NERVOUS SYSTEM. 



NERVOUS SYSTEM. 



It deserves particular remark, that in the cases in which the impres- 

 sion is conveyed only to the spinal chord, as in beheaded animals, the 

 motions that result from the reflected influence evince design. They 

 are in fact instinctive. Thus, when a bird's or any other lower animal's 

 (as a frog's) head is cut off, the body endeavours to escape, and evinces 

 method in its attempts ; if a limb is touched, it is drawn away from 

 the contact ; if a part of the body is irritated, the foot will sometimes 

 be raised in an endeavour to remove the source of irritation. These 

 actions are similar to those of monsters born without brains, many of 

 which have been known to cry, to take milk, and perform many other 

 instinctive acts. From such facts as these it is inferred that the brain- 

 less animal* perform all their functions unconsciously. We can however 

 draw a distinction between the functions performed under the influence 

 of the action of the nerves of special sense (the sensory ganglia) and 

 those performed by the ganglia supplying the stomach, heart, lungs, 

 and other organs. 



Of the nature of the agent which, passing along the nervous fila- 

 ments, produces sensation or excites motion, nothing satisfactory is at 

 present known. The imperceptible velocity of its passage naturally 

 suggested the idea of an electric current, and except by the supposition 

 of the motion, or the vibration of the particles of some such subtle 

 material as electricity or the other imponderable agents are conceived 

 to be, its velocity is scarcely to be accounted for. Some idea may be 

 formed of its rapidity of passage along the nerves to and from the 

 brain, by a rough calculation which the writer lately made, that when 

 a person plays rapidly on the piano, upwards of 1000 distinct move- 

 ments are performed within the minute ; each of which baa its time, 

 place, and strength exactly ordered. At the same time that the mind 

 imparts this number of impressions to the muscles, it is receiving not 

 less than 2000 impressions by each of three distinct senses, the eye, 

 the ear, and the touch, and perceives the most minute variations with 

 each. From the similarity of this velocity of the nervous agent, to 

 that of the passage of electricity, and from a few apparent analogies, 

 some have imagined the nervous agent, or fluid, to be identical with 

 the electric ; but the grounds for such an opinion are at present few 

 and uncertain, while many evident facts militate strongly against it, 

 as the equal conducting power of all the moist tissues as well as the 

 nervous ; the improbability that electric fluid should be isolated in 

 the filaments, the inconstancy of the results of experiments in which 

 a current of electricity is used to replace a removed portion of a 

 nerve, ic. 



The more probable theory is that the nervous force is correlative 

 with the forces engaged in the nutrition of the body. We find light 

 and heat necessary to the production of the materials from which 

 the nervous system is formed. These are correlative with the chemical 

 forces brought into action in forming the compounds of our food, as 

 sugar, starch, and protein. The sugar and starch, on the dissolution 

 of their physical conditions in contact with oxygen, yield heat to the 

 body ; and it is quite as probable that protein, under the same circum- 

 stances, should exhibit muscular and nervous force. At any rate this 

 theory is rendered very probable by a large and increasing number 

 of facts. In animals capable of developing electricity it is clearly 

 correlative with the nervous force. [ELECTRICITY IN ORGANIC BKINUS.] 



The nervons force appears to be generated in the brain, spinal chord, 

 and in all parts where there is ganglionic or gray nervous matter, and 

 from these centres distributed to the nerves. Thus, if the trunk of 

 a mixed nerve be divided, that part which is separated from the 

 nervous centre soon loses that which may be called its stock of excita- 

 bility, while that which remains attached to the centres retains its 

 excitability as if no injury had been inflicted. For the maintenance 

 of the excitability on which the reflex actions depend, the spinal chord 

 alone seems to be necessary; for it is not more rapidly expended 

 after the removal of the brain than when the brain is present. For 

 the maintenance of the excitability for other actions the brain is 

 essential. 



Each impression made on the sensitive nerves, and each excitant to 

 motion, may be considered to cause a certain consumption of the 

 nervous influence, which it is the office of the nervous centres to 

 replace ; and a healthy condition of the nervous system may reason- 

 ably be conceived to depend on a due proportion between the waste 

 and the supply. When the former has been excessive, weakness or 

 fatigue of the senses, or of the power of muscular motion, is produced, 

 which a period of sleep or rest from excitement is necessary to replace. 

 The necessity of such rest is indicated to us by the fatigue at the 

 of each day, and cannot long be safely resisted ; for after the 

 oss of the night's rest, the excitant necessary to produce a certain 

 effect is found fa become greater in a rapidly increasing ratio through 

 every hour of the succeeding day. During rest the brain may be con- 

 sidered as producing the excitability by which the nerves may act 

 during the period of exertion ; and hence exertion is not less necessary 

 for health than rest ; and may disorders show that excitability may, 

 for want of being wanted by exertion, accumulate. Hence much of 

 that which passes under the popular name of nervousness a condition 

 in which a given excitant produces a greater effect than is natural or 

 healthy ; and this (although the use of terms usually applied to 

 material things may give too definite an idea of it) we may reasonably 

 believe to result from an accumulation of nervous influence, as fatigue, 

 or the need of a greater excitement to produce a certain effect, results 



from its waste. The influence of exercise of the nerves follows the 

 same rule as that of exercise of the muscles or any other tissue ; by 

 it, within certain limits, the power of perceiving impressions and of 

 exciting motion is progressively increased ; the excitability of each 

 organ or of the whole system being, within those limits, capable of 

 adaptation to the need of the individual ; heuce the power by practice 

 of attaining to perfection of touch, or of hearing, or sight, or any 

 other sense. Impressions are distinctly felt by the practised sense 

 which are completely imperceptible to that which has been only 

 casually employed. In like manner, when the organ of one sense is 

 destroyed, and thus one outlet for excitability is closed, the rest acquire 

 increased acuteness ; hence the accuracy of the hearing and touch in 

 the blind, of the sight in the deaf, &c. 



