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.
Fig. 3. Dorsal View of the Heart of Limulus (after Carlson). The anterior end is uppermost; In, lateral nerve-strand; mn, median nerve-cord.
If a vigorous Limulus is opened from the dorsal side and the heart exposed, it 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 coordination. It is quite clear from these observations that the heart-beat of Limulus is absolutely dependent upon an extra-cardiac nervous mechanism 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 neurogenic 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
