410 Walter J. Meek. 



aifected by the stain, and this is what one would expect if the struc- 

 ture were connective tissue. Being thin it would stain so faintly 

 that it would make but little impression. 



These results enable us to formulate our conception of the heart 

 musculature. The heart wall is made up of branching, anastomos- 

 ing trabeculse which are individualized by connective tissue sheaths. 

 Within these trabecule are the naked strands of muscle which also 

 branch and fuse and thus form a true heart syncytium. Possibly 

 the connective tissue sheath takes the place of a sarcolemma by 

 functioning as a dialyzing membrane for the exchange of the nutri- 

 ment. If it is related in any way to the development of the sarco- 

 lemma in higiier forms it must bo noted that the relation here is to 

 the tralieculse and not directly to the iibers within. 



In Limulus then we have the same general conditions that are 

 found in the lower vertebrate heart. There are no free endings of 

 muscle fibers, but these even disappear with the higher reptiles. 

 The connective tissue sheath seems more closely appressed than in 

 the case of most forms figured by Marceau. With these two differ- 

 ences, neither of which can be fundamental, the heart muscle can 

 scarcely be told from that of the lower vertebrates. To convince 

 oneself of this it is only necessary to compare Marccau's figures of 

 the lower vertebrates, particularly the fishes, frog, and turtle, with 

 those figured for Limulus. 



Physiologically the important fact brought out by a study of the 

 Limulus heart musculature is that it is a syncytium practically indis- 

 tinguishable from that of the vertebrate heart. Carlson (1904) has 

 shown conclusively that the normal myocardium of Limulus is 

 incapable of physiological conduction. When the nerve ganglion is 

 removed the muscle responds to a local stimulus by a local contrac- 

 tion and there is no conduction whatever of the contraction to a 

 more distant part of the organ. We have here then a heart, syncytic 

 in structure, with a protoplasmic continuity throughoiit, which nev- 

 ertheless does not conduct under normal conditions. This fact, that 

 muscular continuity may be associated with the absence of muscular 

 conduction, renders invalid the argument drawn from the syncytic 

 structure in favor of the myogenic theory of conduction in the 

 vertebrate heart. 



