CH. XVII.] CLASSIFICATION OF NERVE-CELLS 203 



and the piece of the nerve-fibre still attached to it do not remain un- 

 affected. If regeneration of the fibre, and restoration of function 

 takes place, no change is observable. But if regeneration does not 

 occur (and it never takes place in the central nervous system), the 

 cell and its processes undergo a slow chronic wasting ; one of the 

 earliest signs of this disuse atrophy is chromatolysis. Warrington 

 has recently stated a still more interesting fact, namely, that section 

 of the posterior roots causes chromatolysis in the anterior horn cells 

 of the same side ; this indicates that the loss of sensory stimuli pro- 

 duces a depression of the activity and metabolic functions of the 

 spinal motor cells. We shall see later on that this accords quite well 

 with the physiological effects observed under these conditions. 



Classification of Nerve-cells according to their Function. 



In addition to the anatomical classification of the nerve-cells 

 already given, Schafer separates them into four chief classes on a 

 physiological basis : 



1. Afferent or sensory root cells. 



2. Efferent root cells. 



3. Intermediary cells. 



4. Distributing cells. 



1. Afferent root cells. Originally such cells are situated at the 

 periphery, and are connected with a process or afferent fibre which 

 passes to and arborises among the nerve-cells of the central nervous 

 system. This primitive condition is well seen in the earthworm, and 

 persists in the olfactory cells of all vertebrates. 



As evolution progresses, the peripheral cell sinks below the in- 

 tegument, leaving a process at the surface ; this is seen in the worm 

 Nereis (see fig. 215). Ultimately the body of the cell approaches 

 close to the central nervous system, in the spinal ganglion of the 

 posterior root, and the peripheral sensory nerve-fibre is correspond- 

 ingly longer. 



The afferent root cells, such as those of the spinal ganglia and 

 the corresponding ganglia of the cranial nerves, are peculiar in 

 possessing no dendrons. 



2. Efferent root cells. The anterior horn cells of the spinal cord 

 are instances of these ; their axons go directly to muscle fibres. 



3. Intermediary cells. These receive impulses from afferent 

 cells, and transmit them either directly, or indirectly through other 

 intermediary cells to efferent cells. The majority of the cells of the 

 brain and cord come under this heading ; they serve the purposes of 

 association and co-ordination, and form the basis of psychical 

 phenomena. 



4. Distributing cells. These are the cells of the sympathetic 



