THE PHENOMENON OF CONDUCTION. 



83 



Excitatory 



Inhibitory 



Excitatory 



Inhibitory 



I 

 r Motor J. 



Secretory 



Inhibito-ii 



tor 

 Inhibito-si 



cretory 



Seusory 



Inhibito-re- 



liex 



f Motor. 



Vasomotor. 



Cardioiuotor. 



Visceromotor. 



Pilomotor. 



Salivary. 

 ) (iastric. 

 ) Pancreatic. 

 '■ Sweat. 



f Subdivisions corresponding to the varieties ol mcK 

 I tor fibers above. 



/ Subdivisions corresponding to the varieties of se« 

 ( cretory fibers above. 

 i Visual. 



Auditory. 



Olfactory. 



Gustatory. 



Pressure. 



Temperature. 



Pain. 



Hunger. 



Thirst, etc. 



According to the efierent fibers affected. 



Inhibitory effects upon the conscious sensations are 

 not demonstrated. 



The reflex fibers that cause unconscious reflexes 

 are known to be inhibited in some cases at least. 



That the final action of a peripheral nerve fiber is determined 

 by the tissue in which it ends rather than by the nature of the 

 nerve fiber itself or the nature of the impulse that it carries is indi- 

 cated strongty by the regeneration experiments made by Langley.* 

 For instance, the chorda tympani nerv^e contains fibers which cause 

 a dilatation in the blood-vessels of the submaxillary gland, while 

 the cervical sympathetic contains fibers which cause a constriction 

 of the vessels in the same gland. If the lingual nerve (containing 

 the chorda tympani fibers) is divided and the central end is sutured 

 to the peripheral end of the severed cervical sympathetic, the 

 chorda fibers will grow along the paths of the old constrictor fibers 

 of the sympathetic. If time is given for regeneration to take place, 

 stimulation of the chorda now causes a constriction in the vessels. 

 The experiment can also be reversed. That is, by suturing 

 the central end of the cervical sympathetic to the peripheral end 

 of the divided lingual the fibers of the former grow along the paths 

 of the old dilator fibers, and after regeneration has taken place 

 stimulation of the sympathetic causes dilatation of the blood- 

 vessels in the gland. These results are particularly instructive, as 

 vasoconstriction is an example of the excitatory effect of the nerve 

 impulse, being the result of a contraction of the circular muscles 

 in the vessels, while vasodilatation is an example of inhibitory 

 action, being due to an inhibition of the contraction of the same 

 muscles. Yet obviously these two opposite effects are determined 

 not by the nature of the nerve fibers, but by their place or mode 

 of ending in the gland. 



Separation of the Afferent and Efferent Fibers in the Roots 



of the Spinal Nerves.— According to the Bell-Magendie law, 



* Langley, "Journal of Physiology," 23, 240, 1898; ibid., 30, 439, 1904; 

 "Proceedings Royal Society," 73, 1904. 



