648 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



fact the variations in the physiological (chemical) condition of the nerve-cells 

 are equally important, and neither factor can be studied independently. 



The term central stimulation is sometimes employed. For example, the 

 spasmodic movements of the young child, when there is no change noticeable 

 in the external stimuli acting upon it, are sometimes attributed to this cause ; 

 but these, although doubtless due to central changes, altering the irritability 

 of the cells, are most properly classed with the reactions which follow the 

 external stimulus. The misconceptions here to be avoided are those of sup- 

 posing that the nervous system is at any time unstimulated, and that the 

 evident responses follow a change of the external stimulus only. 



Strength of Stimulus and Strength of Response. Where the impulse 

 does not traverse more than one nerve-element, there is a direct relation be- 

 tween the strength of the stimulus and the strength of the response. The 

 negative variation in the isolated nerve increases with the intensity of the 

 stimulus which is sent through it. The same is true for submaximal stimuli 

 applied to the nerve when the nerve is still attached to a muscle, and the 

 height of the muscular contraction is measured. 



When, however, the impulse in one cell-element is used to arouse an impulse 

 in another, as in all experiments where the nerve-cells are arranged in a physio- 

 logical serias, the strength of the impulse from the second is less easy to pre- 

 dict. This is explained as due to variations in the ease with which the impulse 

 in one element stimulates the next, and also to the variations in the second 

 cell of those conditions which determine the intensity with which it may 

 discharge. 



When an impulse has once entered the central system the arrangement 

 of the pathways involves the distribution of it to a larger and larger number 

 of elements. This may be illustrated by Figure 169. 



At the same time that the impulse is thus distributed it tends to die out. 

 If, as we assume, it is a wave of molecular change that passes along the neuron, 

 then when the neuron divides the energy in the main stem is distributed to 

 the mass of substance which forms the branches, and if the mass of these, 

 as is usually the case, is greater than that of the main stem, then the energy 

 in any branch will be less than in the main stem. 



In the case of some of the cells about which the branches of the neuron 

 end the impulse will not be adequate to cause in them a discharge, although 

 it may still produce a certain amount of chemical change in them. The 

 impulse thus tends to disappear within the system, by producing in part chem- 

 ical changes strong enough to cause a discharge, and in part similar changes 

 of a less intensity. 



Diffusion of Central Impulses. Thus the general result of sending an 

 impulse into the central system is that it tends to be distributed and at the 

 same time to become weaker. Finally, by one or more of the central paths it 

 reaches an efferent cell which is in a condition to discharge so as to produce 

 an evident reaction. 



If the previous description has been correct, two very important events 



