THE ANALYSIS OF BEHAVIOUR 137 



dog and the effects of the shock of the operation have passed 

 away, suitable stimulation of the skin of the side just behind 

 the shoulder is followed by the response called the " scratch 

 reflex": the hind-leg of the same side carries out a series of 

 kicking movements somewhat similar to those performed when 

 the normal animal scratches himself as the result of irritation by 

 a flea. Even in man something of the same kind may occur. 

 Thus, in cases of hemiplegia, when the connection of the cortex 



_ Afferent 



, fibres s^Area of 



f ' \ 



stimulated 



nerves 



Seg mefl fs 



_of the cord 



~ Supplying 



I i mbs 



FIG. 42. THE " SCHEMATIC " SPINAL REFLEX, BEING THE NERVOUS 

 PATHS INCLUDED WHEN THE STIMULUS is THROWN INTO A DIFFERENT 

 SEGMENT FROM THAT INNERVATING THE MUSCLES THAT ACT. 



with the cord is rendered ineffective by a " stroke," reflexes in 

 the paralysed leg may be obtained, and these are doubtless due 

 to the centres in the spinal cord. 



Reflex actions, complex and purposeful in character, may thus 

 be carried out by the nerve centres in the spinal cord. 



In such cases the stimuli (irritation by chemical substance, an 

 electric current, mechanical pinching, pricking, etc.) applied to 

 the skin are simple physical ones, and the response is generally 

 such an action as would be carried out by the muscles concerned, 

 in the normal animal, for some useful purpose. It always has 

 a certain " inevitability " that is, it " comes off," as a rule, 

 when the stimulus is applied. There is determinism, or at least 

 a large measure of determinism. It can usually be predicted. 



