COURSE OF NERVE IMPULSES. 189 



the movements of the right and left sides is controlled by means of fibers in the 

 thoracic cross commissures, probably by the crossed collaterals of the motor 

 neurones. 



The Swallowing Reflexes. — There are no nerves or sense organs visible 

 near the lips, or in the soft skin about the mouth; and no reflexes could be pro- 

 duced by touching these parts with food. " Swallowing" apparently depends on 

 preliminary stimulation of the coxal taste organs. The chewing movements are 

 often interrupted by a tetanic spasm of the legs and a prolonged "bite" of the 

 great mandibles on the sixth pair. During this period, a muscular spasm of the 

 stomodaeum appears to take place, by means of which the materials that have 

 been tucked into the mouth by the coxal spurs are swallowed. The reflex is 

 probably initiated in the stomodaeal ganglion by stimuli received from the anterior 

 end of the gustatory tract. 



Course of Nerve Impulses in the Gustatory, Chewing, and Swallowing 

 Reflexes. — We may picture the probable course of the nerve impulses in the 

 chewing reflexes as follows : Stimulation of the gustatory cell, g.o, may i . discharge 

 an impulse by the first set of collaterals directly to the motor neurones of its own 

 segment. 2. To produce the continued rhythmic discharge, it is apparently 

 necessary for the impulse to be conveyed to the secondary, or cheliceral center, 

 and then back to the motor neurones of the chewing muscles. 3. Irrjpulses may 

 be carried from the secondary center to the tertiary center in the hemispheres; 

 this center appears to exercise a depressing, or inhibitory, control of the motor 

 neurones, especially of those supplying the flexors and extensors of the leg. 4. 

 The only way to arouse the motor neurones of a given leg or jaw to normal action 

 is ^•ia its own sensory fibers, either directly through the primary center, or indirectly 

 through the secondary center, or both. 5. The linear coordination of gustatory 

 movements on one side is affected by the cheliceral center of the same side. 6. 

 Bilateral coordination is affected via the thoracic cross commissures. 



The remarkable difference in the action of the same leg muscles, when stimu- 

 lated via different sensory channels, i.e., gustatory and general cutaneous, may be 

 due to several causes the nature of which is very obscure. They are indicated 

 to some extent by the known structure, and to some extent by the nature of the 

 reaction. The general cutaneous tracts, for example, are not so sharply defined 

 and are not composed of such distinct segmental fascicles as the gustatory tracts, 

 which may account for the fact that stimulation of a definite group of temperature 

 organs on one side of the shield usually produces a reaction in several legs of the 

 apposite side, not in one. Moreover stimulation of the temperature organs pro- 

 duces a rather ill defined leg movement, that at once ceases when the stimulus is 

 removed. If the taste organs are stimulated, a definite rhythmic action follows 

 that does not cease at once when the stimulus is removed. This may be due in 

 part to the fact that the stimulating action of the substance may in this case con- 

 ;inue after the source of it has been removed, or in part to the presence of a "center" 

 which continues to act after the peripheral stimulus ceases. But even these con- 



