Volume II Part 18 (1/2)

It will not suit my purpose to offer more than the simplest ill.u.s.tration of the application of the foregoing facts. When an impression, either by pressure or in any other way, is made on the exterior termination of a centripetal fibre, the influence is conveyed with a velocity such as has been mentioned into the vesicle to which that fibre is attached, and thence, going forth along the centrifugal fibre, may give rise to motion through contraction of the muscle to which that fibre is distributed.

[Sidenote: Reflex action of the nervous system.] An impression has thus produced a motion, and to the operation the designation of reflexion is commonly given. This reflexion takes place without consciousness. The three parts--the centripetal fibre, the vesicle, and the centrifugal fibre--conjointly const.i.tute a simple nervous arc.

[Sidenote: Gradual complexity of the nervous system.] A repet.i.tion of these arcs, each precisely like all the others, const.i.tutes the first step toward a complex nervous system. Their manner of arrangement is necessarily subordinated to the general plan of construction of the animals in which they occur. Thus, in the Radiates it is circular; in the Articulates, linear, or upon an axis. But, as the conditions of life require consentaneousness of motion in the different parts, these nerve arcs are not left isolated or without connexion with each other. As it is anatomically termed, they are commissured, nerve fibres pa.s.sing from each to its neighbours, and each is thus brought into sympathy or connexion with all the others.

[Sidenote: First appearance of special ganglia.] The next advance is a very important one, for it indicates the general plan on which the nervous system is to be developed: it is the dedication of special nerve arcs to special duties. Thus, in the higher articulates and molluscs, there are such combinations expressly for the purpose of respiration and deglut.i.tion. Their action is altogether of the reflex kind; it takes place without consciousness. These ganglia are commissured for the sake of sympathetic action, and frequently several of them are coalesced for the sake of package.

This principle of dedication to special uses is carried out in the introduction of ganglia intended to be affected by light, or sounds, or odours. The impressions of those agencies are carried to the ganglion by its centripetal fibres. Such ganglia of special action are most commonly coalesced together, forming nervous ma.s.ses of conspicuous size; they are always commissured with those for ordinary motions, the action being reflex, as in the preceding case, though of a higher order, since it is attended with consciousness.

[Sidenote: They are automatic mechanisms.] Such being the elementary construction of a nervous system, it is plain that animal tribes in which it exists in no higher degree of complexity must be merely automata. In this remark many insects must be included, for the instinct they display is altogether of a mechanical kind, and, so far as they are concerned, without design. Their actions are uniformly alike; what one does under given circ.u.mstances, under the same circ.u.mstances another will certainly do. They are incapable of education, they learn nothing by experience, and the acts they are engaged in they accomplish as well at the first trial as ever after.

Of parts like those described, and of others of a higher order, as will be presently seen, the most complex nervous system, even that of man, is composed. [Sidenote: Evidence to be used in these investigations.] It might, perhaps, be expected that for the determination of the duty of each part of such complex system the physiologist must necessarily resort to experiment, observing what functions have been injured or destroyed when given portions have been removed by his knife. At the best, however, evidence of that kind must be very unsatisfactory on account of the shock the entire system receives in vivisections, and accordingly, artificial evidence can, for the most part, be used only in a corroborative way. But, as Cuvier observed, the hand of Nature has prepared for us these very experiments without that drawback. The animal series, as we advance upward from its lowest members, proves to us what is the effect of the addition of new parts in succession to a nervous system, as also does any individual thereof in its successive periods of development. It is one of the most important discoveries of modern physiology that, as respects their nervous system, we can safely transfer our reasonings and conclusions from the case of the lowest to that of the highest animal tribes.

The articulata present structures and a mode of action ill.u.s.trating in a striking manner the nervous system of man. Lengthwise upon their ventral region is laid a double cord, with ganglia, like a string of beads; sometimes the cords are a little distance apart, but more generally they are coalesced, each pair of ganglia being fused into one. [Sidenote: First introduction of governing ganglia.] To every segment of the body a pair is supplied, each pair controlling its own segment, and acting toward it automatically, each also acting like any of the others. But in the region of the head there is a special pair, the cephalic ganglia, receiving fibres from the eyes and other organs of sense. From them proceed filaments to the ventral cord, establis.h.i.+ng communications with every segment. So every part has two connexions, one with its own ventral ganglia, and one with the cephalic.

It is not difficult to determine experimentally the functions of the ventral ganglia and those of the cephalic. If a centipede be decapitated, its body is still capable of moving, the motion being evidently of a reflex kind, originating in the pressure of the legs against the surface on which they rest. [Sidenote: But thus far actions are only instinctive.] The ventral cord, with its ganglia, is hence purely an automatic mechanism. But if, in making the decapitation, we leave a portion of the body in connexion with the head, we recognize very plainly that the cephalic ganglia are exercising a governing power.

In the part from which they have been cut off the movement is forward, regardless of any obstacle; in that to which they are attached there are modifications in the motions, depending on sight or other special senses; obstacles are avoided, and a variety of directions pursued. Yet still the actions are not intelligent, only instinctive. The general conclusion therefore is, that the cephalic ganglia are of a higher order than the ventral, the latter being simply mechanical, the former instinctive; but thus far there is no trace of intelligence.

