NERVOUS SYSTEM OF ARTICULATA. 



257 



Portion of the gang] ionic tract of Po- 

 lydesnuis maculatus ; b, inter-ganglionic 

 cord ; c, anterior nerves ; d, posterior 

 nerves; f, k, fibres of reinforcement; 

 g. h, commissural fibres ; i, longitudinal 

 fibres, softened and enlarged, as they 

 pass through ganglionic matter. 



and contains no vesicular matter ; whilst the other known as the ganglionic 

 tract has vesicular matter deposited at intervals amongst its fibres, some of 

 which are continuous with the brain, whilst others do not reach it. (Fig. 128, 

 A.) Every nerve that is given off from this ventral column, is connected with 

 both tracts ; and thus it has two sets of roots, one proceeding to the brain, the 

 other entering the ganglion near which it arises. Of this last division, a part 

 crosses to the opposite side, forming the 

 commissural fibres which unite together the 

 lateral halves of the cord ; whilst another 

 bundle of fibres runs along the side of the 

 ganglionic tract, for a greater or less propor- 

 tion of its length, and then emerges again 

 forming part of another nervous trunk. In 

 Fig. 127 is seen Mr. N.'s representation of 

 one of the ventral ganglia, and part of the 

 cord, of Polydesmus maculatus; showing 

 the longitudinal and commissural fibres, to- 

 gether with those to which he has given the 

 name of fibres of reinforcement. These 

 lateral fibres, which do not pass on to the 

 brain, but issue again from the ventral cord 

 at a point a little distant from their entrance, 

 seem to be more numerous in the hinder part 

 of the body of the Centipede tribe, than in 

 its front portion : and thus it is, that the whole 

 size of the cord remains nearly the same 

 along its entire length ; whilst that of the por- 

 tion which passes backwards from the brain, must be continually diminishing, 

 as it gives off fibres to the nerves. 



327. After what has been said of the offices which the ganglia perform in 

 the Mollusca, and of the relation which they bear to the cephalic mass, we 

 shall have little difficulty in understanding the character of the nervous appa- 

 ratus in the Articulata, if our minds be unoccupied by any preconceived 

 notion. When we examine into the actions of the ventral cord, we perceive 

 that those of all its ganglia are similar to each other ; being related only to 

 the movements of their respective segments, and of the members which belong 

 to them. In fact, these ganglia may be regarded as so many repetitions of 

 the pedal or locomotive ganglion of the Mollusca. It is easily proved, that 

 the movements of each pair of feet may be produced by that ganglion alone, 

 with which it is connected ; since a single segment, isolated from the rest, 

 will continue to perform these movements for some time, under favourable 

 circumstances. But it is evident that they must be placed, in the living ani- 

 mal, under some general control ; by which the consentaneousness of action, 

 that is essential to regular locomotion, may be produced. This is proved by 

 the experiments to be presently quoted. We can scarcely account for the 

 exercise of such a general control, otherwise than by attributing it to the 

 fibrous portion of the cord,* which directly connects each of the nervous 



* It is believed by Mr. Newport, that the fibrous portion of the ganglionic trad, which lies 

 nearest the surface of the body, may be the channel by which sensory impressions are con- 

 veyed to the brain; whilst the fibrous tract itself may convey downwards the motor impulses 

 which originate in the cephalic ganglia. The chief reason for this supposition, is the corre- 

 spondence in position, relatively to each other, and to the rest of the body, between the 

 fibrous and ganglionic columns in Articulata, and the portions of the Spinal Cord of Verte- 

 tebrata, from which the anterior or motor roots, and the posterior or sensory, respectively 

 arise. But the fibres which are peculiar to the ganglionic tract, obviously form a distinct 

 system. 



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