334 EPITHELIAL AND ENDOTHELIAL TISSUE. 



ing and anastomosing striped fibers, with very small sarcous 

 elements (see page 272, Fig. 116). The fibers are arranged in 

 bundles and surrounded by a connective-tissue envelope, the 

 external perimy sium, which sends prolongations between each 

 single muscle-fiber and its neighbors ; this last formation consti- 

 tutes the internal perimysium. The points of attachment for the 

 muscular fibers of the heart are the fibrous rings at the auriculo- 

 ventricular openings and the tendons of the papillary muscles. 

 The perimysium is abundantly supplied with blood-vessels and 

 lymphatics. Hyrtl discovered that the heart of many low verte- 

 brates f. i., the frog is destitute of blood-vessels, and that sin- 

 uous prolongations from the cavities of the heart take their place. 

 The endocardium and the pericardium have a capillary system 

 with wide meshes. In reptiles and most of the fishes, accord- 

 ing to the same observer, the muscle of the heart is composed of 

 two distinctly separated layers a broad inner, which is without 

 blood-vessels, and a narrow outer, layer, which has a well-devel- 

 oped capillary system. 



The endocardium is composed of a layer of fibrous connective 

 tissue, varying in width in different portions and largely inter- 

 mixed with elastic substance. The inner surface of this structure 

 is covered by a delicate layer of large and flat endothelia, which 

 is attached to a homogeneous, so-called hyaline membrane. The 

 densej fibrous connective-tissue partitions termed the valves, 

 originating from the rings of the ostia, are also covered with 

 endothelia. According to Gussenbauer, there are found in the 

 peripheral portions of the valves, on the surfaces looking toward 

 the auricles, numerous circular and radiating muscle-bundles, 

 which come from the muscles of the septum of the auricles. The 

 muscle layer of the auricles, however, is much less developed 

 than that of the ventricles, while the elastic layer beneath the 

 endothelia is very marked. In many places, the structure of the 

 wall of the auricles is similar to that of the large arteries. Pur- 

 kin je's filaments in the endocardium of cattle and other animals 

 are considered as peculiar forms of development of striated mus- 

 cle-fibers. 



The pericardium is a connective-tissue formation, composed of 

 coarse interlacing bundles in the parietal, and of delicate inter- 

 lacing bundles in the visceral, portion. The free surface of both 

 portions is covered with flat endothelia. The plexuses of blood- 

 vessels and the lymphatics are broader in the pericardium and 

 endocardium than in the muscle of the heart. According to 



