98 NERVOUS SYSTEM. 



ganglia and communicating nerves which lie on either side of the 

 vertebral column and extend throughout its entire length. Other 

 ganglia occur in connection with some of the cranial nerves, more 

 especially the vagus and trigeminus. There are ganglia and plex- 

 uses connected with the various organs (e.g. cardiac and solar), and 

 still others in the substance of some of the organs (e. g, stomach and 

 intestines). The sympathetic system has numerous communications 

 with the cerebro-spinal system. 



What is a reflex action ? 



An action which results from a centripetal nerve-impulse passing 

 to a nerve-centre in a ganglion, and there transforming to a centrifu- 

 gal impulse passing to a muscle. Such an action may be simple and 

 involve a single muscle, or complex and involve many : thus, a ray 

 of light falling upon the retina causes a simple reflex contraction of 

 a single muscle, and the iris contracts. As an illustration of a com- 

 plex reflex action, however, irritation of the larynx causes not only 

 a closing of the glottis, but a contraction of all the muscles involved 

 in forced expiration or coughing. 



Are all reflex actions involuntary ? 



Yes, but many of them may be checked or prevented by a vol- 

 untary effort. 



Do the reflexes depend upon the cerebro-spinal nervous system 



or upon the sympathetic? 



They are more noticeable in the cerebro-spinal system, but they 

 may belong to either, or may be mixed, the impulse going by the 

 one system and returning by the other. Examples : sneezing, 

 coughing, swallowing are cerebro-spinal reflexes ; the vaso-motor 

 reflexes are largely sympathetic, but the centripetal nerve is often 

 cerebro-spinal, as in the secretion of saliva or in blushing. 



What relations exist between the stimulus and the resulting 



reflex ? 



A stimulus which is mild causes a reflex of the muscle of the 

 same side, but as the stimulus is increased the muscles of the op- 

 posite side may be involved, the reflex of the irritated side remain- 

 ing the stronger. As the irritation is increased the reflex involves 

 more muscles ; that is, the stimulus spreads to a greater number 

 of cells in the ganglion and more efferent fibres are involved. 

 Those relations are obtained largely by experiments upon decap- 

 itated frogs. 



