_An enquiry concerning the real cause of the Night-mare._
Having mentioned the most remarkable opinions, that have occurr’d to
me concerning the cause of this Disease, and shewn them all defective,
I shall next consider several circumstances attending an horizontal
position of the Body in sleep, in which alone this disease is felt; and
endeavour from thence to investigate the real cause of it.
Sleep is the balmy anodyne of nature; and was intended, by the all-wise
Author of our being, to ease the toils of the body, dispel the cares of
the mind, and to repair the losses sustain’d by the fatigue of the day.
In it we see every external stimulus remov’d, the Senses lock’d up,
and every Muscle relax’d, except the Heart, the Sphincters, and those
concern’d in respiration. Nutrition is then principally perform’d, and
then only the Fluids glide equably through the Vessels.
As many of the voluntary Muscles are imployed in keeping the Body
erect, ’tis necessary that the Body should be in an inclin’d or
horizontal position, in order to relax them, and promote the salutary
end of sleep. Accordingly we find, that most of the brute, as well as
the human Species, chuse some easy posture of this kind to sleep in.
When the human Body lies horizontally, the Blood must flow in greater
abundance to the Head; and with a greater momentum, cæteris paribus,
through the Carotid and Vertebral Arteries, than when the Body stands
erect; because the Blood, moving through these tubes in an horizontal
direction, will not so much resist the force of the Heart, as when it
ascends perpendicularly contrary to its own gravity.
No one, I presume, will doubt the truth of this proposition, who
reflects, that it is much easier to move any spherical body on
an horizontal plane, than to raise the same body up against a
perpendicular wall.
Neither will it be denied, that the quantity as well as the velocity of
Blood, flowing into the Carotid and Vertebral Arteries, is increas’d by
the horizontal position of the Body, if it be consider’d, that these
tubes (particularly the left Carotid) arise from, and proceed almost
parallel with the axis of the Aorta, where the velocity of the Blood
rushing out of the Heart is greatest. Whence it follows, from Sir Isaac
Newton’s second general law of motion, and from a well known axiom in
hydraulics, that these Arteries must receive more Blood in the same
time, than any other branches of the Aorta of the same diameter.
As the Blood must lose most of the motion which it receives from the
Heart, in passing through the infinite vascular ramifications, and
fine filtres of the Brain, there scarce appears, even in an erect
position of the Body, any propelling power to push it back again to the
Heart, except we admit the pulsation of the small Arteries belonging
to the coats of the Sinuses, and its own gravity. But in an horizontal
position, the Blood has not the advantage of its gravity to accelerate
its motion through the Jugular Veins; therefore it must move slower,
and must be more subject to obstruction in the vessels of the Brain.
Hence we see the use of pillows is to promote and facilitate the return
of the Blood through the Jugular Veins: hence we may also observe,
the uneasiness and danger attending the too common method of making
the feet of beds higher than the heads, since a stoppage of the Blood
is always productive of dangerous consequences; of which any one may
be soon convinc’d by stooping the Head for a short time; and it will
appear, that the Blood is by this means collected in the Veins of the
Face, which will produce a Vertigo, and, if long continued, may bring
on an Apoplexy. Hence we sometimes hear of people dropping down dead,
upon stooping to buckle their shoes. These instances should deter some
from putting their pillows under their feet, in order to make the Blood
settle in their faces, and to decorate the external part of their Heads
at the expence of the internal.
Notwithstanding the inconveniences and bad effects which may arise
from the Blood’s delay in the Brain, yet, its being sent to the Head
in sleep in a greater quantity, may serve many necessary purposes,
and render sleep more beneficial and refreshing to animals. First,
by distending the Blood-vessels of the Cerebrum, increasing the
pressure on that part, and by that means producing sleep. Secondly, by
promoting the secretion, and preparing a store of animal spirits to
supply the expence of the ensuing day. Thirdly, by gently encreasing
the pressure of the Blood-vessels on the Cerebellum, and perhaps
determining a greater quantity of the nervous influence to the Heart,
respiratory Muscles, and other parts, whose Nerves spring from that
fountain of life. This pressure on the Cerebellum may concur with the
rarefaction of the fluids, to render the motions of these organs more
regular and vigorous in sleep.
To this mechanical pressure on the Cerebellum, the illustrious Van
Sweiten seems to attribute the motion of the Heart: “Cerebelli enim
actio in Cor per Nervos, pendet ab ipsa actione Cordis per Arterias[8].”
