398 



ANATOMY FOR NURSES 



[Chap. XIX 



be entirely wanting, thus saving the brain an enormous amount 

 of wear and tear. 



The simplest of these nervous mechanisms is the reflex arc, 

 and the simplest form of nervous activity is " reflex action." 

 Tw^o neurones enter into the formation of a reflex arc, a sensory 

 neurone and a motor neurone. On applying an appropriate 

 stimulus to the peripheral end of the sensory neurone, an impulse 

 is generated which passes along the sensory neurone to the nerve- 



Fro. 189. — Reflex Arc. S, sensory neuron arising in tactile corpuscles. M, mo- 

 tor neuron ending in muscle fibres. R, interlacing of dendrites. (Collins.) 



centre, and back again to the periphery by the motor neurone ; 

 and, since the motor neurone terminates in a muscle (or some 

 similar mechani.sm), we get a muscular response as the indirect 

 result of stimulating the sensory nerve. 



This is a reflex act, and usually the exchange between the sensory 

 and motor impulse takes place in the spinal cord. The sensory 

 impulse, after delivering its stimulus to the motor neurone, may 

 continue on up the spinal cord to terminate in the brain and 

 give to the individual the consciousness of the stimulus, or, on 

 the other hand, the sensory impulse, after arousing the motor 

 act, may cease, and no impulse be transmitted to the brain, 



