CIRCULATORY AND LYMPHATIC SYSTEMS 273 



as those of the right. The heart receives its nutrient blood from 

 the coronary arteries, the blood being returned by the coronary veins 

 into the right auricle. 



403. Internally, the heart is divided into four cavities — two 

 auricles and two ventricles ; the auricles and ventricles being separated 

 from each other by a partition — the an riculo -ventricular septum — 

 whilst the cavities are lined by a delicate membrane, called the 

 endocardium. 



404. The Auricles are the upper cavities of the heart, and form 

 its base, and open into the ventricles by orifices guarded by valves — - 

 the valve on the right side being in three segments, and named the 

 tricuspid (Plate XXXIV., B, 12) ; while that on the left is in two, 

 and is called the bicuspid, or mitral. On the inside of the 

 auricle, chiefly in the appendix of each, are noticed fleshy elevations, 

 called musculi pectinati (Plate XXXIV., B, 2), while there is a depres- 

 sion on the auricular septum, which is the remains of the foramen ovale, 

 through which the blood courses in foetal circulation. 



i t>' 



405. The Ventricles are the lower chambers of the heart, the 

 left forming the apex. Irside the ventricles are seen the fleshy 

 columns — papillavy muscles (Plate XXXIV., B, 14) — from which run 

 white fibrous cords, called chorda tendinece (Plate XXXIV., B, 13), to 

 be attached to the auriculo-ventricular valves, their purpose being to 

 limit the range of movement of the valves, and prevent them from 

 being swept into the auricles during contraction of the ventricles. 



406. Into the right auricle we have opening the anterior and 

 posterior venae cavse (Plate XXXIV., B, 1 and 5) and the coronary 

 sinus (into which the coronary veins open), while the four pulmonary 

 veins open into the left auricle. 



407. The Pulmonary Artery (Plate XXXIV., B, 9) arises from 

 the right ventricle, and carries venous blood to the lungs to be 

 oxygenated, while the pulmonary veins (Plate XXXIV., A. /.), 

 which are generally four in number, bring back the purified blood 

 from the lungs to the left auricle of the heart. 



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