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 dorsal ly 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 venae cavae (Fig. 

 1 1 6, d and e) (praecava and postcava). It passes thence into 

 the right ventricle, and from the right ventricle to the lungs by 



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