68 



THE BIOLOGY OF AN ANIMAL. 



illustrated by the diagram (Fig. 30). Co-ordination be- 

 tween S and M (two organs) is not effected by a direct nervous 



connection, but indirectly 

 through a nerve-centre, C^ 

 which is a nerve- cell or group 

 of nerve-cells situated in one 

 of the ganglia, with which both 

 S and M are separately con- 

 nected by nerve-libres. If S 

 be thrown into action, an affer- 

 ent mipulse travels to C^ ex- 

 cites the nerve-centre, and 



Fig. 30.— Diagram of simple reflex action. caUSCS an efferent impulse tO 



S skin to which stimulus is applied ; a/, ^^.^^^^ ^^^ ^^ j^ ^j^-^j^ -g ^j^^^.^, 



the afferent nerve-fibre ; C, nerve-centre ; ' 



e/, efferent nerve-fibre; M, muscle in hy thrOWll intO actioil alsO, Or 



which the efferent fibre ends. . TXi i • xx i.' 



IS modmed m respect to actions 

 ^.Iready going on. Thus the actions of S and M are co-ordi- 

 nated through the agency of C\ the whole chain of events 

 •constituting a reflex action. 



For example, let S be the skin and If a certain group of 

 muscles. If the skin be irritated, afferent impulses travel in- 

 wards to nerve-centres in the ganglia (C), which thereupon send 

 forth efferent impulses to the appropriate muscles. Muscular 

 contractions result, and the worm draws back from the unwel- 

 come irritation. 



This chain of events involves three distinct actions on the 

 part of the nervous system which must be carefully distinguished, 

 viz. : (a) the afferent impulse ; (h) action of the centre ; (c) 

 the efferent impulse. It must not be supposed that the afferent 

 Impulse passes unchanged out of the centre as the efferent impulse, 

 i.e., is simply "reflected," like a ball thrown against a wall, as 

 the word ' ' reflex ' ' seems to imply. The afferent impulse as such 

 ends with the nerve-centre, wliich it throws into activity. The 

 efferent impulse is a new action set up by the agency of the 

 centre. 



There is reason to believe that many if not all nerve-centres 

 are connected with a number of different afferent and efferent 

 paths, and also with other centres, as sliown in the diagram 

 Fig, 31. Efferent impulses may therefore be sent out from 