[Sidenote: Nervous anatomy of vertebrates, as man.] In man these typical parts are all present, and discharge the functions specified. His spinal cord answers to the ventral cord of the articulates. It has its lateral communications in the same way, and each segmental portion presents the same reflex action. Toward its upper part it dilates to form the medulla oblongata, sending forth nerves for respiration and deglut.i.tion.

[Sidenote: Their automatic apparatus.] Of these the action is still reflex, as is proved by the involuntary movements of respiration and deglut.i.tion. A portion of food being placed in the pharynx, contraction instantly occurs, the will having no kind of control over the act of swallowing. [Sidenote: Their instinctive apparatus.] Above or in front of this enlargement is a series of ganglia, to which converge the nerves of special sense--of hearing, sight, smell; these are, therefore, the equivalents of the cephalic ganglia of insects, their function being also the same. In the lowest vertebrates, as in the amphioxus, the nervous system consists of nothing more. It may therefore be said to have only two parts--the cord and the sensory ganglia, and to have two functions--the automatic, attributable to the former, and the instinctive, attributable to the latter.

[Sidenote: Their intellectual apparatus.] But as we advance from the low vertebrates upward in the animal scale, we begin to detect new organs; on the medulla oblongata a cerebellum, and on the sensory ganglia a cerebrum. From this moment the animal displays reasoning powers, its intelligence becoming more strikingly marked as the development of the new organs is greater.

[Sidenote: Functions of the brain.] It remains to determine with exactness the function of one of these new parts, the cerebrum; the other portion, the cerebellum, being of minor interest, and connected, probably, with the locomotive apparatus. For the same reason it is unnecessary to speak of the sympathetic nerve, since it belongs to the apparatus of organic life. Confining our attention, therefore, to the true brain, or cerebrum, we soon recognize that the intelligence of an animal is, in a general manner, proportional to the relative size of this organ as compared with the sensory ganglia. We are also struck with the fact that the cerebrum does not send forth to other portions any independent fibres of its own, nor does it receive any from them, its only means of communication being through the parts that have been described--that is to say, through the sensory and automatic apparatus.

[Sidenote: Its relations to the instinctive and automatic portions.] The cerebrum is therefore a mechanism of a higher order, and its relations.h.i.+p with the thalami optici and corpora striata indicate the conditions of its functions. It can only receive impressions which have come through them, and only act upon the body through their intermedium.

[Sidenote: Its secondary and tertiary lobes.] Moreover, as we ascend the animal scale, we find that these cerebral parts not only increase in size, but likewise, in their turn, give rise to offshoots; secondary lobes emerging posteriorly on the primary ones, and, in due season, tertiary lobes posteriorly on the secondary. To these, in human anatomy, the designations of anterior, middle, and posterior lobes have been respectively given. In proportion, as this development has proceeded, the intellectual qualities have become more varied and more profound.

[Sidenote: Action of the spinal cord alone.] The relation of the cerebrum to the cranio-spinal axis is manifested by the circ.u.mstance that the latter can act without the former. In sleep the cerebrum is, as it were, torpid, but respiration, deglut.i.tion, and other reflex actions go on. If we touch the palm of a sleeping infant our finger is instantly grasped. [Sidenote: Conjoint action of the brain and cord.] But, though the axis can work without the cerebrum, the cerebrum can not work without the axis. Ill.u.s.trations of these truths may be experimentally obtained. An animal from which the cerebrum has been purposely removed may be observed to perform actions automatic and instinctive, but never intelligent; and that there is no difference between animals and man in this respect is demonstrated by the numerous instances recorded in the works of medicine and surgery of injuries by accident or disease to the human nervous system, the effects corresponding to those artificially produced in experiments on animals. This important observation, moreover, shows that we may with correctness use the observations made on animals in our investigations of the human system.

[Sidenote: Three distinct parts of the nervous system of man.] In the nervous system of man our attention is therefore especially demanded by three essentially distinct parts--the spinal cord, the sensory ganglia, and the cerebrum. [Sidenote: They are the automatic, the instinctive, the intellectual.] Of the first, the spinal cord, the action is automatic; by its aid we can walk, from place to place, without bestowing a thought on our movements; by it we swallow involuntarily; by it we respire unconsciously. The second portion, the sensory ganglia, is, as we have seen, the counterpart of the cephalic ganglia of invertebrates; it is the place of reception of sensuous impressions and the seat of consciousness. To these ganglia instinct is to be referred.

Their function is not at all impaired by the cerebrum superposed upon them. The third portion, the cerebrum, is anatomically distinct. It is the seat of ideas. It does not directly give rise to motions, being obliged to employ for that purpose its intermediate automatic a.s.sociated apparatus. [Sidenote: Dominating control of the latter.] In this realm of ideas thoughts spring forth suggestively from one another in a perpetual train or flux, and yet the highest branch of the nervous mechanism still retains traces of the modes of operation of the parts from which it was developed. Its action is still often reflex. Reason is not always able to control our emotions, as when we laugh or weep in spite of ourselves, under the impression of some external incident. Nay, more; the inciting cause may be, as we very well know, nothing material--nothing but a recollection, an idea--and yet it is enough. But these phenomena are perhaps restricted to the first or anterior lobes of the brain, and, accordingly, we remark them most distinctly in children and in animals. As the second and third lobes begin to exercise their power, such effects are brought under control.

