THE CIRCULATORY SYSTEM. 
I. THE HEART. COR. 
The heart lies in the mediastinum, enclosed in the pericar- 
dial sac, and projects rather more toward the left than toward 
the right. It is an ovoid or pear-shaped organ, with its long . 
axis directed approximately craniocaudad. Its caudal end or 
apex is, however, directed slightly ventrad and to the left, 
while the larger cranial end or base faces slightly dorsad as 
well as craniad. 
Laterally and dorsally the heart is largely covered by the 
lungs. The ventral side and a considerable portion of the 
lateral surface are, however, not thus covered, so that they lie 
against the thoracic wall. The heart extends from about the 
fourth or the fifth to the eighth rib, and its apex touches the 
diaphragm. 
The cavity of the heart is divided by a longitudinal dorso- 
ventral septum into lateral halves—a right and a left side. 
Each side is again divided by a transverse dorsoventral septum 
(auriculoventricular) into two chambers, one of which, the 
auricle, lies at the base; the other, the ventricle, lies at the 
apex of the heart. There are thus right and left auricles and 
right and left ventricles. An external groove, partly filled 
with fat, separates the auricular portion of the heart from the 
ventricular part; this groove is known as the sulcus coronarius. 
Each auricle or ventricle has a single set of blood-vessels 
either leaving it or entering it. Thus the blood enters the 
right auricle by the inferior and superior ven cave (Fig. 
116, @ and e) (precava and postcava). It passes thence into 
the right ventricle, and from the right ventricle to the lungs by 
274 
