On the Anatomy and Physiology of the Organs of the Senses. 215 



Between the receipt of an impression by a sensorial organ, and the mea- 

 sures practised in consequence of it by a voluntary muscle, so short a period 

 elapses, that it appears to be instantaneous j it is worth while to analyse 

 this process, and to see what steps must occur before the motion can be 

 given. 



An ingenious and able physiologist. Dr. Roget, has divided these steps 

 into twelve. Let us suppose the case of a fly settling upon the eyelid, and 

 to be got rid of by a wink or muscular contraction of the part. 



1. His figure, falling upon the cornea, impinges upon the retina, or ex- 

 panded nerve below. 



2. An impression or picture is formed upon that organ. 



3. That impression, received by the optic nerve, is transmitted along the 

 nerve as a vibration, to the brain. 



4. There is a physical change produced thereby in the sensorium. 



5. The change from matter to mind takes place, the physical impression 

 being presented as a sensation. 



6. The reception of the sensation by the mind, constitutes a perception. 



7. At this stage the mind deliberates upon what course is to be adopted, 

 which being guided in a great measure by experience, is called association. 



8. The result of its deliberations is issued as volition. 



9. The communication of which to the sensorium, involving a physical 

 change, constitutes the ninth stage. 



10. That change is propagated along the nerve of motion, and 

 J 1. The muscle encircling the orbit is incited to contract : 



12. This contraction producing the desired effect, completes the series 

 of changes, and whisks off the fly, all unconscious of the elaborate train 

 which he has put in motion, although there is reason for concluding that 

 his own organisation admits of one precisely similar. 



The velocity with which these physico-psychological changes occur is no 

 doubt very striking if it be compared with purely physical motions, but it 

 ceases to be so if we compare it with the ordinary velocity of those of the 

 mind : to the rapidity of these operations, that is of thought, that is to say 

 to the vast number of ideas that may be clustered into a minute space of 

 time, there does not appear to be any limit, since it is certain that the suc- 

 cession of ideas usually spread over a long period of waking time, may 

 under certain conditions of the mind, as for instance during a dream, be 

 exhibited in an instant. 



The assistance, for the most part unacknowledged, derived by one sense 

 form the impressions received by another, is very considerable; and these 

 senses are not bestowed upon the infant in full perfection, that is to say, 

 the want of the habits of comparing present perceptions with former ones 

 presented by the memory, renders the actual perception of less value : and 

 it is only when use has become to us a second nature, and the actual and 

 past perceptions arc compared, and the conclusion deduced without any 



