PHYLOGENE TIC ORIGIN OF NER VO US S YSTEM 145 



to such small muscle fibres as those in the blood vessels ; but as 

 already mentioned, in the invertebrate central nervous system, one 

 large nerve cell supplies with motor nerves a great quantity of 

 muscles by means of continual division of its motor fibre before it 

 reaches the muscles, so that a large nerve cell may indicate the 

 innervation by it of a Marge number of muscles even though 

 they are small in size. 



Another striking peculiarity of the cells in the ganglia of the 

 leech has been observed. Retzius has examined the cells in 

 the ganglion of the leech by the methylene blue method, and 

 given illustrations of the course of the axons of the cells in the 

 ganglion. Each ganglion gives origin on each side to two 

 nerves, an anterior and a posterior nerve, and the nerve fibres 

 from many of the cells can be traced into one or other of these 

 two nerves. In the group of the leeches, Retzius finds that there 

 are nerve cells on each side, whose axis cylinder process divides 

 into two in the ganglion itself, and of the two fibres so formed, 

 the one passes into the anterior nerve and the other into the 

 posterior nerve. One of such cells is the colossal cell. One then 

 of these cells with splitting processes is certainly the same as a 

 cell containing adrenalin. 



The contractile vessels of the leech beat with great regularity ; 

 and my son has been unable to find any ganglion cells in their 

 walls, though he has traced nerve fibres to them from both the 

 anterior and posterior nerves. By the use of curare, which 

 paralyses both the longitudinal and circular muscles of the leech, 

 he has been able to see the effect upon the rhythm of the longi- 

 tudinal vessel of stimulation of the anterior nerve and posterior 

 nerve respectively, and has found undoubted acceleration upon 

 stimulation of the anterior nerve but never on stimulation of the 

 posterior nerve. The effect in the latter case was irregular and 

 generally doubtful in curarized leeches, but in non-curarized 

 leeches slowing was always obtained ; the slowing was in some 

 cases very well marked, but never reached to absolute stoppage 

 of the beats. Curare therefore affects the inhibitory mechanism, 

 just as it does in the vertebrate heart, masking its effect. Cer- 

 tainly the heart of the leech is affected by muscarine and atropine 

 in the same way as in the vertebrate : muscarine will cause absolute 

 stoppage which can be recovered from by a subsequent dose of 

 atropine. Finally adrenaline in a neutral solution (adrenaline 



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