476 THE NER I'O US S 1 'S TEM. 



4. The rii^ht half of the brain is in relation with the left 

 half of the ventral chain, and vice versa. There is a complete 

 decussation of fibres between the supra-cesophageal centres 

 and the ganglia 1 of the ventral chain, with which the infra- 

 (usophageal ganglia must probably be included. 



This statement is supported by the experiments of Faivre on 

 Dytiscus ; and by those of Yung and others on the Crustacea. 



Yung says, however, ' Each half of the brain (supra-cesopha- 

 geal centre) acts upon the corresponding side of the body,' 

 although he also states that lesion of one of the lobes of the 

 brain produces circus movements (mouvements de mancgi .1 

 from the injured to the sound side of the body. 



As all observers have stated that such movements occur, it 

 appears to me that this entirely negatives Yung's contention. 

 Faivre accounts for the circus movements on the supposition 

 that paralysis of the appendages of the uninjured side occurs, 

 and this seems to be the case. 



Yung, on the other hand, evidently supposes that the lesion 

 acts as a stimulus. This is most improbable, as Faivre has 

 shown that even the slightest lesions give rise to rapid abroga- 

 tion of function in all aquatic insects, from the destructive 

 action of the water which acts upon the injured centre ; more- 

 over, the same movements occur when the entire hemisphere 

 is removed and in the same direction. This fact alone shows 

 that Yung has arrived at a false conclusion. 



Anatomical investigations lead to the conclusion that the 

 most extensive system of transverse commissural fibres exists 

 in the supra-cesophageal centres, and that by far the greater 

 portion of the conducting tracts decussate around the oeso- 

 phagus; indeed, there is no evidence of the direct relation of 

 the two halves of the supra-cesophageal portion of the brain 

 with the corresponding halves of the metameral ganglia. 



5. The classical view that the dorsal tracts of the ventral 

 cord are afferent and the ventral tracts are efferent is probably 

 correct. The experiments of Faivre and Longet are in favour 

 of this conclusion; but Vulpian, Lemoine [174] and Yung [179] 

 could find no difference between the stimulation of the dorsal 



