NERVOUS SYSTEM OF ARTICULATA. 485 



tained by the careful dissections of Mr. Newport, that of the fibres con- 

 stituting the roots, by which the nerves are implanted in the ganglia, 

 some pass into the vesicular matter of the ganglion, and, after coming 

 into relation with its vesicular substance, pass out again on the same 

 side (Fig. 152, /, Jc) ; whilst a second set, after traversing the vesicular 



Portion of the ganglionic tract of Polydesmus moculatus :b, inter-ganglionic cord; c, anterior nerves; d, 

 posterior nerves ; /, k, fibres of reflex action ; g, h, commissural fibres ; i, longitudinal fibres, softened and en- 

 larged, as they pass through ganglionic matter. 



matter, passes out by the trunks proceeding from the opposite side of 

 the same ganglion ; and a third set runs along the portion of the cord 

 which connects the ganglia of different segments, and enters the rier- 

 vous trunks that issue from them, at a distance of one or more ganglia 

 above or below. Thus it appears, that an impression conveyed by an 

 afferent fibre to any ganglion, may excite a motion in the muscles of 

 the same side of its own segment ; or in those of the opposite side ; or 

 in those of segments at a greater or less distance, according to the 

 point at which the efferent fibres leave the cord. 



858. The general conformation of Articulated animals, and the 

 arrangement of the parts of their nervous system, render them pecu- 

 liarly favourable subjects for the study of the reflex actions ; some of 

 the principal phenomena of which will now be described. If the head 

 of a Centipede be cut off, whilst it is in motion, the body will continue 

 to move onwards by the action of its legs ; and the same will take place 

 in the separate parts, if the body be divided into several distinct por- 

 tions. After these actions have come to an end, they may be excited 

 again, by irritating any part of the nervous centres, or the cut extre- 

 mity of the nervous cord. The body is moved forwards by the regular 

 and successive action of the legs, as in the natural state ; but its move- 

 ments are always forwards, never backwards, and are only directed to 

 one side when the forward movement is checked by an interposed ob- 

 stacle. Hence, although they might seem to indicate consciousness 

 and a guiding will, they do not so in reality ; for they are carried on, 

 as it were, mechanically ; and show no direction of object, no avoidance 

 of danger. If the body be opposed in its progress by an obstacle of 

 not more than half of its own height, it mounts over it, and moves directly 

 onwards, as in its natural state ; but if the obstacle be equal to its own 

 height, its progress is arrested, and the cut extremity of the body re- 



