CHAP, v.] THE SPINAL CORD. 587 



trunk. If, in a brainless frog, the area of skin supplied by one of 

 the dorsal cutaneous nerves be separated by section from the rest 

 of the skin of the back, the nerve being left attached to the piece 

 of skin and carefully protected from injury, it will be found that 

 slight stimuli applied to the surface of the piece of skin easily 

 evoke reflex actions, whereas the trunk of the nerve may be stimu- 

 lated with even strong currents without producing anything more 

 than irregular movements. In ordinary mechanical and chemical 

 stimulation of the skin it is a series of impulses and not a single 

 impulse which passes upwards along the sensory nerve, the changes 

 in which may be compared to the changes in a motor nerve during 

 tetanus. In every reflex action, in fact, the central mechanism 

 may be looked upon as being thrown into activity through a 

 summation of the afferent impulses reaching it. Hence while a 

 reflex action is readily called forth by even feeble single induction- 

 shocks applied to the skin if they be repeated sufficiently rapidly, 

 a solitary induction-shock is ineffectual unless it be strong enough 

 to cause profound changes in the skin or nerves. 



When a muscle is thrown into contraction in a reflex action, 

 the note which it gives forth does not vary with the stimulus, but 

 is constant, being the same as that given forth by a muscle thrown 

 into contraction by the will. From which we infer that in a reflex 

 action the afferent impulses do not simply pass through the centre 

 in the same way that they pass along afferent nerves, but are 

 profoundly modified. And this explains why a reflex action takes 

 always a considerable time, and frequently a very long time, for its 

 development. When the toes of a brainless frog are dipped in 

 dilute sulphuric acid, several seconds may elapse before the feet 

 are withdrawn. Making every allowance for the time needed for 

 the acid to develope sensory impulses in the peripheral endings of 

 the afferent nerve, a very large fraction of the period must be 

 taken up by the molecular actions going on in the nerve-cells. In 

 other words, the interval between the advent at the central organ 

 of afferent, and the exit from it of efferent impulses, is a busy time 

 for the nerve-cells of that organ; during it many processes, of 

 which we have at present very little exact knowledge, are being 

 carried on. 



The character of the movement forming part of a reflex 

 action is also influenced by the intensity of the stimulus. A 

 slight stimulus, such as gentle contact of the skin with some 

 body, will produce one kind of movement ; and a strong stimulus, 

 such as a sharp prick applied to the same spot of skin, will 

 call forth quite a different movement. When a decapitated 

 snake or newt is suspended and the skin of the tail lightly 

 touched with the finger the tail bends towards the finger; when 

 the skin is pricked or burnt, the tail is turned away from the 

 offending object. And so in many other instances. Further we 

 have already pointed out (p. 110) that while the effects of a weak 



