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

by Erasmus Darwin

Increased Volition.

GENUS [I].

Increased Actions of the Muscles.

We now step forward to consider the diseases of volition, that superior faculty of the sensorium, which gives us the power of reason, and by its facility of action distinguishes mankind from brute animals; which has effected all that is great in the world, and superimposed the works of art on the situations of nature.

Pain is introduced into the system either by excess or defect of the action of the part. (Sect. IV. 5.) Both which circumstances seem to originate from the accumulation of sensorial power in the affected organ. Thus when the skin is exposed to great cold, the activity of the cutaneous vessels is diminished, and in consequence an accumulation of sensorial power obtains in them, because they are usually excited into incessant motion by the stimulus of heat, as explained in Sect. XII. 5. 2. Contrarywise, when the vessels of the skin are exposed to great heat, an excess of sensorial power is also produced in them, which is derived thither by the increase of stimulus above what is natural.

This accounts for the relief which is received in all kinds of pain by any violent exertions of our muscles or organs of sense; which may thus be in part ascribed to the exhaustion of the sensorial power by such exertions. But this relief is in many cases so instantaneous, that it seems nevertheless probable, that it is also in part owing to the different manner of progression of the two sensorial powers of sensation and volition; one of them commencing at some extremity of the sensorium, and being propagated towards the central parts of it; and the other commencing in the central parts of the sensorium, and being propagated towards the extremities of it; as mentioned in Sect. XI. 2. 1.

These violent voluntary exertions of our muscles or ideas to relieve the sensation of pain constitute convulsions and madness; and are distinguished from the muscular actions owing to increased sensation, as in sneezing, or coughing, or parturition, or ejectio feminis, because they do not contribute to dislodge the cause, but only to prevent the sensation of it. In two cases of parturition, both of young women with their first child, I have seen general convulsions occur from excess of voluntary exertion, as above described, instead of the actions of particular muscles, which ought to have been excited by sensation for the exclusion of the fetus. They both became insensible, and died after some hours; from one of them the fetus was extracted in vain. I have heard also of general convulsions being excited instead of the actions of the musculi acceleratores in the ejectio feminis, which terminated fatally. See Class [III. 1. 1. 7].

These violent exertions are most frequently excited in consequence of those pains, which originate from defect of the action of the part. See Sect. XXXIV. 1. and 2. The pains from excess and defect of the action of the part are distinguishable from each other by the former being attended with increase of heat in the pained part, or of the whole body; while the latter not only exist without increase of heat in the pained part, but are generally attended with coldness of the extremities of the body.

As soon as these violent actions of our muscular or sensual fibres for the purpose of relieving pain cease to be exerted, the pain recurs; whence the reciprocal contraction and relaxation of the muscles in convulsion, and the intervals of madness. Otherwise these violent exertions continue, till so great a part of the sensorial power is exhausted, that no more of it is excitable by the faculty of volition; and a temporary apoplexy succeeds, with snoring as in profound sleep; which so generally terminates epileptic fits.

When these voluntary exertions become so connected with certain disagreeable sensations, or with irritations, that the effort of the will cannot restrain them, they can no longer in common language be termed voluntary; but nevertheless belong to this class, as they are produced by excess of volition, and may still not improperly be called depraved voluntary actions. See Sect. XXXIV. 1. where many motions in common language termed involuntary are shewn to depend on excess of volition.