RELATIONSHIP OF NEURONES. 431 



G. GENERAL SURVEY OF THE RELATIONS OF THE 



NEURONES TO ONE ANOTHER IN THE 



CENTRAL NERVOUS SYSTEM. 



The following figures illustrate the modern theories with re- 

 gard to the relationship of the neurones in a sensorimotor reflex 

 cycle. The pathway along which the impulse from the stimulated 

 area of the body is transmitted to the motor nerve end-organ tra- 

 verses two neurones (primary neurones) which are in contact by 

 means of their telodendria situated within the gray matter of the 

 spinal cord. The cell-body of the sensory neurone lies within the 

 spinal ganglion ; that of the motor neurone, in the anterior horn of 

 the spinal cord. The dendrite of the sensory neurone commences 



mN 



M 



Fig. 346. Schematic diagram of a sensorimotor reflex arc according to the modern 

 neurone theory ; transverse section of spinal cord : mN, Motor neurone ; sA T , sensory 

 neurone ; C l , nerve-cell of the motor neurone ; C' 2 , nerve-cell of the sensory neurone ; 

 </, dendrite ; , neuraxis of both neurones ; /, telodendria ; M, muscle-fiber ; /$, skin 

 with peripheral telodendrion of sensory neurone. 



as a telodendrion in the skin or perhaps also in more deeply seated 

 structures, and transmits a cellulipetal impulse, while its cellulifugal 

 neuraxis and telodendrion (the latter in the gray matter of the cord) 

 transfer the impulse to the cellulipetal telodendrion of the motor 

 neurone. The cellulifugal neuraxis of the latter finally ends as a 

 telodendrion in the muscle. (Figs. 346 and 347.) 



In the case of longer tracts the conditions are somewhat more 

 complicated, as, for instance, in tracing the impulse along the sen- 

 sory fibers to the cortex of the brain, and from there along the 

 motor fibers to the responding muscle. In such cases secondary 

 neurones are called into play by means of their telodendria, which 

 are necessarily in contact with the primary neurones just described. 



