406 Walter J. Meek. 



acid and teased with needles gives such pictures as represented by 

 Fig. 2. The trabeculse are at once apparent. They are of all sizes 

 and run for the most part circularly around the lumen of the heart. 

 These strands branch freely in all directions and join neighboring 

 strands. Preparations teased in this way show even more plainly 

 than the cross section described above, that the heart is a great net- 

 work of muscle, whose strands form a spongy wall or a thick solid 

 one according as they are mingled together. In the heavier heart 

 walls the strands branch with rather acute angles, and in life the 

 meshes between are no doubt mostly potential spaces. In the parts 

 near the lumen the meshes are large. Often many trabeculse fuse 

 and form large sheets of muscle, which are really only large trabec- 

 ulse, since they again divide and the parts reunite with neighboring 

 strands. (See Fig. 3.) 



On teasing these trabecule with fine needles it is seen that they 

 tend to split into smaller strands along lines fairly well determined 

 at least for short distances. These smaller strands which may be 

 regarded as the real heart fibers branch and anastomose with each 

 other within the primary trabeculse. This arrangement is particu- 

 larly visible in tissue macerated in Zenker's solution, stained, and 

 examined in glycerine under high power. Fig. 4 shows this arrange- 

 ment semi-diagrammatically. 



So far as can be determined neither the trabeculae nor the secondary 

 anastomosing strands composing the larger trabeculse, end freely. 

 Marceau (1904) has described such a condition in the lower verte- 

 brates. He finds a large number of heart fibers ending freely among the 

 trabeculse in the Fishes, Batrachians, Saurians, and Ophidians. In 

 the Chelonians and Crocodilians the free endings are quite rare, 

 and in the higher vertebrates they do not occur at all. In teased 

 specimens of Limulus heart muscle the endings found are always 

 such as those shown in Fig. 2. The smaller branches here represent 

 the heart fibers and it is very apparent that the blunt ends are due 

 to transverse rupture by the needles. Conical or filiform termina- 

 tions, which would be the characteristic shape or natural endings, 

 have not been found. If they do occur they must be somewhat rare. 

 In this particular the heart of Limulus is like the higher vertebrates. 



