle from which the aorta takes its origin; these may be called the two arterial zones. The remaining two may be termed the auriculo-ventricular zones ; they mark the connection between the auricles and ventricles, surround the auriculo-ventricular orifices, and give attachment to the bases of the tricuspid and mitral valves: they are composed of pale, condensed, tendinous fibres; they have the same form as the auriculo-ventricular openings, which they surround; and they receive and are continuous with those expansions of the chordae tendineae, which are placed between the laminae of the endocardium composing the mitral and tricuspid valves, and which thus add considerably to their strength. These zones may be best seen by dissecting from the interior of the heart. The endocardium, or lining membrane, is in intimate connection with the inner surface of these zones, and is thicker here than in other situations.

According to Bouillaud, the cavity of each ventricle is composed of two very distinct regions, one communicating with the corresponding auricle, and the other with the artery arising from its base; and these two portions are not constituted exactly alike in the right and left sides. In the right ventricle, the arterial portion is united with the auricular portion, by means of an angle projecting into the ventricle, the sinus of which is consequently turned upwards, embracing the aorta. In the left ventricle, the arterial and auricular regions are very nearly parallel to each other, so that their axes approach one another as they proceed from the base to the apex of this cavity: they are separated by the anterior lamina of the mitral valve, and by two large fleshy columns, which are inserted into it by means of numerous tendons. Inferior, posterior, and a little to the left of this septum, is the auricular region of the ventricle ; and superior, anterior, and internal to it, is the arterial or aortic portion. These two regions communicate with each other freely at the interval between the two large columns above mentioned. It is in the auricular region of the ventricle that we principally find the fleshy columns; in fact, a large portion of the arterial region is altogether destitute of them; and the same remark will apply to the right ventricle : those that arc found in the arterial region are small and interlaced, and are not, like the large ones, inserted into the valves. The left ventricle contains fewer carneae columnae than the right; they are, however, more voluminous.