COMPARATIVE ANATOMY AND PHYSIOLOGY. 109 



different ganglia have little communication with each other. The highest of 

 them, situated just beneath the oesophagus, is connected with the cephalic 

 masses by two cords, between which that canal passes, encircled, as it were, 

 in a ring. 



143. The most detailed account of the conformation of the Nervous Centres 

 in the Articulata, is that recently given by Mr. Newport, in regard to the 

 Julus, and other animals of the class MYRIAPODA.* Their general arrange- 

 ment corresponds with that which has been just described in the larva of the 

 Sphinx Liguslri; but the number of ganglia is much greater. In each 

 lateral half of the cord, two distinct tracts or layers of fibres can be detected ; 

 of these, one known as the fibrous tract is continuous with the brain, and 

 contains no gray matter ; whilst the other known as the ganglionic tract 

 has gray matter deposited at intervals amongst its fibres, some of which are 

 continuous with the brain, whilst others do not reach it. (Fig. 13, A.) Every 

 nerve that is given off from this ventral column, is connected w r ith both tracts ; 

 and thus it has two sets of roots, one proceeding to the brain, the other e^ter- 

 ing the ganglion near which it arises. Of 



this last division, a part crosses to the oppo- Fig. 12. 



site 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 proportion of its length, and then emerges 

 again, forming part of another nervous trunk. 

 In Fig. 12 is seen Mr. N.'s representation of 

 one of the ventral ganglia, and part of the 

 cord, of Polydesmus maculalus ; showing the 

 longitudinal and commissural fibres, together 

 with those to which he has given the name of 

 fibres of reinforcement. These lateral fibres, *r 



which do not pass on to the brain, but issue Portion of the ganglionic tract of Pf> . 

 again from the ventral cord at a point a little lydesmus macuiatus; 6, imer-gangiionic 



distant from their entrance, Seem tO be more cord; c, anterior nerves; d, posterior 

 numerous in the hinder part Of the bdldy of nerves; /, k, fibres of reinforcement; 



the Centipede tribe, than in its front portion : f- /J ' cora ssu ; al fi , bres ; '' lo s hudi " al 



, , i . , 1 11- c i i fibres, softened and enlarged, as they 



and thus it is, that the whole size of the cord pass lhrough ganglionic matter . 

 remains nearly the same along its entire length; 



whilst that of the portion which passes backwards from the brain, must be con- 

 tinually diminishing, as it gives off fibres to the nerves. 



144. 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 move- 

 ments 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. If 

 an Earth\vorm be cut in two, whilst crawling, each portion will continue to 

 advance, though the anterior one only will permanently preserve its vitality. 



* Philosophical Transactions, 1843. 

 10 



