535 



PARALLELS. 



PARALYSIS. 



286 



affirmative, it does not follow that comparisons of infinites can be 

 successfully introduced into elementary teaching. 



PARALLELS, in the attack or siege of a place (SlEGE), are wide 

 trenches affording the besieging troops a free covered communication 

 between their batteries and approaches, and a protection for the guard 

 of the trenches and the covering parties in the day time. What were 

 in former times termed lines of contravallation are now termed 

 parallels. There are usually three or four parallels made in the attack 

 of a place, though of course the number will depend wholly on the 

 strength of , and the artillery possessed by, the garrison ; the saliency of its 

 main works, and of ite outworks and detached works, should it possess 

 any ; the vigour of the defence and the nature of the ground in the 

 neighbourhood, whether providing good natural cover for bodies of 

 covering troops or not. Though the siege of different places will vary 

 much according to these points, methods of attack are laid down 

 in works on fortification on certain assumptions, in order to give an 

 idea of the general method, and from which to deduce a standard of 

 strength of different systems. In these there are generally three 

 parallels ; the first at 600 yards from the place, another at about 300 

 yards, and a third at the foot of the glacis. The second parallel is con- 

 structed by Jt>/ii>;/ tap, the third by regular sap, while the first, being 

 supposed out of the range of musquetry, grape-shot, and canister, by 

 ordinary trench-work. These distances and methods may be much 

 modified in practice, especially by the introduction of arms of pre- 

 cision, but as an assumption either for instruction or for comparing 

 different systems they are still retained. The method of tracing, fte., 

 will be found under SIEGE. 



PARALYSIS (a Greek word, irapi\vans, 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 the 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 

 befal a motor nerve, or a centre of origin for motor nerves, the part 

 supplied therefrom will lose its motion, but 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 anmthesi'i, and 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. In 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 [AMACROSIS] ; in loss of smell, 

 or anosmia, from affection of the olfactory nerve ; in deafness, from 

 disease of the auditory nerve ; and loss 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 ptotit, 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, 

 hemijileyia, in which one side half of the body is deprived of sensa- 

 tion or motion, or of both ; parapley!a,'m 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 circuin- 

 rtances in their cau.se or symptoms ; as lead-palsy, which is produced 

 by the influence of lead, either locally applied, as to the hands of 

 painterri, or received into the system generally ; cree/Ji'iijr-palsy, which, 

 "ncing in a limited part of the body, gradually extends over a 

 large portion of it ; diaL-iny-joHny, 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. 



Tli.- 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 [HEAD, INJURIES OF], 

 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 [ENCE- 

 PHALITIS ; MENINGITIS] 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 



ducing not a complete loss, but an impairment of nervous power. 

 Injuries of the same kind affecting only one side of the brain produce 

 lierniplegia, the loss of power existing on the side of the body opposite 

 to that on which the brain is compressed, in consequence of the 

 decussation 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 

 off 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. 



There are some other but rarer and generally less serious cases of 

 paralysis, in which no material change is discoverable in the structure 

 of the nervous system. These are called functional or idiopathic 

 paralysis ; but it may be reasonably doubted whether they do not all, 

 or for the most part, depend on some alteration not yet discovered, 

 and perhaps inappreciable by our present means of investigation. Such 

 cases occur in some anomalous forms of nervous diseases, as iu hysteria, 

 and appear to be connected by sympathy with disorder of the 

 uterine or digestive functions; they are also not unfrequently pro- 

 duced by the introduction of poisons, as in those who work with 

 lead or mercury. 



General paralysis, in which all sensation and voluntary motion are 

 impaired or lost, is most commonly the result of apoplexy, or of 

 severe injury to the head, producing concussion or compression of 

 the brain ; and indeed it may be said to exist in all cases of complete 

 coma, or insensibility. More rarely it is produced gradually ; the 

 patient losing in succession the power of motion in all his muscles, and 

 at last existing only with an internal life, but disabled from all active 

 communication with the external world. This state is usually the 

 result of disease of the brain or spinal cord gradually spreading 

 through their substance ; and especially of disease of the latter spread- 

 ing upwards. It not unfrequeutly comes on in the inmates of lunatic 

 asylums. 



The most frequent if not the only cause of paraplegia is injury or 

 disease of the spinal cord ; and this may originate in its own struc- 

 ture, or be produced by injuries or diseases of the vertebral column or 

 other parts surrounding it. Paraplegia, when the result of disease, 

 commonly affects first the lower extremities and the parts below the 

 level of the haunches, because the first part affected in the greater 

 number of diseases is the lower portion of the spinal cord, frcm 

 which the nerves supplying the lower extremities and pelvic organs 

 are derived. When produced by injuries of the vertebral column 

 however, the paraplegia may from the first affect the body to a much 

 higher level ; its extent being always determined by the height in 

 the vertebral column at which the injury is inflicted. In either case 

 the disease, whether original or consequent upon injury, tends to 

 spread up the cord ; and with a corresponding progress, the paralysis 

 rises up the trunk, and affects successively the chest and arms, termi- 

 nating in death when the disease has reached the origin of the 

 phrenic nerves, upon which the movements of the diaphragm 

 depend. 



In this form of paralysis, when all sensation and voluntary motion 

 are destroyed, involuntary movements are produced by irritating the 

 skin of the insensible parts. These motions depend on the reflex action 

 of the spinal cord [NERVOUS SYSTEM, NAT. HIST. Div.] ; the power 

 of conducting impressions to aud from the brain is lost, but the power 

 which the cord possesses of exciting motions when impressions are 

 made upon it, remains. The patient thus affected is indeed, as far as 

 his nervous system is concerned, in the same condition as a beheaded 

 animal ; and the actions produced by irritating the parts below the 

 divided or injured portion of the cord are of the same kind as those 

 observed after decapitation. 



ID paraplegia the parts deprived of nervous connection with the 

 brain are, even more than in any other form of paralysis, liable to 

 mortification when long subjected to pressure ; and hence the slough- 

 ing of the back, which so commonly tends to shorten the patient's life. 

 The sensibility to the pain produced by long-continued pressure being 

 destroyed, the patient would not be inclined, even if he were able, to 

 make those frequent changes of position by which a healthy person 

 avoids its dangers. 



Hemiplegia, in which the paralysis is confined to one side of the 

 body, usually affects (as paraplegia does) the sensitive and the motor 

 nerves simultaneously. This is by far the most frequent form of para- 

 lysis, and is that by which those popularly called paralytic are usually 

 affected. Hemiplegia generally occupies exactly one-half of the body, 

 the middle vertical plane separating the healthy from the paralytic 

 portion. It is at once easily recognised in any patient by the flatness 

 and smoothness of the affected side of the face, by the angle of the 

 mouth being drawn over towards the opposite side, and by a general 

 wry position of the features. The arm hangs powerless by the side ; 

 and in walking, the patient drags the affected leg after him, or, 

 raising it from the ground by depressing the opposite side of the 

 pelvis, lets it swing forward with its toe pointed towards the ground 

 like a pendulum. Hemiplegia is most frequently the consequence of 



