310 FUNCTIONS OF THE CORD. 



veyed by an afferent fibre to any ganglion may excite motion in the mus- 

 cles 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 ; and as the function of 

 these ganglia is altogether related to the locomotive actions of the seg- 

 ments, we may regard them as so many repetitions of the pedal ganglia 

 of the mollusca, their multiplication being in precise accordance with that 

 of the instruments which they supply. 



" The general conformation of articulated animals, and the arrangement 

 of the parts of their nervous systems, render them peculiarly favorable 

 subjects for the study of the reflex actions, some of the principal phe- 

 nomena of which will now be described. The mantis religiosa custom- 

 arily places itself in a curious position, especially when threatened or at- 

 tacked, resting on its two posterior pairs of legs, and elevating its thorax 

 with the anterior pair, which are armed with powerful claws ; now if the 

 anterior segment of the thorax, with its attached members, be removed, 

 the posterior part of the body will still remain balanced upon the four 

 legs which belong to it, resisting any attempts to overthrow it, recover- 

 ing its position when disturbed, and performing the same agitated move- 

 ments of the wings and elytra as when the unmutilated insect is irritated ; 

 on the other hand, the detached portion of the thorax, which contains a 

 ganglion, will, when separated from the head, set in motion its long arms, 

 and impress their hooks on the fingers which hold it. If the head of a 

 centipede be cut off while it is in motion, the body will continue to move 

 onward by the action of the legs, and the same will take place in the 

 separate parts if the body be divided into several distinct portions. 

 After these actions have come to an end, they may be excited again by 

 irritating any part of the nervous centres, or the cut extremity of the 

 nervous cord. The body is moved forward by the regular and successive 

 action of the legs, as in the natural state, but its movements are always 

 forward, never backward, and are only directed to one side when the for- 

 ward movement is checked by an interposed obstacle. Hence, though 

 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 op- 

 posed in its progress by an obstacle of not more than half of its own 

 height, it mounts over it, and moves directly onward as. in its natural 

 state ; but if the obstacle be equal to its own height, its progress is arrest- 

 ed, and the cut extremity of the body remains forced up against the op- 

 posing substance, the legs still continuing to move. If, again, the nerv- 

 ous cord of a centipede be divided in the middle of the trunk, so that the 

 hinder legs are cut off from connection with the cephalic ganglia, they 

 will continue to move, but not in harmony with those of the fore part of* 



