AFFERENT AND EFFERENT NERVATURES 69 



textures proper depend. Thus far the characteristics of 

 the two systems of innervation, sensory and motor (i.e. 

 so far as their histological development and structure are 

 concerned), are entirely parallel ; at this point in their 

 comparison, however, there ensues a distinction, which 

 amounts to an absolute difference, because nerve stimuli 

 are transmitted from without inwards to the cell by the 

 one, and from within outwards to the muscles by the other. 

 It cannot be said, therefore, that nerve stimuli are always 

 transmitted in the line of growth of the transmitting fibre 

 only, but according to the direction of incidence of the 

 functional transmission or circulation of nerve impulse or 

 energy. An exception, however, to this latter rule is 

 claimed to the extent that, in certain physiological as well 

 as pathological conditions, a nerve fibre can and does 

 transmit a duplex current, i.e. an afferent fibre can be made 

 the vehicle of transmitting an efferent impulse, as in 

 herpes zoster, and an efferent fibre can be made the vehicle 

 of transmitting an afferent impulse, as it may be conceived 

 to do in connection with the phenomenon of the "sixth" 

 or "muscular sense." 



In contending for the truth of these assumptions, it is 

 necessary to reiterate that a nerve fibre, instead of being 

 a solid and homogeneous texture, is composed of two 

 keratinous tubes, through which run, or are circulated, 

 the medullary and axis cylinder substances, these sub- 

 stances being the products respectively of the cell and its 

 nucleus, from which they are poured into the lumina of 

 their respective tubules and circulated to their remotest 

 terminals, where they are disposed of as skin ingredients 

 and sarcous tissue elements. Instead, thus, of the nerve 

 fibre being a stationary non-mobile strand of neural sub- 

 stance, histologically attached by its two extremities to 

 nerve cell and innervated texture, it is composed of two 

 telescoped tubes, an outer and an inner, transmitting 

 neuroglial substance, known as medullary and axis cylinder 

 substances, from brain, cord, or ganglia, to the limits of 

 the systemically innervated tissues, peripherally and cen- 

 trally, of the entire organism. 



Being, if this be true, active agents in the distribution 

 of neural tissue pabulum, as well as the transmitters of 



