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TEXT-BOOK OF PHYSIOLOGY. 



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cavities, the right auricle and the right ventricle constitute the venous heart; 

 the left auricle and the left ventricle, the arterial heart. 



The right auricle is quadrangular in shape and presents on its posterior 



aspect two large openings, the terminations 

 of the two final trunks of the venous system, 

 the superior and inferior vena cava (Fig. 116). 

 Below the auricle communicates with the ven- 

 tricle by a large opening which, from its posi- 

 tion, is termed the auriculo-ventricular open- 

 ing. The walls of the auricle are extremely 

 thin, not measuring more than two millimeters 

 in thickness. 



The right ventricle, as shown on cross- 

 section, is crescentic in shape owing to the 

 projection of the ventricular septum. It pre- 

 sents at its upper left angle a cone-shaped 

 prolongation, the conus arteriosus. From this 

 prolongation, and continuous with it, arises 

 the pulmonary artery. The wall of the ven- 

 tricle measures in the middle about four milli- 

 meters in thickness. The inner surfaces of 

 the ventricle show: (i) a complicated system 

 of muscle ridges and bands, the columns car- 

 nece (fleshy columns), and (2) a set of muscle 

 projections, the musculi papillares (papillary 

 muscles), which arise by a broad base from 

 the walls of the ventricle and project upward 

 toward the auriculo-ventricular opening. 

 From the apex of each papillary muscle there 

 are given off fine tendinous cords, the chorda 

 tendinea, which become attached above to the 

 under surface of the auriculo-ventricular valve. 

 The left auricle, similar in general shape to 

 the right, presents posteriorly four openings, 

 the terminations of the four final trunks of the 

 venous system of the lungs, the pulmonary 

 veins. Below is found the corresponding 

 auriculo-ventricular opening. The wall of the 

 auricle measures about 3 mm. in thickness. 

 The left ventricle (Fig. 117) is conic in shape 

 from above downward and oval or circular in 

 shape on cross-section. At its upper inner 

 angle it presents a circular orifice, the mar- 

 gins of which give attachment to the walls of 

 the aorta, the main arterial trunk of the sys- 

 temic circulation. The inner surfaces of the 

 ventricle show a similar though better developed system of columnae car- 

 neae, musculi papillares, chordae tendineae, etc. The wall of the left ven- 

 tricle measures about 11.5 mm. in thickness in the middle. 



FIG. 115. DIAGRAM OF THE 

 CIRCULATION, i. Heart. 2. 

 Lungs. 3. Head and upper ex- 

 tremities. 4. Spleen. 5. Intestines. 

 6. Kidney. 7. Lower extremities. 

 8. Liver. (After Dalton.) 



