264 



TEXT-BOOK OF PHYSIOLOGY. 



The heart is a double organ, consisting of a right and a left half, 



separated by a vertical septum. The 

 general cavity of each side is partially 

 subdivided by a transverse constriction 

 into two smaller cavities, an upper and 

 a lower, known respectively as the 

 auricle and the ventricle. 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 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 termi- 

 nations of the two final trunks of the 

 venous system, the superior and in- 

 ferior vena cav<z (Fig. 112). Below, 

 the auricle communicates with the ven- 

 tricle by a large opening which, from 

 its position, is termed the auriculo- 

 ventricular opening. The walls of the 

 auricle are extremely thin, not meas- 

 uring more than two millimeters in 

 thickness. 



The right ventricle, as shown on 

 cross-section, is crescentic in shape 

 owing to the projection of the ven- 

 tricular septum. It presents at its 

 upper left angle a cone-shaped pro- 

 longation, the conus arteriosus. From 

 this prolongation, and continuous with 

 it, arises the pulmonary artery. The 

 wall of the ventricle measures in the 

 middle about four millimeters in thick- 

 ness. The inner surfaces of the ven- 

 tricle show: (i) a complicated system 

 of muscle ridges and bands, the col- 

 umns cornea (fleshy columns), and 

 (2) a set of muscle projections, the 

 musculi papillar es (papillary muscles), 

 which arise by a broad base from the 

 walls of the ventricle and project upward toward the auriculo- 



FIG. in. DIAGRAM OF CIRCULA- 

 TION, i. Heart. 2. Lungs. 

 3. Head and upper extremi- 

 ties. 4. Spleen. 5. Intestine. 

 6. Kidney. 7. Lower extremi- 

 ties. 8. Liver. (Dalton.) 



