THE PHYSIOLOGY OF THE HEARTBEAT 173 



nerve cord containing ganglion cells and giving off fibers which proceed 

 in part directly to the heart and in part to lateral cords (Fig. 41). Re- 

 moval of this median nerve cord is followed by total abolition of the 

 heartbeat; the heart becomes perfectly quiescent like an unstimulated 

 skeletal muscle. In appraising the evidence at its true value, it must be 

 noted that although by stimulation of the nerve fibers contraction of the 

 heart can be produced, the contraction is like that of a skeletal mus- 

 cle it is not rhythmic ; and moreover and this is most important if 

 the various physiologic properties of muscle as described below be stud- 

 ied (page 176), it will be found that in all of them the quiescent heart 

 muscle behaves, not like the heart muscle of other animals, but like that 

 of skeletal muscle. This evidence, therefore, while indisputably showing 

 that the heart of Limulus depends for its rhythmic .power upon neigh- 

 boring nerve structures, does not justify the assumption that this will 

 be the case in the heart of animals having different physiologic properties. 

 2. The disposition of the nervous structures in the heart, especially of 

 the frog and turtle, exactly corresponds to the degree of development of 



Fig. 41. Heart and cardiac nerves of Limulus polyphetmis. (Carlson.) aa, anterior ar- 

 teries; la, lateral arteries; In, lateral nerves, mnc, median ganglionic chain; os, ostii or afferent 

 stomata, each pair of which corresponds to one of the segments into which the Limulus heart 

 is divided. 



the rhythmic power of the different parts of the heart ; thus, the greatest 

 rhythmic power is manifested by the sinus and the least by the tip of the 

 ventricle at the bulbus arteriosus. In the former position the nerve 

 structures are very prominent; in the latter, no nerve cells and but few 

 nerve fibers can be detected. This proof is, however, easily assailed. 

 In the first place, it may merely be a coincidence that the disposition of 

 the nerve structures and the development of rhythmic power correspond. 

 The unequal rhythmic powers may depend primarily on a difference 

 in structure of the muscle fibers themselves, such differences having 

 been shown to exist between the muscle cells of the sinus and those 

 of, say, the ventricle. The former cells, for example, have much less' 

 developed crossed striation and their protoplasm is much more gran- 

 ular; in short, they are much more embryonic in type than the cells from 

 the tip of the ventricle. 



If a jury had to return a verdict from evidence of so conflicting a char, 

 acter, it would no doubt be equivalent to that of the Scottish court "not 



