122 PSYCHOBIOLOGY 



adopted here, considers all normal actions as the termini of reflexes: it 

 holds, for example, that voluntary actions, such as dropping, a letter in the 

 box after deliberating whether to post it or not, are just as much ' reflex 

 actions ' as are the blinking of the eye when a cinder strikes it, and the 

 deep inhalation which follows the perception of a faint pleasant odor. 



THE FUNCTIONAL UNITY OF THE CENTRAL NERVOUS SYSTEM. 



Before considering further the dependence of consciousness on organic 

 reflexes, it is necessary to look at the nervous system from the point of 

 view of its mode of function. 



An afferent impulse over a single neuron is capable of being trans- 

 mitted to any efferent neuron of the centralized nervous system, including 

 the visceral division (but excluding possibly the local system: plexuses 

 of Auerbach and Meissner), or to a large number of such neurons. That 

 is to say: the irritation of an afferent neuron may, through the successive 

 passing of the irritation to various intermediate (associative and com- 

 missural) neurons cause the irritation of any efferent neuron, or of a num- 

 ber of such neurons, and so many modify the activity of any muscle or 

 gland, or of a large number of effectors. 



Impulses are constantly passing inward over the afferent chains. Even 

 in sleep, the only afferent neurons which suspend their activity are those 

 from the retina, and from some of the striped muscles, and possibly from 

 some small areas of the skin. The afferent terminals in smooth muscle, 

 and in the muscles of the breathing mechanism, are still being stimulated, 

 with about normal response. Conversely, there is a continuous and widely 

 distributed outgo, maintaining the tone of the muscles, and stimulating or 

 inhibiting contraction and secretion. Even in sleep, the control of the 

 visceral organs, and of respiration, cannot be suspended, and the tone of 

 the striped muscle generally must be maintained. 



The neural mechanism, therefore, must not be regarded as a collection 

 of potential or actual arcs, but as one enormously complicated arc, in 

 which, for legitimate purposes of description we distinguish multitudinous 

 paths from sensory periphery to motor periphery. These individual arcs 

 are not fictitious, but are to a certain extent abstractions. 



The following example of a reflex may make the relation of total sys- 

 tem and particular function more clear. The organism may be so dis- 

 posed that a stimulus to the eye produces a specific movement of the hand ; 

 this is the case in a simple reaction measurement when the reactor is in- 

 structed to press a rubber bulb immediately on seeing a light. In this 

 case there is probably a discharge through a series of neurons running 

 from the retina of the eye through the mid-brain (and possibly through 



