Sleep, with an overpowering influence, wraps us into forgetfulness of ourselves. It envelopes in its sombre drapery, the votaries of pleasure and the subjects of sorrow. A helpless - inanimate - unconscious submission is yielded to its dominion, and a solemn calm rests alike upon all. It dispenses to the fatigued frame - refreshment; to the diseased body - a balm; and to the care-worn, a short oblivion of their woes.

Is this neutrality of our hopes and fears and passions undisturbed? Does this power, controlling our sensibilities, shield us from harm? Do we remain unmolested in this defenceless condition, regaining bodily activity and mental energy? Or is this state also subject to evils? Alas! it is; for dreams do come, that, in their varied structure, harass the inmost soul with dread, and

"On his nightmare through the evening fog, Flits the squab Fiend o'er fen, and lake, and bog."

Incubus and nightmare are terms applied to a disease, deeply interesting both to the physician and to the patient. To the physician, because no satisfactory explanation of its phenomena has been given, all being mere speculation not founded on facts, neither supported by correct pathological reasoning: and to the sufferer, for it seizes him under circumstances frightful in themselves, occurring mostly at dead of night, when assistance is least easily obtained, and he being rendered totally helpless.

It makes its attack on the system in that stage of sleep when the external functions are impaired, and the imagination is free from those restraints imposed upon it in the state of wakefulness by the judgment. The mind, roving through various scenes and producing effects only experienced in dreams, and best described by reference to them, arrives at a condition in which fear is the most prominent emotion. The dreamer often believes himself shipwrecked, and left to the fury of the winds and waves; or he is fast approaching the brink of a dread precipice, without the power to turn aside, and over which he must unavoidably fall; or he is pursued by wild beasts intent on devouring him, and through all, he feels spell-bound and unable to help or defend himself: he struggles with all his power to be released from this frightful situation, but apparently to no purpose, until at last when he considers his destruction inevitable, a sudden bound frees him from his condition, and a dream is disclosed, which he believes to have been the cause of his sufferings.

The effects produced by a paroxysm of incubus are very great. Immediately after recovery, the pulse is a little quickened; a tremor of the abdominal muscles is experienced, and extreme lassitude is felt throughout the whole frame. A want of energy characterizes this state; and as the first shades of sleep again descend upon him, he very perceptibly feels the approach of the disease a second time, but under different circumstances. From a lack of energy to change his position and shake off the predisposition now formed, he remains quiet, perfectly conscious of the advancing symptoms which are gradually stealing over and strengthening upon him, until the power of voluntary motion is again suspended, and he is in a condition differing from the first, inasmuch as he is conscious of his situation. He now attempts to change his position, but without success, and it is only after repeated trials that he succeeds in accomplishing his object. The same lassitude continues as after the first attack, and the second will be followed by a third with the same symptoms, and the third by a fourth, and so on until his exertions are sufficient to throw off the disease, by producing an action throughout the whole voluntary muscular texture.

There is no particular position necessary for the occurrence of this disease. It makes its attacks in any position that a person can lie in, and the sitting position is not exempt from its effects.

This disease may be divided into the first and second paroxysms, or that which is accompanied by a dream, and the succeeding attacks in which we comprehend our situation.

In addition to the feelings described in the first paroxysm, there is the sensation of a load upon the chest, and some fancy it a monster attempting to suffocate them. From this paroxysm very little information can be obtained; the person on awaking attributes all his sufferings to the dream which he considers the cause of his distress:

It is by attending strictly to the phenomena of the succeeding attacks, that a knowledge of the pathology of the disease can be obtained. In these attacks many faculties of the mind are active, and restrained to their proper course by the judgment. This is displayed in the exertion to move one part of the body and then another alternately, knowing that if we succeed, relief will be obtained. Neither is the memory dormant, for when previously we have been told that some of the senses are not impaired, and that we can exercise certain muscles, we are sure to make the trial. A person laboring under the second paroxysm, if the room be sufficiently light, can see whatever comes directly in front of him; he can hear the breathing of his bedfellow, and is conscious of conversation when it takes place in his presence; he has the sense of touch, for he is aware of his contact with the bed-clothes, and also of irregularities in their position under him; he can move his tinder jaw with ease perpendicularly, but has no power to produce a lateral motion; he knows that he breathes, but with much difficulty; he has the power of natural voice, but not of speech, and volition is perfect, but the organs of locomotion are not obedient to its mandates.

These are the facts upon which we are to determine its pathological character, and upon which we are to build our hope of success in removing the obscurity hitherto surrounding the location and cause of this disease.

The remote causes of this affection are violent mental agitation - fear - also great fatigue of body, and in truth whatever wearies the mind or body beyond their healthy endurance. But the most prolific cause is overloading the stomach before retiring to rest. The proximate cause consists in one part of the system requiring a greater amount of nervous power than is naturally appropriated to it; and as the extremities of the nerves cannot provide the deficiency, this extra quantity must be supplied from some other part; consequently the part from which the deficiency is made up is left in an unnatural condition, and thus results this affection.