Page:Mind (New Series) Volume 12.djvu/499

 PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 485 the one hand serves to maintain or to reinforce conscious- ness, and, 'on the other, returns to its original starting-point in the form of a fresh movement." l Allowing for a certain looseness of the phrasing, this sentence seems to me to represent the process correctly. Kibot distinguishes two parts of the effect produced by the inflow of neurin by the nerves of the muscular sense, (1) part of it at once re-enters the motor tract to maintain the contraction ; (2) part serves to re-enforce consciousness. The former pro- cess, the return of the excitement to the motor tract, implies the existence of the ' motor circle ' as it has been called by James. The principle of the motor circle is now pretty well established as obtaining very generally, if not universally, throughout the cord and subcortical levels. It may be schematically represented as follows (fig. 9) : A stimulus at S excites the sensory neurone a, which discharges through a central neurone b into motor neurone c, thus bringing about a reflex contraction of some muscle ra ; the contrac- tion of the muscle stimulates its afferent neurone d, and this then discharges into the central neurone 6, and so re- enforces the excitement of this reflex path and maintains the contraction. 2 And this arrangement seems to be repeated in the second or sensory-reflex level, for, as Ebbinghaus puts it, 3 " Those parts of the cerebral cortex, which centrifugally are connected with the cells of origin of a particular move- ment-complex in the subcortical centres, contain also the end-station for the kinsesthetic excitations arising from the execution of just this movement and passing centripetally to the cortex". So that this path (k in fig. 9) in the Kolandic cortex, whose activity determines the sensation of movement, constitutes a loop upon the motor-circle, leading off from the afferent neurone d, and returning to the motor neurone c of that circle. The motor effects of a sensory, say a visual, stimulus thus tend to maintain themselves and the corresponding kinaes- thetic sensation by a circular activity, and they also support and maintain sensation in general by contributing to the influx of neurin to the afferent side of the nervous system. They do not however re-enforce in this way all sensations equally, 1 Psychology of Attention, p. 20. 2 For the evidence of the prevalence of this arrangement see Chaveau, " On the Sensorimotor Nerve-circuit of Muscles," Brain, vol. xiv. ; Sher- rington, Marshall Hall Address, 1899, 'The Spinal Animal,' Medico- Chirurgical Trans., vol. Ixxxii. ; James, Princ. of Psychology, vol. ii., p. 583. 3 Grundzuge d. Psychologie, Bd. i., S. 692.