In the origination of a thought there are two distinct conditions: the state of the organism as dependent on antecedent impressions, and on the existing physical circumstances.
In the cephalic ganglia of insects are stored up the relics of impressions that have been made upon the common peripheral nerves, and in them are kept those which are brought in by the organs of special sense--the visual, olfactive, auditory.

The interaction of these raises insects above mere mechanical automata, in which the reaction instantly follows the impression.
In all cases the action of every nerve-centre, no matter what its stage of development may be, high or low, depends upon an essential chemical condition--oxidation.

Even in man, if the supply of arterial blood be stopped but for a moment, the nerve-mechanism loses its power; if diminished, it correspondingly declines; if, on the contrary, it be increased--as when nitrogen monoxide is breathed--there is more energetic action.

Hence there arises a need of repair, a necessity for rest and sleep.
Two fundamental ideas are essentially attached to all our perceptions of external things: they are SPACE and TIME, and for these provision is made in the nervous mechanism while it is yet in an almost rudimentary state.

The eye is the organ of space, the ear of time; the perceptions of which by the elaborate mechanism of these structures become infinitely more precise than would be possible if the sense of touch alone were resorted to.
There are some simple experiments which illustrate the vestiges of ganglionic impressions.