The study of the functions of the spinal chord in health is especi- 

 ally important in relation to the numerous diseased actions of the 

 nervous system. In the previous remarks we have seen that the 

 spinal chord is the seat of an important series of reflex functions. It 

 must not however be supposed that the spinal chord alone is the seat 

 of reflex functions, the masses of nervous matter which lie at tho 

 base of the brain, the medulla oblongata, and the nervous centres 

 giving origin to the nerves of special sense, and the brain itself, are all 

 seats of special reflex functions. The reflex actions more especially 

 presided over by the spinal chord in man, appear to be the expulsive 

 movements of the organs of organic life and the muscular movements 

 of the posterior extremities. The expulsive movements alluded to, 

 and which can be performed independent of any action of the brain 

 or will, are defecation, urination, parturition, and other functions con- 

 nected with the reproductive processes. These functions are performed 

 in diseased conditions such as apoplexy, and in the anaesthetic state 

 produced by chloroform, or ether, without the consciousness of the 

 individual. So also the movements of the posterior extremities occur 

 as the result of stimulation when the individual is entirely uncon- 

 scious. Amongst the lower animals muscular movements are manifested 

 in the Invcrlebrata after the removal of the cephalic ganglia, and in. 

 the Va-tebrata after the destruction of the brain. Thus frogs will 

 leap when irritated after their brains are removed, and birds are known 

 to fly after their heads have been cut off. From these facts we may 

 infer that the muscular movements in man, of the lower extremities 

 especially, may be the result of the reflex action of nerves proceeding 

 from the spinal chord, and supplying these parts. It is very certain 

 that the movements of the body are continued in walking whilst the 

 mind is entirely abstracted. 



In morbid conditions of the nervous system, the functions of the 

 spinal chord are frequently deranged. In many diseases convulsions 

 occur, which are the result of the derangement of the spinal chord 

 and its nerves. [CONVULSIONS, in ARTS AND Sc. Div.] Convulsive 

 diseases may arise from three causes : 1, from irritation of the exoitor 

 nerves ; 2, from deranged or morbid conditions of the spinal chord 

 or medulla oblongata ; or 3, from a combination of these two. Such 

 conditions of the nervous system are seen in the convulsions of hydro- 

 phobia, tetanus, epilepsy, and hysteria. When these diseases destroy 

 life, it is usually by suspending the respiratory movements, the muscles 

 effecting which being fixed by the spasms prevent the ingress and 

 egress of air to and from the lungs. 



The Medulla Oblongata [BRAIN] is distinct from the spinal chord 

 and the brain, although it has an intimate relation with both. From 

 this relationship arises one of its special functions, which appears to be, 

 to bring the parts of the nervous system above and below it into 

 intimate connection. It also serves as a centre for the reflex opera- 

 tions of the nerves which issue from it. These reflex actions are 

 connected with the most important functions in the life of animals 

 Ueglvtition and Respiration. This explains the fact that the activity 

 of almost every other portion of the nervous system may be suspended, 

 and yet life continue, but should the functions of the medulla 

 oblongata cease, death ensues. 



" The chief excitor nerve of the Respiratory Movements, is the 

 afferent portion of the par vagum ; but the afferent portion of the 

 fifth pair is also a powerful excitor ; and the afferent portions of all 

 the spinal nerves, conveying impressions from the general surface of 

 the body, are also capable of contributing to the excitement necessary 

 for the production of the movement. The chief motor nerves are the 

 phrenic and intercostals, which, though issuing from the chord at a 

 considerable space lower down, probably originate in the medulla 

 oblongata. The motor portions of several other spinal nerves are also 

 partly concerned, as are also the facial nerve, the motor portion of tho 

 par vagum, and the spinal accessory. The ordinary movements of 

 respiration involve little action of any motor nerves but the phrenic 

 and intercostal ; and it is only when an excess of the stimulus (pro- 

 duced for example by two long a suspension of the aerating process) 

 excites extraordinary movements, that the nerves last enumerated are 

 called into action. 



" The acts of prehension of food with the lips, and of mastication, 

 though usually effected by voluntary power in the adult, seem to be 

 capable of taking place as a part of the reflex operation of the medulla 

 oblongata in the infant, as in the lower animals. This is particularly 

 evident in the prehension of the nipple by the lips of the infant, and 

 the act of suction which the contact of that body (or of any resembling 

 it) seems to excite. The experiments provided for us by nature, in 



