62 



TEE POPULAR SCIENCE MONTHLY 



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In.i 



ing ganglion-cells and connected with two parallel lateral nerve-strands 

 that lie near the sides of the heart. This whole nervous mechanism 



may be dissected off from the heart, leaving this 

 organ in other respects intact. 



If a vigorous Limulus is opened from the 

 dorsal side and the heart exposed, at will be seen 

 to contract at the rate of about twenty beats 

 per minute, and this is likely to continue under 

 the conditions of simple exposure for some twelve 

 to fifteen hours. If now the median nerve-cord 

 and the lateral strands are dissected away, the 

 heart comes to a standstill and never again shows 

 a natural beat, though a stimulus applied directly 

 to its substance will cause it to contract. If 

 instead of removing the nerves, the median and 

 lateral strands are cut through at any plane, 

 care being taken not to injure the underlying 

 heart-muscle, the two regions of the heart thus 

 established beat independently and coordination 

 of the heart as a whole is lost. If the nervous 

 connections are left intact but the muscular heart 

 is completely cut across in several places, the 

 whole organ continues to beat in complete co- 

 ordination. It is quite clear from these observa- 

 tions that the heart-beat of Limulus is absolutely 

 dependent upon an extra-cardiac nervous mech- 

 anism and that this beat is carried out in exact 

 accordance with the neurogenic theory. Since 

 the artificial stimulation of a cardiac nerve in 

 Limulus is followed by tetanus in the region of 

 the heart under the control of this nerve, the 

 conclusion is justified that the heart-muscle of 

 Limulus is comparable rather with the skeletal 

 muscles of this animal than with the so-called 

 organic muscles, for skeletal muscles show tetanus 

 when thus stimulated. 



As Carlson himself remarks, however, the 

 fact that the heart-beat of Limulus is neuro- 

 genic does not prove that the heart in other 

 animals necessarily functions in a like way. 

 In fact it is comparatively easy to point to 

 another example in which the evidence for the myogenic beat 

 is just as strong as that already presented for the neurogenic 

 beat. This example is the tunicate heart. The tunicate heart, as for 



Fig. 3. Dorsal View 

 of the Heakt op Lim- 

 ulus (after Carlson). 

 The anterior end is 

 uppermost ; In, lateral 

 nerve-strand ; mn, me- 

 dian nerve-cord. 



