THE CENTRAL NERVOUS SYSTEM 799 



the optic nerve, heat that of the end-organs of the nerves by 

 which we perceive the sensation of warmth, mechanical pressure 

 that of the nerves by which we perceive the sensation of pressure. 

 Other kinds of stimuli are either entirely inactive or much less 

 effective in evoking the particular sensory response. There is 

 every reason to believe that the receptor in the reflex arc occupies 

 the same position in regard to adequate stimuli as it does when 

 it functions as a sense-organ. 



Sherrington has shown that the different kinds of nerve- 

 endings in one and the same area of the skin (in the dog) 

 must be assumed to possess totally different spinal connections, 

 since the movements elicited by stimuli suitable for one form of 

 nerve-ending are quite different from those elicited by stimuli 

 suitable for another. 



The ' extensor- thrust ' is a reflex obtained in the hind-leg of 

 the dog, and characterized by a brief, strong extension at the 

 hip, knee, and ankle. It is only elicited by a certain kind of 

 mechanical stimulation, best in the spinal dog i.e., in a dog 

 whose brain has been destroyed or severed from the cord by 

 pushing the tip of the finger between the plantar cushion and 

 the pads of the toes. It cannot be obtained by electrical stimu- 

 lation or by any kind of direct stimulation of afferent nerve 

 trunks. The same is true of the pinna-reflex in the cat i.e., the 

 backward crumpling of the ear elicited by squeezing or tickling 

 its tip. The scratch-reflex,* a scratching movement of the hind- 

 foot, is much more easily elicited in the spinal dog by mechanical 

 stimulation (rubbing, tickling, or tapping) applied to the skin 

 of the back behind the shoulder than by electrical stimulation, 

 which often fails to evoke it at all. The puzzling fact that, 

 according to surgical experience, many of the internal organs 

 e.g., the ureters and bile-ducts can be handled, cut, and sutured 

 without pain, while the passage of a renal calculus or a gall- 

 stone may cause excruciating agony, becomes explicable in view 

 of the apparently slight difference which sometimes distinguishes 

 an adequate from an inadequate stimulus. Thus Sherrington 

 has shown that very distinct reflex effects e.g., a rise of blood- 

 pressure can be obtained by sudden distension of the bile-duct 

 by the injection of salt solution into its lumen. Distension is 

 here the adequate form of mechanical stimulation, and it is the 

 form induced by the passage of a calculus, while nerve-cutting, 

 although a mechanical stimulus, is not an adequate one. 



Conduction in reflex arcs shows certain peculiarities when 

 compared with the conduction in nerve-trunks already studied 

 (p. 687) : (i) The direction of the reflex conduction cannot be 



* The scratch-reflex is very easily obtained in cats during resuscitation 

 after a period of cerebral anaemia. 



