SPINAL COED AS NEEVE CENTEE. 617 



In the transmission of these impulses the cells seem to have a 

 certain directing and controlling influence which deserves special 

 attention, as it gives us the key to the more complex mechanisms 

 of the higher centres. Although the various powers exerted by 

 the cells of the spinal cord are so intimately associated together 

 as to be practically inseparable, it is found convenient to consider 

 their functions under distinct headings. The following division 

 of their duties will be found useful in working out their general 

 function, viz. : 



1. Their influence on afferent impulses. 



2. Their influence over efferent impulses. 



3. Their automatic or independent power of originating im- 

 pulses. 



When an impulse the result of some slight stimulus, such as 

 the prick of a pin arrives at the cells of the spinal cord, it is 

 at once communicated to the other cells in the immediate neigh- 

 borhood, and reaching some cells connected with motor nerves it 

 gives rise to a movement of the muscles of the neighborhood 

 from which the impulse first started. At the same time impulses 

 travel to the brain, and there give rise to a consciousness of the 

 various events taking place, i.e., a local stimulation and a local 

 movement. The action of the cells of the cord takes place with- 

 out the aid of the will, and generally occurs before one is con- 

 scious of it ; it is therefore called an involuntary act, and on 

 account of being a throwing back of the impulse, these kinds of 

 movements are called reflex acts. 



Reflex action forms one of the commonest duties of the cells 

 of the spinal cord. Even the gentlest stimulation commonly 

 gives rise to a complex movement, the execution of which requires 

 many muscles to act together with, as it were, a common object. 

 Thus the sudden prick of a pin in the finger will cause a person 

 to withdraw the hand quickly from the irritating object. If a 

 greater or more prolonged stimulus be applied, much more ex- 

 tensive movements occur, and these are in the same way accom- 

 plished by the well-arranged cooperation of many muscles, acting 

 together in a such a way that a definite and familiar action is per- 

 formed. For example, if the burning head of a match adhere 



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