MINUTE STRUCTURE OF THE NERVOUS TISSUES 33 



The neurons belonging in the class of adjusters far 

 outnumber the others in any system which can be said 

 to be highly organized. Such neurons are, by the necessi- 

 ties of the case, intermediate between other neurons. 

 They are stimulated through their dendrites and they 

 are assumed to discharge through their axons. A single 

 adjuster neuron may have a long axon and thus span a 

 wide space, or it may be restricted in its reach to a micro- 

 scopic scale. When the processes are short a large number 

 of neurons in series may be concerned in carrying im- 

 pulses for a very limited distance. Probably this kind of 

 transference from cell to cell is measurably slower than 

 the uninterrupted flow through the white matter. 



Efferent neurons receive through their dendrites 

 stimuli which may have been furnished directly from 

 receptor 'elements or which may have arrived by way of 

 the adjusters. The efferent fibers extend usually to a 

 considerable distance from the seat of their origin and 

 terminate by making connections with contractile or 

 secreting cells. When mention is made of efferent 

 fibers one is most apt to think of those which excite the 

 skeletal muscles motor fibers as commonly understood. 

 Neurons serving this purpose run an unbroken course 

 from cells in the lower part of the brain to muscles in the 

 head and neck; others similarly pass from the spinal cord 

 directly to muscles in the trunk and limbs. Efferent 

 fibers of other classes are usually, if not always, arranged 

 in tandem fashion, with a synapse in their course, so that 

 two orders have to be traversed by impulses bound from 

 the brain or cord to many peripheral structures. 



We have pictured the nervous elements in the way 

 which has been considered acceptable until quite recently. 

 But, as we have already hinted, the neuron theory is 

 under a fire of criticism. We must take pains to suggest 

 possible departures from the doctrine which may come 

 to be established in time. It has been said before that 

 continuity of protoplasm from neuron to neuron is 

 assumed by many. The view that a synapse is a bridge 



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