78 MANUAL OF ELEMENTARY ZOOLOGY 



from the gall-bladder without either the will or the know- 

 ledge of the animal. For a reflex action three things are 

 necessary : (i) an afferent nerve, (2) a portion of the central 

 nervous system, known as the centre of the reflex, (3) an 

 efferent nerve. This apparatus is known as the reflex arc. 

 For some reflex actions the centre is in the brain, but for 

 many it is only necessary that a part of the spinal cord 

 should be intact. Thus a frog from which the brain has 

 been removed will, if its spinal cord be uninjured, lift its 

 leg to wipe off an irritant, such as a drop of acid, from its 

 flank. A voluntary action is one in which the will intervenes, 

 and a choice is made, as when the animal decides between 

 two directions in which it can escape an enemy, or wanders 

 to seek food when it is hungry. Voluntary actions may or 

 may not follow immediately upon an external stimulus, but 

 when they do so the same stimulus is not always followed 

 by the same response. For nearly all, if not for all, volun- 

 tary actions it is necessary that some part of the cerebral 

 hemispheres should be uninjured. 



In the last paragraph we have had to mention conscious- 

 ness as accompanying certain events in the 



Consciousness. a i_ ■ 



nervous system. A conscious being is one 

 that is aware of events. The events which its conscious- 

 ness immediately accompanies take place in its own nervous 

 tissue. Awareness of other events, within or without the 

 body, is of course due to nervous events which they cause. 1 

 A vast number of events in the human body are signalled 

 in consciousness, but no event is so signalled unless it 

 affect the nervous system. On the other hand, innumer- 

 able events such as the opening of the bile-duct, which 

 we mentioned above, take place in the viscera and else- 

 where quite unconsciously, although they affect the nervous 

 system. It is, of course, impossible for us to be certain 

 that a frog possesses consciousness, but just as each of 



1 This is not to say that the conscious being knows that the event 

 of which it is aware are in the first place only events within itself. 

 Ordinarily, the processes in sense organs and nervous system are not 

 regarded, and consciousness is accepted as first-hand evidence of 

 external things. Still less is it realised that consciousness does not 

 necessarily present the likeness of things outside it, not even of the bodies 

 with whose working it is linked, but rather signs which stand for such 

 things, and that after this fashion alone does it know material things. 



