THE CIRCULATION OF BODY FLUIDS 109 



portions, alternate segments being then stripped off, the heart- 

 beat continues only in those portions to which the remaining 

 ganglionic tissues adhere, the rhythm of the pulsating regions 

 being now unco-ordinated. 



The last-mentioned experiment leads to a consideration of 

 another peculiarity of the cardiac rhythm in Limulus. In 

 vertebrates, not only is it true that the beat arises spontaneously 

 in the muscle-cells of the sinus venosus, or cardinal sinuses 

 (Fishes) ; but the conduction of the excitation from one part of 

 the heart to another takes place through the muscular tissue. 

 The muscular continuity of the auricles and ventricles in the 

 higher vertebrates is effected through a bundle of modified 

 muscle fibres, the bundle of His. Carlson has shown that 

 section of the heart of Limulus without damage to the median 

 ganglion does not interrupt the synchronism of the two halves ; 

 but section of the median ganglion alone abolishes the co- 

 ordination of rhythm, each half now beating with a rhythm of 

 its own. It appears fairly certain, therefore, that the origin 

 and co-ordination of the heart beat in Limulus depends upon 

 the rhythmical discharge of nervous stimuli from the cardiac 

 ganglion ; there is no precise parallel to such a mechanism 

 in vertebrates outside the central nervous system itself. It 

 is to be noted that stimulation of the heart-muscle or median 

 ganglion itself with an interrupted current produces a tetanus ; 

 the period during which the heart remains refractory to a 

 second stimulus is not protracted to the extent so charac- 

 teristic of cardiac muscle in the vertebrate. 



Some investigators have sought to apply the same interpre- 

 tation of the origin and conduction of the heart-beat to decapod 

 Crustacea. It is to be noted, however, that the median gang- 

 lion of Limulus is an organ sui generis. There may, of course, 

 be ganglion cells in the heart-muscle of Crustacea ; but there 

 is no structure comparable to the cardiac ganglion of Limulus 

 in their gross anatomy, Limulus is not very closely related 

 to Crustacea in the phyletic scale. Moreover, the embryonic 

 heart of Limulus, which is at first composed of smooth muscle, 

 beats before any nerve-fibres reach it : ontogenetically it is 

 a myogenic heart. And it is quite likely that one would find 



