FIRST then, the[63]_VERTIGO_ is a deprav’d _Imagination_, attended with the vitiated SENSES of _Hearing_ and _Seeing_; proceeding from the violent Commotion of the _Animal Spirits_. _Secondly_, The _SCOTOMIA_ is also a deprav’d _Imagination_, accompany’d with loss of _Sight_, and sometimes of the _Motion_ of the _Animals_ affected with it, because of some _Interruption_ in their circular Passage. In the _first Case_, the _PATIENT_ imagines his Head only to be turn’d round, or winded about: In the _other_, he thinks that _circular Motion_ to consist in the _external Objects_.
NOW the _VERTIGO_ is attended with the vitiated SENSE of _Hearing_, as well as _Seeing_: Because, as something like a _Cloud_, _Smoak_, or _Web_, seems to appear before the PATIENT’s _Eyes_; so there is a certain _Whistling_, _Hissing_, or Tinkling always in his _Ears_. The SCOTOMIA, in like manner, is attended with Loss of _Sight_, and _Motion_; because of some _Disorder_ of the _Ventricles_ of the BRAIN obstructing or impeding the _Transition_ of the SPIRITS.
IN the VERTIGO, an _Agitation_ happens in the _Membranes_ of the VEINS and ARTERIES, as also in the _Membranes_ of the BRAIN; by which violent _Motion_, the very _Continent Parts_ are vehemently shaken and concuss’d, and at length so disquieted, that the PATIENT thinks his HEAD wheels round about.
IN the SCOTOMIA, the _Animal Spirits_ having in themselves the _Species_ of all Sensibles, and those _Species_ being presented to the _Imagination_: As such _Images_ of external _Objects_ are moved in it, so the _Party_ thinks the same _real Objects_ to be moved. For, according to _Avicen_, it is the same thing, whether that which is _seen_, or that by which we _see_, is moved: As seeing _Land_ from on board a _Ship_, in a smooth Sea, being insensible of the _Ship’s Way_, we imagine (and sometimes very strongly too) that the LAND is in _Motion_.
THAT by which we _see_, is the _visible Species_ reserved in the _Spirit_: Hence when this _Species_ is moved, the _external Object_ seems also to be _moved_.
BUT in explaining the _Circular Motion_ of the _Animal Spirits_, we must consider it to be _twofold_; namely, _Natural_ and _Preternatural_: The _Natural Motion_ is that which begins in the _Carotide Arteries_, thence tending to the _Plexus Choroideus_, or the _anteriour Ventricles_ of the BRAIN; from _These_ to the _middle_; from the _middle_ to the _Posteriours_; and from the _posteriour Ventricles_ of the BRAIN, the _Spirits_ are imparted to the NERVES. The _Preternatural Motion_ is just the _Reverse_ of this Case.