136 THE MECHANISM OE LIEE 



towards the point from which most light comes. This response 

 is called a fhototaxis, and it may be defined as the directed move- 

 ments of an organism in response to a directed light stimulus. 

 It is invariable and determined, and a logical hypothesis — that 

 is, one which is consistent with what we know about light and 

 its general efiect on living tissues — can be made to account for it. 



The Spinal Reflex Action. — "Pith" a frog — that is, cut 

 through the spinal cord — immediately behind the brain, and then 

 destroy the latter by pushing a blunt wire into it. The body of 

 the animal is now solely under the control of the spinal cord and 

 its ganglia; to convince oneself that this is the case, the head 

 may be removed. If, now (or, rather, after the effects of the 

 " shock " have passed off), a drop of strong vinegar be placed 

 on the back of the animal, one of the hind-legs will be bent 

 forward and the acid will be wiped off, A reflex arc of some 

 complexity is here involved: afferent impulses pass into the 

 spinal cord from the receptors in the skin irritated by the acid, 

 and these impulses are received by the nerve cells in the segment 

 of the cord which innervates the part of the skin stimulated. 



From the segment stimulated tracts of fibres convey the 

 impulses received to the grey matter of the segments from which 

 the legs are supplied with motor nerves. The latter are then 

 stimulated, and the antagonistic muscles contract and relax, 

 carrying out the series of movements described (Fig. 42). 



Now here, again, there is determinism, or, at least, the results 

 of stimulating the skin can be predicted. But there is now a 

 difference between the response in this case and the " mechani- 

 cally " repeated one that occurs in the muscle-nerve preparation: 

 when the drop of acid is placed on the right flank of the headless 

 frog, the right leg is used to wipe it off. Now let this leg be cut 

 away or forcibly held, and the left one is used to make the same 

 kind of response. Something occurs, then, in this case which 

 we have not, so far, observed. Evidently there are two mechan- 

 isms, one for each side of the body, but in what we may call 

 ordinary circumstances one of them is passive. Let, however, 

 the " normal " mechanism be prevented from operating, and 

 then that one which was passive before now responds vicariously. 

 We have to deal here with a " regulation." 



Such a " spinal reflex " — that is, a co-ordinated action carried 

 out by the nervous ganglia of the cord — can be elicited from 

 higher animals than the frog. When the cord is severed in the 



