fatigued it, is withdrawn; but that it still remains liable to be
excited into action by any other colours except the colour with which it
has been fatigued. Thus the yawning and stretching the limbs after a
continued action or attitude seems occasioned by the antagonist muscles
being stimulated by their extension during the contractions of those in
action, or in the situation in which that action last left them.

5. A quantity of stimulus greater than the last, or longer continued,
induces variety of convulsions or fixed spasms either of the affected organ
or of the moving fibres in the other parts of the body. In respect to the
spectra in the eye, this is well illustrated in No. 7 and 8, of Sect. XL.
Epileptic convulsions, as the emprosthotonos and opisthotonos, with the
cramp of the calf of the leg, locked jaw, and other cataleptic fits, appear
to originate from pain, as some of these patients scream aloud before the
convulsion takes place; which seems at first to be an effort to relieve
painful sensation, and afterwards an effort to prevent it.

In these cases the violent contractions of the fibres produce so much pain,
as to constitute a perpetual excitement; and that in so great a degree as
to allow but small intervals of relaxation of the contracting fibres as in
convulsions, or no intervals at all as in fixed spasms.

6. A quantity of stimulus greater than the last, or longer continued,
produces a paralysis of the organ. In many cases this paralysis is only a
temporary effect, as on looking long on a small area of bright red silk
placed on a sheet of white paper on the floor in a strong light, the red
silk gradually becomes paler, and at length disappears; which evinces that
a part of the retina, by being violently excited, becomes for a time
unaffected by the stimulus of that colour. Thus cathartic medicines,
opiates, poisons, contagious matter, cease to influence our system after it
has been habituated to the use of them, except by the exhibition of
increased quantities of them; our fibres not only become unaffected by
stimuli, by which they have previously been violently irritated, as by the
matter of the small-pox or measles; but they also become unaffected by
sensation, where the violent exertions, which disabled them, were in
consequence of too great quantity of sensation. And lastly the fibres,
which become disobedient to volition, are probably disabled by their too
violent exertions in consequence of too