206 THE NERVOUS SYSTEM. 



Those of the second type have a short neurite which does 

 not emerge from the nerve-centres, but not far from its 

 origin divides into its terminal branches or fibrils, some- 

 times forming a basket-like network enveloping another 

 nerve-cell. Neurons of the second type are situated entirely 

 within the nerve-centres, those of the first type partly within 

 the nerve-centres, partly without, in the general body-struct- 

 ures. 



A neuron typically consists of a nerve-cell or cell-body, one 

 or more dendrites, and a neurite prolonged as the axis-cylinder 

 of a nerve-fibre to the distal part of the body (or to 

 another part of the nerve-centres), where it has a special 

 termination. 



From the nature of nervous action, as we conceive it, a 

 neuron must have a mechanism both for receiving nervous 

 impulses and for sending them out again. Each neuron is 

 supposed to be complete in itself, and to have no anatomical 

 connection or continuity with other neurons ; the processes of 

 one nerve-cell are believed not to be united to those of 

 other cells. 



As it is evident that some communication of nervous impulses 

 takes place from one neuron to another, it is supposed that 

 such communication occurs through contact of the processes 

 or other parts of different cells with one another. If the cell- 

 processes are retracted (as by amoeboid movement) or diseased, 

 the communication may be cut off and altered nervous condi- 

 tions result. 



Another conceivable mode of transfer of nervous impulses 

 from one cell to another, possible without direct contact or 

 continuity, might be by a sort of induction, analogous to 

 electric induction. 



Function of dendrites and neurites : It is by some supposed 

 that the dendrites are afferent or centripetal (cellulipetal) in 

 their action, conducting impulses to the nerve-cell, and that 

 the neurites are efferent or centrifugal (cellulifugal), convey- 

 ing impulses from the cell and degenerating when their con- 

 nection with the cell is severed ; this supposition is clear 

 enough with respect to motor and other efferent nerve-cells 

 and fibres, but it is not so obvious in the case of sensory and 

 afferent fibres, 