Tho’ the contraction of the Heart is evidently the efficient cause of
the Blood’s motion, and consequently of the secretion of these spirits
in the Cerebellum, yet, without these spirits, the action of the Heart
could not be performed. These two causes appear to act in a circle, and
mutually depend on each other. Hence Hippocrates divin’d, ὁλον το ζωμα
κυκλος εστι. These also convey the idea of a perpetuum mobile; since,
as long as life lasts, an animal is really such, and far excels any
machine that human art has been yet able to make, or (in the opinion of
many philosophers) will ever invent.
The laborious Hoffman ascribes a great deal to this pressure on the
Brain, where he says, “[9]Declivior cubitus sanguinis regressum
quodammodo impedit, quia per venas jugulares descendere debet, quod
elatiori capite commodius peragitur. Hinc, capite nimis demisso ac
depresso, profundiores somnii cum insomniis, fiunt, universo corpore
torpor inducitur. Eadem ratione, si quis facie prona velut in mensa,
in somnum delabitur. Ob difficiliorem sanguinis regressum, gravitatem
capiti, et ingenio stupiditatem accersit.”
“[10]Sed etiam mechanicæ causæ somnum producunt, compressio nempe Duræ
Matris, aut Cerebri, quæcunque nata a Sanguine effuso, inpacto Osse,
aquæ in Ventriculis copia.”
These, I hope, are sufficient to shew how far the motion of the fluids
may be affected by the horizontal position of the Body; which, if duly
consider’d, might be of great service in the practice of Physic; and
perhaps many effectual derivations might be made, without drawing a
drop of Blood. I saw a remarkable instance of this kind in a gentleman
of a full habit, who, being ill of a Fever, talk’d rationally and rav’d
alternately, as his head was elevated or depress’d. In acute Diseases,
when the motion of the Blood is very rapid through the whole Body, the
Brain must suffer greatly, on account of the horizontal position, to
which people in such cases are confin’d; because, the Blood rushing
violently into the Arteries of the Brain, and its return being retarded
by the Jugular Veins, will remarkably contribute to produce delirious
symptoms, so frequent in acute Disorders, which might be in some
measure prevented, by raising the Head; for, by that means, the motion
of the Blood through the Jugular Veins will be increas’d, the pressure
on the Brain will be eas’d, and a safe and sudden derivation from
the Head may be made, which may produce very happy effects, where no
evacuation could be safely attempted.
Let us next take a view of the Heart, and consider how it may be
affected by the various positions of the Body, particularly the supine
one, in which the Night-mare generally invades.
The Heart is placed above the Diaphragm: the greater part of it lies
in the left cavity of the Breast: its apex or point is turn’d towards
the extremity of the sixth true Rib, where its pulsations are commonly
felt: it adheres to the Lungs by its large vessels, and is connected to
the Diaphragm by the Pericardium[11].
Thus the Heart is suspended in the Breast; and therefore must be
subject to the laws of pendulous bodies, which alter their situation
according to the different directions of their centers of gravity.
From the above just description of the human Heart, ’tis evident, that
when the Body is erect, the parts of the Heart which are commonly
called the right and left, ought to be more properly call’d the
anterior and posterior.
Hence, when the Body is plac’d on the Back, these become the superior
and inferior parts of the Heart.
That the Heart alters its situation in the Breast according to the
different positions of the Body, and the different directions of its
center of gravity, may be prov’d by the following easy experiments.
If the Finger be applied to that part of the Ribs where the pulsation
is felt in an erect position; and if, at the same time, the Diaphragm
be contracted strongly, the beatings become immediately weaker, because
the Heart is pulled downwards by the Diaphragm.
If one lies on the left side, the point of the Heart is felt beating
nearer the Spine of the Back; if we turn on our Backs, it is scarce
perceptible; and if we lie on the right side, it intirely vanishes.
These alterations of the Heart’s situation in the Breast, are more
remarkable in some persons than in others; and in general I have found,
by repeated tryals, that they were most considerable in those who were
most subject to the Night-mare.
When the Body lies supine, the Heart necessarily falls on the Vertebræ
of the Spine; and therefore, by its own gravity, must compress the left
Auricle and Pulmonary Veins, which, at that time, lie directly under
its basis; and, by that means, the course of the Blood through the
Lungs will be stop’d. Thus the Blood will be collected in the Pulmonary
Vessels, and the right, or rather superior Ventricle, not being able
to discharge itself into the Pulmonary Artery, will be oppressed by
the Blood returning from the Extremities; which, being gather’d in
the vessels about the superior part of the Heart, will increase its
gravity, and consequently augment the cause of the obstruction. In
this manner the return of the Blood from the Head will be prevented,
the tender dilatable vessels of the Brain will be over-distended,
the nervous influence obstructed, and the vital motions, in a great
measure, if not altogether, stopt. This I take to be a real fit of the
Night-mare, and in this manner it appears to be produc’d.