PARALYSIS (a Greek word, raftsusis, which signifies literally a " loosening" or " relaxation") is the diseased condition in which the natural power of sensation or motion is lost in any part of tho body.
The principal forms of paralysis are dependent on the fact that although most of the nerves distributed through the various parts of the body contain filaments for performing the functions of motion and sensation within the same sheath, yet the two orders of filaments are distinct in their origins and arise from separate portions or tracts of the brain and spinal chord. If a sensitive nerve, or the tract of nervous matter from which the sensitive nerves of any part arise, be destroyed or seriously diseased, there will be a loss of sensation in that part, but its natural power of motion will remain ; if the same injury betel a motor nerve, or a centre of origin for motor nerves, the part supplied therefrom will lose its motion, hut retain its sensibility ; and if a mixed nerve, or both nervous centres simultaneously, be affected, there will be a loss at once of sensation and of motion. Hence we have two distinct kinds of paralysis—loss of sensation, which is some times called anwsthesia,snd loss of motion,to which the term paralysis, or palsy, is by some exclusively applied.
Each of these varieties of paralysis may vary in its degree of seve rity, or in the extent of the part of the body which it affects. For example, either kind may be complete or incomplete ; in the former, the sensation or loss of motion, or both, are completely destroyed ; in the latter, they are only impaired. la its varieties of extent, paralysis of sensation may affect either a single nerve, as in loss of sight when dependent on disease of the optic nerve [Aueunosis]; in loss of smell, or anosmia, from affection of the olfactory nerve; in deafness, from disease of the auditory nerve; and low of taste, from disease of the nerves appropriated to that function : or it may affect the sensitive nerves of a limb or of a variable portion of the body. In like manner paralysis of motion may affect a single muscle, as in peals, or dropping of the eyelid, from disease of the third nerve ; or it. may occur in a part of the muscles of the face, or the muscles of one or more limbs, or of a part of the trunk and limbs. Lastly, whole regions of the body may be paralysed; and of these cases the chief varieties are, hemiplegia, in which one side—half of the body—is deprived of sensa tion ur motion, or of both ; paraplegia, in which the lower part of the body is paralysed, while the upper retains both sensation and motion ; and general paralysis, in which the loss of nervous power extends over nearly every part of the body.
Other varieties of paralysis are described from peculiar circum stances in their cause or symptoms ; as kad-paley, which is produced by the influence of lead, either locally applied, as to the bands of painters, or received into the system generally ; creeping-palsy, which, commencing in a limited part of the body, gradually extends over a large portion of it; shaking-palsy, in which the loss of motion being Incomplete, any attempt at its exercise is effected by a trembling unsteady action, like that of a fatigued muscle.
The conditions under which these various forms of paralysis arise are numerous. Its most common and general causes are those which mechanically destroy that condition which is essential either to the conduction of sensitive impressions to the brain and to their percep tion by the mind, or to the conveyance of the stimulus of the will through the nerves of motion to the muscles. Thus pressure on the brain, by a fracture and depression of the skull [Heals, INJURIES os], or by a large effusion of blood [APOPLEXY], or by large tumours, or an excessive fulness of the arterial or venous system of the brain [Esc& eiratrris ; .MEarsGrris] may, by preventing the free circulation of the blood through every part of its substance, produce general paralysis, Disorganisation of the brain by softening or other excessive change of structure has the same effect, but often in a less degree, pro' luting not a complete loss, but an impairment of nervous power. injuries of the same kind affecting only one side of the brain produce aemiplegia, the loss of power existing on the side of the body opposite As that on which the brain is compressed, in consequence of the lecussation of the nerves which takes place at the medulla oblongata. :BRAIN, NAT. HIST. Div.] In like manner a similar compression of the spinal cord in any part will produce a paralysis of all the parts of the body whose nerves come aff below the level of the injured part; and a similar obstruction applied to a single nerve will 'affect only that part which it supplies. The effect is the same, whatever be the nature of the cause preventing the performance of the functions of the nerves or their centres ; the results of each differing only in the suddenness or slowness, or the degree of intensity with which its symptoms are produced.