Sleep.
13. _Incubus._ Night-mare.
14. _Lethargus._ Lethargy.
15. _Syncope epileptica._ Epileptic fainting.
16. _Apoplexia._ Apoplexy.
17. _Mors a frigore._ Death from cold.

GENUS II.

_With decreased Actions of the Organs of Sense._

SPECIES.

1. _Recollectionis jactura._ Loss of recollection.
2. _Stultitia voluntaria._ Voluntary folly.
3. _Credulitas._ Credulity.

* * * * *

CLASS III.

DISEASES OF VOLITION.

ORDO I.

_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