ON THE MAMMALIAN NERVOUS SYSTEM. 447 



both ends of a nerve when the middle portion is stimulated. We have seen that 

 similar evidence in the case of the nerve fibres in the cord shows that in them also the 

 effect is pi'opagated from the stimulated region indifferently either towards the centre 

 or the periphery, and it follows from the combination of these two, that if any fibres 

 in a nerve are directly continuous with those in the cord, the excitatory changes 

 following stimulation of these must be propagated along the whole length of the 

 continuous strand and may thus travel out into the fibres of the nerve. Direct 

 continuity has been shown to exist by the degeneration method between the 

 fibres of the posterior root and those of certain portions of the posterior 

 columns, hence it is not surprising that the stimulation of the posterior columns in 

 the cord should cause electrical changes in the posterior root due to the arrival in the 

 fibres of this structure of excitatory processes transmitted from the cord down tracts 

 which, owing to their terminal relations, are usually believed to be solely ascending 

 ones. There is nothing to warrant the belief that these descending impulses in 

 afferent nerves, evoked by excitation of the columns in the spinal cord, are in 

 themselves different from those which are evoked by similar methods of excitation 

 when applied to the peripheral parts of the afferent nerves, and which are propagated 

 in the natural direction, since it seems to be fundamental as regards nerve conduction 

 that fibres can conduct equally in either direction. 



The fact that in one group of nerve fibres (afferent) the starting platforms are at the 

 peripheral end, and the receiving termini at the central, whilst in the other (efferent) the 

 positions are reversed, and that, in consequence, what are called normal nerve 

 impulses proceed in the afferent direction in the one group and in the efferent in the 

 other, apparently has not, physiologically or structurally, altered the fibres (polarised 

 them in any way) so as to make conduction in the one direction more difficult than in 

 the other. As far, then, as the directly continuous afferent fibres are concerned, 

 there is no difficulty either in obtaining electrical results in the posterior roots on 

 exciting their direct prolongations in the. cord, or in interpreting these as indicative 

 of the passage of nerve impulses. 



A much more complicated condition must, however, be now referred to, that, 

 namely, of the nerve fibres which are only indirectly continuous with others in the 

 cord, there being interposed in their path corpuscular elements and unknown 

 channels. 



The simplest of these are the efferent (motor) nerves, and it has already been 

 stated that, as regards these, Chapter IX., Section 2, the corpuscular connection is of 

 such a character that, whilst allowing the passage of impulses from the cord to the 

 nerves, it appears to completely block the passage of impulses from the nerves 

 into the cord. (See also Chapter XI., Section 2 (1).) The stimulation of a mixed 

 nerve (the posterior roots being divided) or an anterior root thus evokes impulses which 

 travel up and break upon the shore of the nerve corpuscles, and either remain on 



