TOUCH AND KINESTHESIS 



391 



Thus, Weddell el al. (267) conclude that the ana- 

 tomical arrangement of the axoplasmic filaments of 

 the encapsulated endings is such that one could expect 

 them — in contrast to the free endings — to be highly 

 sensitive to minimal deformations. A differential 

 sensitivity to mechanical deformation, however, is all 

 that could reasonably be required to declare such an 

 ending as specific for touch or pressure. The differ- 

 ence, then, between the Oxford authors and the 

 orthodox view in this instance reduces itself to the 

 proposition that the Oxford workers presumably as- 

 sume that discharge in some other fibers as well must 

 necessarily occur before a touch sensation is recog- 

 nized while we are inclined to think, with von Frey, 

 that in principle such a sensation could arise if a single 

 appropriate peripheral fiber were activated. 



Concept of Head 



An important difficulty in drawing conclusions 

 from psychophysical experiments is the uncertainty 

 about the classifying of some sensations which may- 

 still be called touch. This uncertainty is clearly at the 

 root of the controversy as to whether or not stimula- 

 tion of the free endings actually arouses tactile sensa- 

 tions. The interpretation of von Frey denies that this 

 is the case and considers such sensations as akin to 

 pain; the O.xford interpretation affirms the tactile 

 quality of such sensations and denies the existence of 

 specific receptors. Head and his collaborators (118) 

 proposed that there are basically two different kinds 

 of sensations subserved by a dual sensory mechanism 

 at the periphery, the more generalized and, as they 

 felt, more primitive or protopathic type and the more 

 specific and advanced or epicritic system. The idea of 

 the duality of cutaneous sensations was greatly elab- 

 orated by Head and this elaboration might have con- 

 tributed to the present eclipse of his concepts. Al- 

 though at first accepted by many, they were soon 

 sharply criticized, and finally Walshe (262) in his 

 review of the subject delivered a cou[) de grace to these 

 concepts by pointing out that the crucial introspective 

 experimental observations of Head were not inter- 

 preted in the same way by any of the suljsequent 

 observers and that his theoretical elaboration was 

 sometimes hazy and contradictory in details, often 

 incompatible with the present knowledge and always 

 speculative. However much one may disagree with 

 some of Walshe' s criticisms the fact remains that none 

 of the experimental observations offered by Head in 

 support of his ideas has been accepted by others. 



The conclusion that Head did not prove his point 



is, however, irrelevant for an inquiry as to whether or 

 not his central idea has merit. It is apparent that a 

 protopathic system, if it exists, is likely to be repre- 

 sented anatomically by free endings. The question 

 may then be asked whether stimulation of such end- 

 ings results in sensations other than pain. There are 

 some indications that this indeed may be the case. 

 Evidence to this effect seems to be at present the chief 

 support for the pattern theory of the Oxford workers, 

 even though such findings could argue, in better 

 harmony with other well established facts, for the 

 existence of protopathic .sensibility. 



Another hint that a protopathic system may exist 

 is offered by studies of action potentials in peripheral 

 nerve. We shall discuss these in some detail later. Here 

 it is sufficient to state that there is evidence that im- 

 pulses evoked by tactile stimuli are conducted in both 

 myelinated and unmyelinated fibers. Finally, the fact 

 that activity aroused by tactile stimuli is conducted 

 within the spinal cord in at least two independent 

 ascending pathways could be utilized to argue that 

 the idea of duality of the tactile system does not appear 

 unreasonable. 



It should be apparent from these remarks that it is 

 not proposed at present to revive the concepts of epi- 

 critic and protopathic sensibility. We wish only to 

 point out that an unqualified rejection of these con- 

 cepts may be premature and that Head's ideas in some 

 form may yet prove useful in the future. 



SOME PROPERTIES OF PERIPHERAL SOMATIC 

 AFFERENT SYSTEM 



Receptors 



It is convenient to consider the receptors both in 

 this section and in the one which deals with the cen- 

 tral events. In order to avoid repetition we shall con- 

 sider here primarilv those properties which have a 

 bearing on their specificity. 



SPECIFICITY OF RECEPTORS. Whatever opinions one 

 may hold about the way tactile stimuli arouse sensa- 

 tions it is fundamental to recognize that there are 

 some receptors which are specifically sensitive to such 

 stimuli. Conclusive evidence in this respect is provided 

 by studies of discharges, usually of single units, when 

 mechanoreceptors are activated by natural stimuli. 

 In many of these studies (4-6, 16-18, 29, 42, 68, 94, 

 124, 127, 161, 187) the existence of a specific receptor 

 is inferred from the behavior of the neural discharge; 



