1 10 FUNCTIONS OF THE NERVOUS SYSTEM 



If a Centipede be divided into several portions under the same circumstances, 

 each will execute motions of progression for some time. But it is evident that 

 these must be placed, in the living animal, under some general control ; by 

 which the consentaneousness of action, that is essential to regular locomotion, 

 may be produced. This is easily proved by experiment. If in a Mantis, for 

 example, the nervous cord be divided between the first and second thoracic 

 ganglia, so as to isolate the ganglionic centres of the posterior legs, the limbs 

 will continue to move energetically, but not with a combined object, and no 

 progression will take place. We can scarcely account for the exercise of this 

 general control, otherwise than by attributing it to the fibrous portion of the 

 cord,* which connects each of the nervous trunks immediately with the cepha- 

 lic ganglia, as in the Mollusca ; and this must, therefore, conduct to the sen- 

 sorium (whose seat is probably in the latter) the impressions which there pro- 

 duce sensations, and must convey downwards the locomotive impulse ; whilst 

 the ganglion of each segment, with the filaments connected with its nucleus, 

 will form the circle necessary for the simply reflex actions of its members. 

 The independence of the segments of the Articulata, as far as their reflex 

 actions are concerned, and their common subordination to one presiding centre 

 of the will, are fully explained on this supposition. It is also quite conform- 

 able to the analogy both of the Mollusca and of Vertebrata. 



145. The number and variety of the reflex actions which take place in the 

 Articulata after decapitation, is very remarkable ; and they seem to have a 

 consentaneousness, proportioned to the closeness of the relation between the 

 nervous centres in the respective species. Thus, in the Centipede, we find 

 the ganglia of the several segments distinct, but connected by a commissural 

 trunk. Here an impression made equally upon the afferent nerves of aU the 

 ganglia, will produce a consentaneous action. Thus, if the respiratory orifices 

 on one side of a decapitated Centipede be exposed to an irritating vapour, the 

 body will be immediately flexed in the opposite direction ; and if the stigmata 

 of the other side be then similarly irritated, a contrary movement will occur. 

 But different actions may be excited in different parts of the cord, by the pro- 

 per disposition of the irritating cause. In the higher classes, however, where 

 the ganglia of the locomotive organs are much concentrated, the same irritation 

 will produce consentaneous motions mseveral members, similar to those which 

 the unmutilated animal performs. In the Mantis religiosa, for example, 

 which ordinarily places itself in a very curious position, especially when 

 threatened or attacked, resting upon its two posterior pair of legs, and elevat- 

 ing its thorax and the anterior pair, which are armed with powerful claws,- 

 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, recovering its posi- 

 tion when disturbed, and performing the same agitated movements of the wings 

 and elytra, as when the unmutilated animal is irritated : on the other hand, the 

 detached portion of the thorax, which contains a 'ganglion, will, when sepa- 

 rated from the head, set in motion its long arms, and impress their hooks on 

 the fingers which hold it. These facts prove unequivocally, that the automa- 



* 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 conveyed 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 sup- 

 position, is the correspondence 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 Vertebrata, 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. 



