A Mechanical Account of Poisons in Several Essays

by Richard Mead

From hence It will follow, That the Motion of the Arterial Fluid must be, Ceteris Paribus, more sedate, even and regular, in the time of Sleeping than Waking; For, besides the various Alterations which in the latter State this receives from the several Passions of the Mind, the very Contractions of the Muscles themselves in Exercises of the Body do differently forward its Course; whereas in Sleep the force of the Heart and Pectoral Muscles being more constant and uniform, gives it a more calm and equally continued Impulse.

Hence also it will come to pass, that the Influx of the Liquor of the Nerves into the Organs of the Body, as also Its Reflux towards the Brain, is in Sleep either none, or very inconsiderable; that is, that this Fluid has at this time but little or no Motion. For ’tis Muscular Action and Sensation that require It to be thus determin’d, this way or that, which are now hardly any. And yet by the arrival of Blood at the Brain, this Juice will still be separated there, fit to be derived into its Canals or Tubes. So that by this means there will be a kind of Accumulation, or laying up in Store, of Spirits for the Offices and Requirements of Waking.

Thus We may in short look upon the time of Watching, as the time of Wearing out, or the Destruction of the Animal Fabrick; and the time of Sleep, as that in which it is repaired and recruited; not only upon the account of what We have just mentioned concerning the Nervous Liquor, but also with respect to all the other Parts, as well Fluid as Solid. For Action does necessarily by Degrees impair the Springs and Organs; and in Motion something is continually abraded and struck off from the Distractile Fibres, which cannot otherwise be restored than by their being at rest from Tension. Besides that, such a regular and steady Course of the Blood, as we have observed to be in Sleep, is by far more fit and proper for Nutrition, or an Apposition of Parts to the Vessels, which an uneven Hurry of It is more apt to tear off and wash away.

The Case being thus, it is very plain that whatsoever can induce such a Disposition of the Fluids and Muscular Parts of the Body, as this We have described, will so far cause Sleepiness. And in like manner, when any thing interposes and hinders this Composedness and Tranquillity, the removing of the Impediment will be the causing of Sleep; inasmuch as this is only reducing the Animal Œconomy to its right State, in which by natural Order there must be a Succession of Sleeping and Waking.

Thus it appears how necessarily continued Exercises do make Us Sleepy, since These do exhaust the Juice of the Nerves; that is, both lessen its Influx into the Organs of Motion, and incline the Mind not to determine it any longer that way, upon the account of the Pain and Uneasiness, with which too violent a Tension of the Parst is always attended; which therefore we must needs desire to Relax, or lay to Rest.

That Sleepiness which follows upon a fulness of the Stomach after Eating or Drinking, is owing to a different Cause; and does indeed so nearly fall in with the Effects of Opiate Medicines, that it requires a particular Consideration.

As Hunger, or the Emptiness of the Stomach, is a painful Sensation; so the satisfying or removing of This, is a pleasing or agreeable One. Now all Pain is a Stimulus upon the Part affected; and This, we all know, being attended with Contractions of the pained Membranes, causes a greater Afflux than ordinary of the Nervous Juice that way. On the other Hand, Pleasure, or a delightful Sensation in any part, is accompanied with a smooth Undulation, and easie Reflux of the Liquor of the Nerves towards the Brain. This is, as it were, the Entertainment of the Mind, with which being Taken up, it does not Determine the Spirits to the Organs of Motion; That is, there is such a Relaxation of the Muscular Fibres, and such a Disposition of the Nervous Fluid, as we have observed to be necessary to Sleep.

This is the Reason of that Chilliness in the Limbs, which we commonly Complain of after a good Feast.

If it seem strange that a Pleasure in the Stomach should so powerfully Influence the Mind; let it be considered, on the other Hand, how violent Effects, an uneasie and disagreeable Sense in the same Part does produce; what a terrible Agony Two or Three Grains of Crocus Metallorum throws the whole Fabrick into; how readily the Fluid of the Nerves is with a more than Ordinary Impetus determin’d and commanded into the Muscles of the Stomach and Abdomen, in order to throw off the Enemy, and remove the ungrateful Sensation.

Now the Consequences which we have ascribed to a pleasing Sense in this Part, are only just the contrary of these we find the opposite Affection of Pain induces. And indeed Pleasure and Pain are Two great Springs of Action in the Animal Œconomy; The Changes they make in the Fabrick are the Causes of many Effects which seem surprising, because we do not regard the Mechanism by which they are produced: but these must be more considerable in the Stomach than any where else; This Part being, for very wise Purposes, of so acute a Feeling, that some Philosophers have for this Reason thought It to be the Seat of the Soul.