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SECONDLY,

OF

INDIRECT NERVOUS STIMULI,

SECT. LXXIX.

JAW III.

A DEFECTING EXCITEMENT OF THE MIND ACCU-,

MULATES NERVOUS ENERGY, WHILE IT AT THE

SAME TIME WEAKENS THE NERVES.

HAVING treated of direct nervous stimuli, and their effect in strengthening the powers of the understanding, when in due quantity, and their baneful effect when in excess, and having shewn that the nervous system was, in this respect, obedient to nearly the same laws as the fibrous, we are arrived now at that part of our subject which treats of indirect nervous stimuli, which, like other indirect stimuli, accumulates nervous energy.

That the nervous power is accumulated by defect of sensorial action, we have the most striking example in vision. It is, indeed, surprising how far the eye can accommodate itself to darkness, and make the best of a gloomy situation.

When first taken from light and brought into a dark room, all things disappear; but after a few minutes the pupil dilates, and the optic nerve Vol. IV.

5 N

having

having accumulated sufficient sensorial power many objects may be discerned.

Xenophon, in his account of the retreat of the ten thousand, mentions that the Persians and Greeks had always lights in their camps: but as they traversed Thrace, the king Seuthes had lights around his camp, which kept him concealed, and enabled him to perceive any person who should have the boldness to reconnoitre his camp, whereas the Persians and Greeks could only discover the spy in the camp itself. The reason of this is explained by the principle above.

There was a gentleman of great courage and understanding, who was a major under King CHARLES I. This unfortunate man, sharing in his master's misfortunes, and being forced abroad, ventured, at Madrid, to do his king a signal service ; but, unluckily, failed in the attempt. In consequence of this, he was instantly ordered to a dark and dismal dungeon, into which the light never entered, and into which there was no opening but by a hole at the top, down which the keeper put his provisions, and presently closed -it again on the other side. In this manner the unfortunate loyalist continued for some weeks, distressed and disconfolate ; but, at last, began to think he saw fome 8

little

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