784 THE SPINAL CORD. 



selves. Hence the nervous system links together and co-ordinates the 

 activities of the various organs. Its power of spatial transmission is 

 technically known as its " conductivity." 



In the physiological analysis of the reactions of the spinal system 

 the unit reached is " the reflex." Upon it as basis our existing notion 

 of the reactions of the nervous system is built. The whole neural 

 system may be regarded as a superstructure resting upon two nerve 

 cells, the two root cells, one afferent the other efferent. The afferent 

 root cell is the alpha of every functional series of units in the system ; 

 the efferent root cell is the omega to which every series immediately 

 or mediately leads. In every functional series of units there is a place 

 or there are places where conduction involves change in quantity of the 

 state of excitement transmitted, and alteration of its distribution both 

 in space and time. 



The places where such modification of the nerve impulses occur are 

 spoken of as " nerve centres." 



Stephen Hales first proved the spinal cord to be a seat of " reflection " as 

 against other places where nerves seem, to unite (for example, the nerve plexuses). 

 Grainger 1 later indicated that in the spinal cord the grey matter is the seat of 

 the reflection. It was then inferred that the essential element of the centre 

 must be the nerve cell body (perikaryon) ; later, an inextricable net of nerve cell 

 branches was found surrounding and continuous with the bodies of the cells (J. 

 Gerlach) ; to these together were attributed the reflecting and distributing pro- 

 perties of the nerve centre. More recently, from evidence that goes to show that 

 the branches of the individual nerve cells do not anastomose into a continuum, 

 but retain morphological individuality, although functionally linked, stress has 

 been laid on the possible properties of the places of linkage or synapse. The 

 places where nerve cells thus " hold hands " with each other are not necessarily 

 near to the cell body, although usually so. It is as the, with few exceptions, 

 exclusive seat of modification and redistribution of nerve impulses that brain 

 and cord play such pre-eminent part in the functions of the nervous system. 

 Comparable seats of modification occur also at places of linkage outside the 

 central nervous system, e.g. the places of linkage of the spinal nerve cells with 

 the sympathetic nerve cells in the sympathetic ganglia, and of the linkage of 

 spinal nerve cells with muscle cells and epithelial cells in motor end-plates, 

 glands, and sense organs. These seem the nearest approach outside the brain 

 and cord to anything of real likeness to a nervous centre. 



Afferent is the term applied to nerve cells, considered as conductors, along 

 which the waves of change normally excited pass in a direction away from the 

 extrinsic, i.e. non-nervous tissues. Efferent, conversely, is applied to nerve cells 

 which, conduct normally towards extrinsic, i.e. non-nervous tissues. The afferent 

 root cells taken together form one great pathway into an intricate congeries of 

 nerve centres which the cerebro-spinal organ encloses. The efferent root cells 

 conversely together form one great pathway out. And within the cerebro-spinal 

 organ the nerve centres are not only very numerous but are of very various rank. 

 The cerebro-spinal organ is fundamentally a chain of segmental centres, each 

 segmental centre primarily a seat of reflection and distribution connected with 

 the local conducting paths between sense cells and muscle cells of its own 

 segment. The segmental centres have in the course of evolution of the nervous 

 system come to be more and more put en rapport one with another by nerve 

 cells overlying them, and dealing with them in groups. By means of these inter- 

 mediate and overlying conductors, a nerve impulse may, besides embouching 

 upon its own segmental muscle cells, travel over a concatenation of central 

 nerve cells. The reactions which occur along nerve cell chains of a single link 

 1 "Observ. on the Struct, and Functions of the Spinal Cord," London, 1837. 



