198 



Dr. H. Head. 



stimulus to the lieat-, the cold-, and the pain-spots, and yet, under normal 

 conditions, produces a sensation of pleasant warmth only. Between the 

 impact of a physical stimulus on the peripheral end-organs and the simplest 

 changes it evokes in consciousness lie the various phases of physiological 

 integration. We are accustomed to think of the external stimuli we employ 

 in scientific investigation as if they caused unitary effects; but in reality 

 they produce a multitude of diverse physiological reactions. The simplest 

 physical condition acting on the peripheral nervous system may produce 

 afferent impulses which are incompatible from the point of view of sensation. 

 But the stimuli of daily life are not simple ; they are extremely complex and 

 give rise to a multitude of diverse impressions. These must be sorted and 

 regrouped ; some are facilitated, others are suppressed before the final sum is 

 presented to consciousness. Many afferent impulses remain on the physio- 

 logical ■ level and never form the basis of consciousness ; they are destined to 

 control reflex activity or to co-ordinate movements of the body and limbs. 



We must not forget that the larger number of afferent impulses, arising in 

 peripheral end-organs, do not travel the whole length of the central nervous 

 system. Their essential task is to excite and co-ordinate reflex action 

 without of necessity giving rise' to sensation, and the earlier phases of 

 integration are undoubtedly adapted to this end. Such co-ordination could 

 not occur if there was no qualitative selection of afferent impulses at the 

 receptive junctions of the various reflex arcs. The main function of the intra- 

 medullary receptors is to increase the excitability of the reflex arc for one 

 kind of stimulation and to diminish it for all others. This ensures a selective 

 response to the complex results of peripheral excitation, find at the same 

 time precludes many afferent impulses from influencing consciousness. Thus, 

 for example, visceral insensibility is a function of the specific reaction of 

 receptors, and not the consequence of an absence of afferent impulses. 



Each specific group of receptive organs within the central nervous system 

 works on the same principles ; it accepts those elements of a massive 

 reaction which can excite it to activity and rejects those to which it cannot 

 respond. At the same time, there is evidence to show that the synaptic 

 junctions are subject to those refractory states and phases of heightened 

 activity characteristic of all neural action. 



These three processes, selection of certain impulses, rejection of others, 

 together with biphasic states of the receptive mechanism, are sufficient to 

 produce coherent reactions on the reflex levels. Here all functions are 

 simple, but at the same time highly organised. The response is rigidly pre- 

 determined within certain limits ; there is little choice, and the answer is 

 physiologically inevitable. In strict accordance with such an arrangement, 



