THE PHENOMENON OF CONDUCTION. 77 



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 discovery, 

 the motor fibers to the voluntary muscles emerge from the spinal 

 cord in the anterior roots, while the fibers that give rise to sensa- 

 tions enter the cord through the posterior roots. These facts have 

 been demonstrated beyond all doubt. Magendie discovered an 

 apparent exception in the phenomenon of recurrent sensibility. 

 When the anterior root is severed and its peripheral end is stimu- 

 lated only motor effects should be obtained. Magendie observed, 

 however, upon dogs that in certain cases the animals showed signs 

 of pain. This apparent exception to the general rule was after- 

 ward explained satisfactorily. It was shown that the fibers in 

 question do not really belong to the anterior root, that is, they do 

 not emerge from the cord with the root fibers; they are, in fact, 

 sensory fibers for the meningeal membranes of the cord which 

 are on their way to the posterior roots and which enter the cord 

 with the fibers of the latter. Since the work of Bell and Magendie 

 it has been a question whether their law applies to all afferent and 

 efferent fibers and not simply to the motor and sensory fibers proper. 

 The experimental evidence upon this point, as far as the mammals 

 are concerned, has accumulated slowly. Various authors have shown 

 that stimulation of the anterior roots of certain spinal nerves may 

 cause a constriction of the blood-vessels, an erection of the hairs 

 (stimulation of the pilomotor fibers), a secretion of sweat, and so 

 on, while stimulation of the posterior roots in the same regions is 

 without effect upon these peripheral tissues. One apparent excep- 

 tion, however, has been noted. A number of observers have found 

 that stimulation of the peripheral end of the divided posterior 



