56 ANIMAL BIOLOGY. [Parti. 



reflex action. We have reason for supposing that a wave of 

 change started by the stimulus passes along a nerve-fibre to the 

 spinal cord ; that it there stimulates into activity certain groups 

 of nervous cells within the cord ; and that as a result of such 

 stimulation, a wave of change passes down other nerve-fibres to 

 certain muscles and causes them to contract in an orderly 

 fashion. The nerve-fibre that carries the wave of change in- 

 ward to the spinal cord is called an afferent fibre ; the group of 

 cells within the cord is called a ganglionic centre ; the fibres that 

 carry the waves of change outwards to the muscles are called 

 efferent fibres. The series afferent fibre, ganglion centre, efferent 

 fibre is called a nervous arc. (ttf any part of the nervous arc be 

 incomplete, reflex action does not take place. J This much we 

 may be said to know. How the incoming stimulus wave is con- 

 verted into the outgoing wave, initiating orderly activities, we 

 do not know. We are at present ignorant of the minute phy- 

 siology of the process. But we believe that there is such a 

 minute physiology. 



The experiments of competent and qualified investigators have 

 further taught us that if only the fore- and mid- brain be 

 destroyed, the hind-brain remaining intact, the frog does not die. 

 It breathes ; it may be artificially fed ; and it performs a number 

 of simple reflexes, often with great vigour. And if only the 

 fore-brain be destroyed, the hind- and mid- brain being left intact, 

 the frog will eat if you feed it ; it will jump on the stimulus of a 

 needle-prick ; it will swim if you put it in water ; if you place it 

 upon a board and then very gradually tilt the board, it will slowly 

 climb and balance itself on the edge ; if you put it on its back, it 

 will turn over again and sit up ; the male will croak and clasp your 

 finger if, in the breeding season, you stimulate the pad on the 

 manus. It is thus capable of a number of responsive actions 

 that is, actions in answer to definite stimuli. But all internal 

 spring of action is gone. It is utterly incapable of initiating any 

 action, even the simplest ; and if left to itself, it will remain 

 motionless in one position till it dies. 



The inference from all which is, that hind-brain, mid-brain, 

 and fore-brain have for their physiological function the co- 



