THE NERVOUS SYSTEM 



475 



tibres serve as longitudinal commissural fibres to bring the different 

 segments of the cord into communication (Fig. 329). The shorter 

 fibres lie nearest the gray matter and link together adjacent segments. 

 The longer fibres lie farther from the gray matter and continue 

 through several 'Segments. The origin of these fibres as axones of 

 cells of the gray matter, and the manner in which they re-enter the 

 gray matter as terminals and collaterals 

 have been considered (pp. 450 and 451). 



The fact, alluded to above, that the 

 shorter fibres lie nearest, or mingled with 

 the gray is, in a general way, true through- 

 out the central nervous system. A result 

 of this in the cord is the superficial posi- 

 tion of many of the longest tracts. 



Attention has already been called to 

 the concept of neural arcs which may 

 traverse the cord or both cord and 

 suprasegmental structures (page 427). 



It must be kept in mind that there are prob- 

 ably no isolated neural arcs and that every 

 neural reaction involving any given arc always 

 influences and is influenced by other parts of 

 the nervous system. 



Fig. 329. — Diagram iUustrat- 

 ing Three-neurone Spinal Reflex 

 Arcs of one segment and more 

 than one segment. Groups of 

 neurones are represented by one 

 neurone. g, Spinal ganglion 

 cells; h.c.c, heteromeric column 

 cell; t.c.c, tautomeric column 

 cell; v.r., ventral root. 



From the neurones thus far studied 

 and the tracts which their axones form, 

 the following neural arcs may be con- 

 structed: 



(i) A Two-neurone Spinal Reflex Arc 

 (Fig. 328). — (a) Peripheral afferent neu- 

 rones; their peripheral processes and receptors, the spinal ganglion 

 cells, their central processes with collaterals terminating around 

 motor cells of anterior horn; (6) peripheral efferent neurones, i.e., 

 motor cells of anterior horn with axones passing to effectors. 

 Such a two-neurone reflex arc is chiefly uncrossed and in most cases 

 involves only one segment or closely adjacent segments. As it 

 involves only one synapsis (see chapter X) (in the ventral gray) it is 

 sometimes termed a monosynaptic arc. 



(2) A Three-neurone S pinal Reflex Arc (Fig. 329). — {a) Peripheral 

 afferent neiirones as in (i), but terminating around column cells of the 

 cord, (b) Cord neurones (column cells) — axones in the fundamental 



