484 ELEMENTARY PHYSIOLOGY less. 



cord is a great mixed motor and sensory nerve. But 

 it is also much more. 



Reflex ActioQ through the Spiaal Cord. — If the trunk 

 of a spinal nerve be cut through, so as to sever its 

 connection with the cord, an irritation of the skin to 

 which the sensory fibres of that nerve are distributed 

 produces neither motor nor sensory effect. But if the cord 

 be cut through anywhere so as to sever its connection with 

 the brain, irritation applied to the skin of the parts sup- 

 plied with sensory nerves from the part of the cord below 

 the section, though it gives rise to no sensation, may pro- 

 duce violent motion of the parts supplied with motor nerves 

 from the same part of the cord. 



Thus, in the case supposed above, of a man whose legs 

 are paralj'sed and insensible from spinal injui;y, tickling 

 the soles of the feet will cause the legs to kick out convul- 

 sively. And as a broad fact, it may be said that, so long 

 as both roots of the spinal nerves remain connected with 

 the cord, irritation of any afferent nerve is competent to 

 give rise to excitement of some, or the whole, of the efferent 

 nerves so connected. 



It the cord be cut across a second time at any distaiice 

 below the first section, the efferent nerves below the second 

 cut will no longer be affected by irritation of the afferent 

 nerves above it — but only of those below the second 

 section. Or, in other words, in order that an afferent 

 impulse may be converted into an efferent one by the 

 spinal cord, the afferent nerve must be in definite 

 material communication with the efferent nerve, by means 

 of a neuron in the sjjinal cord. The nature of these 

 communications we have already seen (p. 472). 



This peculiar power of the cord, by which it is com- 

 petent to convert afferent into efferent impulses, is that 

 which distinguishes it physiologically, as a central organ, 

 from a nerve, and is called reflex action. It is a power 

 possessed by the grey matter, and not by the white 

 substance of the cord. 



The number of the efferent nerves which may be 



