Zoonomia; Or, the Laws of Organic Life, Vol. II

by Erasmus Darwin

Decreased Action of the Organs of Sense.

SPECIES.

[1]. Stultitia inirritabilis. Folly from inirritability. Dulness of perception. When the motions of the fibrous extremities of the nerves of sense are too weak to excite sensation with sufficient quickness and vigour. The irritative ideas are nevertheless performed, though perhaps in a feeble manner, as such people do not run against a post, or walk into a well. There are three other kinds of folly; that from deficient sensation, from deficient volition, and from deficient association, as will be mentioned in their places. In delirium, reverie, and sleep, the power of perception is abolished from other causes.

[2]. Visus imminutus. Diminished vision. In our approach to old age our vision becomes imperfect, not only from the form of the cornea, which becomes less convex, and from its decreased transparency mentioned in Class [I. 2. 3. 26].; but also from the decreased irritability of the optic nerve. Thus, in the inirritative or nervous fever, the pupil of the eye becomes dilated; which in this, as well as in the dropsy of the brain, is generally a fatal symptom. A part of the cornea as well as a part of the albuginea in these fevers is frequently seen during sleep; which is owing to the inirritability of the retina to light, or to the general paresis of muscular action, and in consequence to the less contraction of the sphincter of the eye, if it may be so called, at that time.

There have been instances of some, who could not distinguish certain colours; and yet whose eyes, in other respects, were not imperfect. Philos. Transact. Which seems to have been owing to the want of irritability, or the inaptitude to action, of some classes of fibres which compose the retina. Other permanent defects depend on the diseased state of the external organ. Class [I. 1. 3. 14]. [I. 2. 3. 25]. [IV. 2. 1. 11].

[3]. Muscæ volitantes. Dark spots appearing before the eyes, and changing their apparent place with the motions of the eyes, are owing to a temporary defect of irritability of those parts of the retina, which have been lately exposed to more luminous objects than the other parts of it, as explained in Sect. XL. 2. Hence dark spots are seen on the bed-clothes by patients, when the optic nerve is become less irritable, as in fevers with great debility; and the patients are perpetually trying to pick them off with their fingers to discover what they are; for these parts of the retina of weak people are sooner exhausted by the stimulus of bright colours, and are longer in regaining their irritability.

Other kinds of ocular spectra, as the coloured ones, are also more liable to remain in the eyes of people debilitated by fevers, and to produce various hallucinations of sight. For after the contraction of a muscle, the fibres of it continue in the last situation, till some antagonist muscles are exerted to retract them; whence, when any one is much exhausted by exercise, or by want of sleep, or in fevers, it is easier to let the fibres of the retina remain in their last situation, after having been stimulated into contraction, than to exert any antagonist fibres to replace them.

As the optic nerves at their entrance into the eyes are each of them as thick as a crow-quill, it appears that a great quantity of sensorial power is expended during the day in the perpetual activity of our sense of vision, besides that used in the motions of the eye-balls and eyelids; as much I suppose as is expended in the motions of our arms, which are supplied with nerves of about the same diameters. From hence we may conclude, that the light should be kept from patients in fevers with debility, to prevent the unnecessary exhaustion of the sensorial power. And that on the same account their rooms should be kept silent as well as dark; that they should be at rest in an horizontal posture; and be cooled by a blast of cool air, or by washing them with cold water, whenever their skins are warmer than natural.

[4]. Strabismus. Squinting is generally owing to one eye being less perfect than the other; on which account the patient endeavours to hide the worst eye in the shadow of the nose, that his vision by the other may not be confused. Calves, which have an hydatide with insects inclosed in it in the frontal sinus on one side, turn towards the affected side; because the vision on that side, by the pressure of the hydatide, becomes less perfect; and the disease being recent, the animal turns round, expecting to get a more distinct view of objects.

In the hydrocephalus internus, where both eyes are not become insensible, the patient squints with only one eye, and views objects with the other, as in common strabismus. In this case it may be known on which side the disease exists, and that it does not exist on both sides of the brain; in such circumstances, as the patients I believe never recover as they are now treated, might it not be adviseable to perforate the cranium over the ventricule of the affected side? which might at least give room and stimulus to the affected part of the brain?