CH. xn.] 



CLASSIFICATION OF NERVE-CELLS 



195 



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. 187). Ultimately in the vertebrates the body of the 



Earth-worm 



Nereis 



Vertebrate 



FIG. 187. Diagram to illustrate the primitive conditions of the afferent nerve-cell, and the manner in 

 which it becomes altered in the process of evolution. (After Retzius.) I, integument ; C, central 

 nervous system ; the arrows show the direction in which the impulse passes. 



cell approaches close to the central nervous system, in the spinal 

 ganglion of the posterior root, and the peripheral sensory nerve-fibre 

 is correspondingly longer. 



The afferent neurons such as those of the spinal ganglia and 

 the corresponding ganglia of the cranial nerves, are peculiar in 

 possessing no dendrons. 



2. Efferent neurons. The anterior horn-cells of the spinal cord 

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



3. Intermediary neurons. 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 coordination, and thus their activity underlies 

 psychical phenomena. 



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

 ganglia ; they are situated outside the central nervous system ; they 

 receive impulses from efferent cells in the central nervous system, 

 and distribute them to involuntary muscles and secreting glands. 



