1 66 



THE CIRCULATION OF THE BLOOD 



The walls of the heart are constructed almost entirely of layers of muscu- 

 lar fibers; but a ring of connective tissue, to which some of the muscular 

 fibers are attached, is inserted between each auricle and ventricle and forms 

 the boundary of the venous opening. Fibrous tissue also exists at the 

 origins of the pulmonary artery and aorta. The muscular fibers of each 

 auricle are in part continuous with those of the others and in part separate; 

 and the same holds true for the ventricles. The fibers of the auricles are, 



FIG. 137. FIG. 138. 



FIG. 137. Cardiac Muscle Cells, Showing their Form, Branches, Nuclei, and Striae. 

 From the heart of a young rabbit. Magnified 425 diameters, a, Line of junction between 

 the cells (intercellular cement); b, c, branches of the cells. (Schafer.) 



FIG. 138. Cardiac Muscle Cells of the Left Ventricle of a Child at Birth (full term), to 

 show the form of the cells, their structural details, their relations to one another, and their 

 general agreement with those of cold-blooded vertebrates. A, Large cell with two nuclei; 

 this cell has nearly the proportions of those of the adult; B, group of cells in their natural 

 relation. At the right of the middle cell are two spaces or fissures, n, Nucleus. The 

 transverse striations cross the nuclei in all the cells, and each nucleus possesses several 

 nucleoli. (Gage.) 



however, quite separate from those of the ventricles, the bond of connection 

 between them being the fibrous and the embryonic muscular tissue of the 

 auriculo-ventricular rings and the bundle of His in the septum. 



The development of the heart shows that it is derived from an embryonic 

 tube, which in its growth becomes twisted upon itself and divided into the 

 two main divisions that we know in the adult. Anatomical dissections have 

 shown that the muscles of the ventricles form spiral sheaths extending from 

 the base of the two ventricles in spiral bands toward the apex. These bands 

 of muscle are wound about the surface of the ventricles in the right-to-left 



