Component Parts.-As has already been stated, the heart is sub¬divided by a muscular septum into two lateral halves, which are named respectively right and left; and a transverse constriction subdivides each half of the organ into two cavities, the upper cavity on each side being called the auricle, the lower the ventricle. The course of the blood through the heart cavities and blood-vessels has already been described
The division of the heart into four cavities is indicated by grooves upon its surface, The groove separating the auricles from the ventricles is called the auriculo-ventricular groove. It is deficient, in front, where it is crossed by the root of the pulmonary artery. It contains the trunks of the nutrient vessels of the heart. The auricular portion occupies the base of the heart, and is su bdivided into two cavities by a median septum. The two ventricles are also separated into a right and left by two furrows, the interventricular grooves, which are situated one on the anterior, the other on the posterior, surface; these extend from the base of the ventricular portion to near the apex of the organ; the former being situated
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