486 OF THE NERVOUS StSTEM AND ITS ACTIONS. 



mains forced up against the opposing substance, the legs still continuing 

 to move. If, again, the nervous cord of a Centipede be divided in the 

 middle of the trunk, so that the hinder legs are cut off from connexion 

 with the. cephalic ganglia, they will continue to move, but not in' har- 

 mony with those of the fore part of the body ; being completely para- 

 lysed, as far as the animal's controlling power is concerned ; though 

 sdll capable of performing reflex movements, by the influence of their 

 own ganglia, which may thus continue to propel the body, in opposition 

 to the determinations of the animal itself. The case is still more re- 

 markable, when the nervous cord is not merely divided, but a portion 

 of it is entirely removed from the middle of the trunk ; for the anterior 

 legs still remain obedient 'to the animal's control; the legs of the seg- 

 ments from which the nervous cord has been removed, are altogether 

 motionless ; whilst those of the posterior segments continue to act, 

 through the reflex powers of their own ganglia, in a manner which 

 shows that the animal has no power of checking or directing them. 



859. The stimulus to the reflex movements of the legs, in the fore- 

 going cases, appears to be ( given by the contact of the extremities with 

 the solid surface on which they rest. In other cases, the appropriate 

 impression can only be made by the contact of liquid ; thus a Dytiscus 

 (a kind of water-beetle) having had its cephalic ganglia removed, re- 

 mained motionless, so long as it rested upon a dry surface ; but when 

 cast into water, it executed the usual swimming motions with great 

 energy and rapidity, striking all its comrades to one side by its vio- 

 lence, and persisting in these for more than half an hour. Other 

 movements, again, may be excited through the respiratory surface. 

 Thus, if the head of a Centipede be cut off, and, while it remains at 

 rest, some irritating vapour (such as that of ammonia or muriatic acid) 

 be caused to enter the air-tubes on one side of the trunk, the body will 

 be immediately bent in the opposite direction, so as to withdraw itself 

 as much as possible from the influence of the vapour ; if the same irri- 

 tation be then applied on the other side, the reverse movement will 

 take place ; and the body may be caused to bend in two or three diffe- 

 rent curves, by bringing the irritating vapour into the neighbourhood 

 of different parts of either side. This movement is evidently a reflex 

 one, and serves to withdraw the entrances of the air-tubes from the 

 source of irritation ; in the same manner as the acts of coughing and 

 sneezing in the higher animals cause the expulsion, from their air-pas- 

 sages, of solid, liquid, or gaseous irritating matters, which may have 

 found their way into them. 



860. From these and similar facts it appears, that the ordinary 

 movements of the legs and wings of Articulated animals are of a reflex 

 nature, and may be effected solely through the ganglia with which 

 these organs are severally connected ; whilst in the perfect being, they 

 are harmonized, controlled, and directed by the instinctive guidance, 

 which depends upon sensations acting through the cephalic ganglia and 

 the fibres proceeding from them. There is strong reason to believe, 

 that the operations to which these ganglia are subservient, are almost 

 entirely of a consensual nature ; being immediately prompted by sensa- 

 tions, chiefly those of sight, and seldom involving any processes of a 



