CHAPTER III. 



CENTEAIt OkGAN AND PeHIPHEBAL NeEVES (PHYSIOLOGICAL). 



Next to a knowledge of the ganglion-cells, and of their grouping to 

 form smaller or larger centers, the most important question is: What is 

 known of these cells physiologically? And, first of all, we know that a 

 motor nerve loses its function when separated from its cell of origin, and, 

 too, that destruction of gray matter, in which sensory fibers end, destroys 

 their function as well. We know, further, that, by irritation of the cells 

 in which a nerve ends, we can produce all the phenomena which are ordi- 

 narily observed in the normal performance of its function. This fact 

 alone has led to the deduction that in the ganglion-cells and their inter- 

 relationships we have the basis of nervous activity. 



Numerous experiments have demonstrated that a sensory impression, 

 which, coming from the periphery, enters the nerve-center, may there excite 

 cells of origin of motor fibers, and bring the end-organs of the latter, the 

 muscles, into activity. This process is well known as reflex action. The 

 examination of such reflexes led subsequently to the additional very inter- 

 esting datum, that the sensory impulse does not always immediately excite 

 a motor discharge, but rather that a certain intensity of the original irrita- 

 tion is necessary, although a feeble irritation, if continued for a time, may 

 finally excite the motor apparatus. The theory is that ganglion-cells have 

 the property of storing up and retaining irritations coming to them until 

 the accumulated irritation is too great or some new irritation arrives from 

 some other direction, when they suddenly discharge. 



The small nerve-center of the crab, already mentioned in Chapter II, 

 receives many fibers from the periphery, and sends out, from the large cells 

 it contains, large fibers to the muscles. A glance at it and at the accom- 

 panying figure shows that a given irritation never affects one cell only, but 

 rather that an impression, which is conducted from one point in the pe- 

 riphery by a single fiber to the nerve-center, may there "charge" a large col- 

 lection of motor-cells. The subsequent discharge in like manner excites 

 not just one motor fiber, but, according to the anatomical relations of the 

 motor cells, brings an entire system of muscles into contraction. In this 

 wise is explained (Exner) how a single sensory impression may lead to a com- 

 plicated movement, in which many different muscles may take part. What 

 kind of motor reaction occurs from a sensory impulse depends upon which 



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