344 PHYSIOLOGY 



Just as excitation causes fatigue and therefore furnishes a hindrance to 

 repetition of the same act, so the reverse process of inhibition, which is a 

 large component of every reaction, is followed by a condition of increased 

 excitability, or diminished resistance to the passage of impulses. In each 

 case there is a tendency for a ' swing- back ' to take place from inexcitability, 

 to over- excitability, from excitation to inexcitability. This ' successive 

 spinal induction,' as it has been termed, may be seen on inhibiting some 

 movement by the excitation of an independent reflex. Thus the scratch 

 reflex is excited, and then while the excitation is still continued the reaction 

 is inhibited by excitation of the extensor or stepping reflex. As soon as the 

 ' stepping ' reflex has passed off, the scratch reflex returns with an intensity 

 greater than before. This successive spinal induction explains the tendency 

 which exists in the spinal cord to an alternation of response ; every act tend- 

 ing to come to an end by fatigue and, as a result of negative spinal induction, 

 inducing the opposed or antagonistic act. Thus if a spinal dog be held up in 

 the vertical position, so that the hind limbs hang freely, these latter execute 

 a series of alternate movements of flexion and extension. The starting-point 

 of these is the stretching of the anterior thigh muscles. Once started 

 they continue of themselves, each act exciting the alternating antagonistic 

 act. 



A reflex act has often been distinguished from other reactions, described 

 as conscious or purposive, by its fatality i.e. by the invariability with 

 which it results on a given stimulus, whether the reaction be for the good of 

 the animal as a whole or not. Thus a decapitated eel will wind itself with 

 equal readiness around a stick or a hot poker. All reactions are, however, 

 purposive. The machinery for them has been evolved and the paths laid 

 down in the spinal cord under the action of natural selection, so that they 

 must act, at any rate in the average of cases, towards the well-being of the 

 animal as a whole. Since the nerve path involved in any reaction includes a 

 number of synapses, each of which may be influenced from other parts of the 

 body in a positive or negative direction, an absolute uniformity of response 

 cannot be predicated for any one reaction. There will be changes in the 

 facility with which it is evoked and changes in its extent, and these will 

 become the more operative the greater the complexity of the arc, and the 

 larger the number of other impulses to which it may be subject. The 

 fatality of response is therefore only shown at its best in the very simplest 

 of reflexes, or the most lowly organised nervous systems. 



The purposive character of the reflexes obtained from the spinal frog has some- 

 times led writers, especially in pre-Darwinian days, to endow the spinal cord with a 

 guiding intelligence. At the present time we recognise that every reaction of a living 

 being must be purposive, in the sense of being adapted to the preservation of the species, 

 if the latter is to survive in the struggle for existence. The question as to whether 

 we are justified in predicating the existence of even a germ of consciousness or volition 

 in the spinal animal must be decided in the negative. " Associative memory would 

 seem to be a postulate for the very existence of perception. Where even simplest 

 ideas are not, there cannot be consciousness. Animal movements that are appropriate 

 not only for an immediate but also for a remote end indicate associative memory. 



