3.  When by our continued posture in sleep, some uneasy sensations are produced, we either gradually awake by the exertion of volition, or the muscles connected by habit with such sensations alter the position of the body; but where the sleep is uncommonly profound, and those uneasy sensations great, the disease called the incubus, or nightmare, is produced.  Here the desire of moving the body is painfully exerted, by the power of moving it, or volition, is incapable of action, till we awake.  Many less disagreeable struggles in our dreams, as when we wish in vain to fly from terrifying objects, constitute a slighter degree of this disease.  In awaking from the nightmare I have more than once observed, that there was no disorder in my pulse; nor do I believe the respiration is laborious, as some have affirmed.  It occurs to people whose sleep is too profound, and some disagreeable sensation exists, which at other times would have awakened them, and have thence prevented the disease of nightmare; as after great fatigue or hunger with too large a supper and wine, which occasion our sleep to be uncommonly profound.  See No. 14, of this Section.

4.  As the larger muscles of the body are much more frequently excited by volition than by sensation, they are but seldom brought into action in our sleep:  but the ideas of the mind are by habit much more frequently connected with sensation than with volition; and hence the ceaseless flow of our ideas in dreams.  Every one’s experience will teach him this truth, for we all daily exert much voluntary muscular motion:  but few of mankind can bear the fatigue of much voluntary thinking.

5.  A very curious circumstance attending these our sleeping imaginations is, that we seem to receive them by the senses.  The muscles, which are subservient to the external organs of sense, are connected with volition, and cease to act in sleep; hence the eyelids are closed, and the tympanum of the ear relaxed; and it is probable a similarity of voluntary exertion may be necessary for the perceptions of the other nerves of sense; for it is observed that the papillae of the tongue can be seen to become erected, when we attempt to taste any thing extremely grateful.  Hewson Exper.  Enquir.  V. 2. 186.  Albini Annot.  Acad.  L. i. c. 15.  Add to this, that the immediate organs of sense have no objects to excite them in the darkness and silence of the night, but their nerves of sense nevertheless continue to possess their perfect activity subservient to all their numerous sensitive connections.  This vivacity of our nerves of sense during the time of sleep is evinced by a circumstance, which almost every one must at some time or other have experienced; that is, if we sleep in the daylight, and endeavour to see some object in our dream, the light is exceedingly painful to our eyes; and after repeated struggles we lament in our sleep, that we cannot see it.  In this case I apprehend the eyelid is in some degree opened by the vehemence of our sensations; and, the iris being dilated, the optic nerve shews as great or greater sensibility than in our waking hours.  See No. 15. of this Section.