THE CIRCULATION OF THE BLOOD 



265 



the right side; the apex downward and to the left side, extending as far as the 

 space between the cartilages of the fifth and sixth ribs. In this situation 

 the heart is enclosed and suspended in an invaginated fibro-serous sac, the 

 pericardium, attached to the great vessels at 

 its base. 



The heart is a hollow, double muscle 

 organ, consisting of a right and a left half, 

 separated by a musculo-membranous septum. 

 The general cavity of each side is subdivided 

 by an incomplete transverse fibrous septum 

 into two smaller cavities, an upper and a 

 lower, known respectively as the auricular 

 and the ventricular cavities. The heart may 

 therefore be said to consist of four cavities, 

 the walls of which are composed of muscle- 

 tissue. Of these four cavities, the right auri- 

 cular and the right ventricular constitute the 

 venous heart; the left auricular and the left 

 ventricular, 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. Below the auricle com- 

 municates with the ventricle by a large open- 

 ing which, from its position, is termed the 

 auricula-ventricular opening. 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 pulmonic 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 columnce car- 

 nea (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 tendinece, which become at- 

 tached 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 



FIG. 109. 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.) 



