THE ASSOCIATION OF IDEAS 37 



consciousness, and, as I shall attempt to show, that they are the 

 essential mechanisms for associative thinking; hence we may for 

 the tune being neglect the first mentioned classes, concerning 

 which, as a matter of fact, we possess little information. 5 



Heterodetic arcs may terminate in any of the three classes of 

 effectors; homeodetic arcs are perhaps of the muscular types only, 

 since there have been no afferent terminals discovered in glands. 

 The peculiarity of the homeodetic arc is that the effect of one 

 reaction initiates another; a heterodetic muscular reaction may 

 therefore be followed by a sequence of homeodetic reactions 

 a sequence which will be brought to an end by another hetero- 

 detic reaction, from muscle to gland; or by "drainage" into 

 another arc system; or possibly by a positive inhibition reaction. 



Restricting our attention once more to the cognitive or striped 

 muscle arcs, it becomes at once apparent that the heterodetic 

 striped-muscle arc conditions perception and the homeodetic 

 arc conditions thought. This deduction from the hypothesis of 

 reaction arcs provides at once two things of which psychology 

 has long been in want; a physiological explanation of the associa- 

 tion of ideas, and an explanation of the nature of so-called " men- 

 tal images." The way in which a given series of homeodetic 

 reactions, once established, may become a habit, thus condi- 

 tioning an associative train of thought, is so obvious that we need 



B The smooth muscles and the glands undoubtedly have important psycho- 

 physiological function, but the details can not at present be determined. It is 

 possible that the experiences of desire and aversion are conditioned by reactions 

 from certain of the smooth muscles ; hunger fromthe muscular coat of the stomach, 

 sexual desire from the involuntary muscles of the genital organs, all other forms of 

 desire being possibly reducible to these basic appetites and thirst at whose reac- 

 tion conditions we can guess with less plausibility. Other affective experiences 

 may also be conditioned by reactions from smooth muscular systems, and from 

 cardiac muscle. The muscular coats of the blood vessels, for example, have long 

 been suspected of participation in the production of pleasure and pain. The 

 arrectores pilorum in the skin may have a specific affective function. We are not 

 at present able to declare that the terminus ad quern of the arc is without impor- 

 tance, and we must therefore admit the possibility that the reactions to smooth 

 muscle and to glands may have effects on consciousness which are characteristic 

 regardless of the terminus a quo. The morphology of glandular and smooth mus- 

 cular arcs must be more fully known before speculation concerning the corre- 

 sponding reactions can be useful. 



