92 ANATOMY FOR NURSES. [Chap. VII. 



defined as the continual response to external stimuli, reflex ac- 

 tion, which is the chief method of response, is the most impor- 

 tant vital phenomenon peculiar to animals possessing any nervous 

 system whatsoever. 



A careful study of Figs. 74 and 75 will make the typical 

 reflex path perfectly intelligible to the student, and should on 

 no account be omitted. 



All nervous action is fundamentally similar to this typical 

 reflex action. Usually the number of neurones involved is 

 greater, often very much greater, than two. The fewer tiie 

 neurones, the simpler and more obviously machine-like the 

 reaction. The more complex the path, the more uncertain and 

 variable the reaction. When the path of the impulse does not 

 mvolve the cerebrum, the reactions are unconscious and com- 

 paratively simple; but if the cerebral cortex be involved, tlie 

 passage of the nerve impulse is accompanied by the phenome- 

 non of consciousness, and the reaction may be exceedingly com- 

 plex, uncertain, and long delayed. These are the characteristics 

 of what we call voluntary reactions. But, although the phrase 

 •' reflex action " is usually confined to those actions which are 

 involuntary and of which we are unconscious, yet all nervous 

 action is essentially the same, differing only in the complexity 

 of the path followed by the impulse. 



We will now conclude with a summary of the functions of 

 the various parts of the nervous system. 



The nerves serve to connect the distant parts of the body 

 with the central nervous system. 



The spinal ganglia contain the cells of origin of all the 

 peripheral sensory nerve fibres. 



The sympathetic ganglia serve to distribute motor, and to 

 collect sensory, impulses. Also in a few cases an afferent im- 

 pulse may pass to a ganglion by the dendrone of one sympa- 

 thetic neurone, and leave it to pass back again to the periphery 

 by the axone of another, the spinal column not being included 

 in the arc. Thus the sympathetic ganglia may occasionally act 

 as a centre for reflex action. 



The spinal cord, medulla, and pons act as centres for the more 

 simple reHexes. In the medulla there are also special centres 

 which govern more complex muscular movements, such as the 

 vaso-motor centre which controls the calibre of the blood-vessels, 



