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, (7, 

 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-fibres. If S 

 be thrown into action, an affer- 

 ent impulse travels to C, ex- 

 cites the nerve-centre, and 



FIG. 30. Diagram of simple reflex action, causes ail efferent impulse to 



S, skin to which stimulus is applied; a/, travp l Ollf fo ir w } } \ ( ,}, fo tllM'P 

 the afferent nerve-fibre ; C, nerve-centre ; ) M ^ 



ff, efferent nerve-fibre; 31, muscle in \)j thl'OWn into action also, OF 



which the efferent fibre ends. . T r i 



is modmed in respect to actions 



already going on. Thus the actions of /S and M are co-ordi- 

 nated through the agency of 6 r ; the whole chain of events 

 constituting a reflex action. 



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

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

 wards to nerve-centres in the ganglia (6 y ), 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 ; (5) 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, which 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 shown in the diagram 

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



