173 



ventricle are about three times as tliick as those of the ri,sjjht, and form 

 the apex. Tlie heart receives its nutrient blood from the coronary 

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

 auricle. 



401. 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 auriculo-ventricular septum ; whilst the 

 cavities are lined by a delicate membrane, called the endocardium. 



402. The Auricles, being in the upper portion of the heart, 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 

 tri-cuspid (Plate XXXI. ,B., 12) ; while that on the left is m two, and 

 is called the bi-cuspid, or mitral. On the inside of the auricles, 

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

 lunsculi pectinati (Plate XXXI., B. 2), while there is a depression on the 

 auricular septum, which is the remains of the foramen ovale, through 

 which the blood courses in fcetal circulation. 



403. Inside the Ventricles are fleshy columns — papillary muscles 

 (Plate XXXI., B. 14) — from which run white fibrous cords, called 

 chorda; tcndinecv (Plate XXXI., 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. 



404. Into the right auricle, we have opening the anterior and posterior 

 (Plate XXXI., B. 1 and 5) venae cavae, and the coronary sinus (into 

 which the coronary veins open), while the four pulmonary veins open 

 into the left auricle. 



405. The Pulmonary Artery (Plate XXXI., B. 9) arises from the 

 right ventricle, and carries venous blood to the lungs to be oxj'genated, 

 while the Pulmonary Veins (Plate XXXI. , A J.), which are generally 

 four in number, bring back the purified blood from the lungs to the 

 left auricle of the heart. 



