74} JELLY-FISH, STAR-FISII, AND SEA-URCHINS. 



But, lastly, the strongest evidence in favour of 

 this view as affoi-Jed by the following observations. 

 At the beginning of this treatise I stated that the 

 distinguishing function of nerve consists in its power 

 of conducting stimuli to a distance, irrespective of 

 the passage of a contraction- wave ; and I may here 

 add that when a stimulus so conducted reaches a 

 ganglion, or nerve-centre, it causes the ganglion to 

 discharge by so-called " reflex action." Now, this 

 distinguishing function of nerve can plainly be 

 proved to be present in the Medusae. For instance, 

 take such a section of Aurelia as this one (Fig. 12), 



that the propai^ation of the wave is at least in some measure 

 dependent on nervons conduction. This fact is, that after a 

 contraction-wave has been blocked by the severity of a spiral or 

 other form of section, it may again be made to force a passage 

 under the influence of vagus stimulatiou. 



Moreover, in a paper still more recently published by Drs. Brunton 

 and Cash on " Electrical Stimulation of the Fi'og's Heart" (Proc. 

 Roy. 8')C., vol. XXXV., No. 227, p. 455, et seq.) it is remarked, 

 " Another interesting consideration is, whether the stimulus which 

 each cavity of the heart transmits to the succeeding one consists 

 in the propagation of an actual muscular wave, or in the 

 propagation of an impulse along the nerves. The observations 

 of Gaskell have given very great importance to the muscular 

 wave occurring in each cavity of the heart of cold-blooded animals 

 as a stimulus to the contraction of the next succeeding cavity. 

 Our observations appear to us to show that, while this is an 

 important factor, it is not the only one in the tiansmission of 

 stimuli. . . . We consider that stimuli are ahso propagated from 

 one chamber of the heart to another through nervous channels : 

 thus we find that irritation of the venus sinus will sometimes 

 produce simultaneous contractions of the auricle and ventricle, 

 ixastead of the ventricular beat succeeding the auricular in the 

 ordinary way. This we think is hardly consistent with the hypo- 

 thesis, that a stimulus consists of the propagation of a muscular 

 wave only from the auricle to the ventricle." 



