ix CAEDIAC MUSCLE AND NERVES 311 



automatic! ty, it does represent the conducting element of 

 excitation. 



The most important and convincing arguments in favour of 

 the neurogenic theory of cardiac rhythm were, however, adduced 

 by Carlson (1904-5). 



He directed all his observations to the invertebrate heart, in 

 which, from the phylogenetic point of view, the automatic activity 

 of the muscle cells should reach their highest development. 



One of the strongest arguments adduced by Engelmann in 

 favour of the myogenic theory, was the non-existence of nerves 

 and ganglion cells in the adult heart of many of the Invertebrata 

 (molluscs, arthropods, tunicates, lower crustaceans). He reasoned 

 from the observations of A. Brandt, C. Eckhard, M. Foster and 

 Dew Smith, W. Biedermann and Ransom. Carlson, however, 

 noticed that these negative results do not hold for all invertebrates. 

 In a great many molluscs and arthropods the heart is visibly 

 invested with nerve-cells and fibres, while, according to Hunter, 



FIG. 137. Heart and cardiac nerves of Limulus polypJiemus. "(Carlson.) an, Anterior arteries ; 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. 



these elements also exist in certain tunicates, e.g. in Molgula. 

 Carlson worked on the heart of an invertebrate, Limulus 

 polyphemus (an arachnid, according to others a crustacean), the 

 American " horse-shoe crab." The heart of this animal (which in 

 the best-developed specimens may be as much as 10-15 cm. long) 

 is in the form of an elongated sac, divided into segments by 

 arterial rami which originate in a double lateral series (Fig. 137). 

 During systole this heart contracts simultaneously in its entire 

 length, or else the wave of contraction is peristaltically propagated 

 with such velocity that the eye is incapable of following its pro- 

 gression. The nerve plexus by which the heart is invested is 

 disposed above the ectocardium in three principal trunks, the 

 median of which may be regarded as an extended nervous ganglion, 

 mixed with nerve fibres, while the two lateral nerves, and the 

 branches by which these are connected with the median ganglion, 

 contain no nerve fibres. In this case, therefore, it is comparatively 

 easy to separate the nervous elements without injuring the 

 muscular walls of the heart, which is impossible in any other 

 animal. 



By a technique as simple as it is conclusive, Carlson has 