[Sidenote: Progressive nervous development in the animal series.] There is, therefore, a regular progression, a definite improvement in the nervous system of the animal series, the plan never varying, but being persistently carried out, and thus offering a powerful argument for relations.h.i.+p among all those successively improving forms, an observation which becomes of the utmost interest to us in its application to the vertebrates. In the amphioxus, as has been said, the cranio-spinal axis alone exists; the Cyclostome fishes are but a step higher. In fishes the true cerebrum appears at first in an insignificant manner, a condition repeated in the early embryonic state both of birds and mammals. An improvement is made in reptiles, whose cerebral hemispheres are larger than their optic lobes. As we advance to birds, a further increase occurs; the hemispheres are now of nearly sufficient dimensions to cover over those ganglia. In the lower mammals there is another step, yet not a very great one. But from the anterior lobes, which thus far have const.i.tuted the entire brain, there are next to be developed the middle lobes. In the Rodents the progress is still continued, and in the Ruminants and Pachyderms the convolutions have become well marked. [Sidenote: It attains its maximum in man.] In the higher carnivora and quadrumana the posterior or tertiary lobes appear.

The pa.s.sage from the anthropoid apes to man brings us to the utmost development thus far attained by the nervous system. The cerebrum has reached its maximum organization by a continued and unbroken process of development.

[Sidenote: The same progressive development occurs in each individual man.] This orderly development of the nervous system in the animal series is recognized again in the gradual development of the individual man. The primitive trace, as it faintly appears in the germinal membrane, marks out the place presently to be occupied by the cranio-spinal axis, and, that point of development gained, man answers to the amphioxus. Not until the twelfth week of embryonic life does he reach the state permanently presented by birds; at this time the anterior lobes are only perceptible. In four or six weeks more the middle lobes are evolved posteriorly on the anterior, and, finally, in a similar manner, the tertiary or posterior ones are formed. And thus it appears that, compared with the nervous system of other animals, that of man proceeds through the same predetermined succession of forms. Theirs suffers an arrest, in some instances at a lower, in some at a higher point, but his pa.s.ses onward to completion.

[Sidenote: It occurs again in the entire life of the globe.] But that is not all. The biography of the earth, the life of the entire globe, corresponds to this progress of the individual, to this orderly relation of the animal series. Commencing with the oldest rocks that furnish animal remains, and advancing to the most recent, we recognize a continual improvement in construction, indicated by the degree of advancement of the nervous system. The earliest fishes did not proceed beyond that condition of the spinal column which is to be considered as embryonic. The Silurian and Devonian rocks do not present it in an ossified state. Fishes, up to the Carboniferous epoch, had a heterocercal tail, just as the embryos of osseous fishes of the present time have up to a certain period of their life. There was, therefore, an arrest in the old extinct forms, and an advance to a higher point in the more modern. The buckler-headed fishes of the Devonian rocks had their respiratory organs and much of their digestive apparatus in the head, and showed an approximation to the tadpoles or embryos of the frog. The crocodiles of the oolite had biconcave vertebrae, like the embryos of the recent ones which have gained the capability of making an advance to a higher point. In the geological order, reptiles make their appearance next after fishes, and this is what we should expect on the principle of an ascending nervous development. Not until long after come birds, later in date and higher in nervous advancement, capable not only of instinct, but also of intelligence. Of mammals, the first that appear are what we should have expected--the marsupials; but among the tertiary rocks, very many other forms are presented, the earlier ones, whether herbivorous or carnivorous, having a closer correspondence to the archetype than the existing ones, save in their embryonic states, the a.n.a.logies occurring in such minor details as the possession of forty-four teeth. [Sidenote: Absolute necessity of admitting trans.m.u.tation of forms.] The biography of the earth is thus, on the great scale, typical of individual life, even that of man, and the succession of species in the progress of numberless ages is the counterpart of the trans.m.u.tation of an individual from form to form. As in a dissolving view, new objects emerge from old ones, and new forms spontaneously appear without the exercise of any periodical creative act.

[Sidenote: Life of man from infancy to maturity in accordance with his anatomy.] For some days after birth the actions of the human being are merely reflex. Its cranio-spinal axis alone is in operation, and thus far it is only an automaton. But soon the impressions of external objects begin to be registered or preserved in the sensory ganglia, and the evidences of memory appear. The first token of this is perhaps the display of an attachment to persons, not through any intelligent recognition of relations.h.i.+p, but merely because of familiarity. This is followed by the manifestation of a liking to accustomed places and a dread of strange ones. At this stage the infant is leading an instinctive life, and has made no greater advance than many of the lower mammals; but they linger here, while he proceeds onward. He soon shows high powers of memory, the exercise of reason in the determinations of judgment, and in the adaptation of varied means to varied ends.

Such is therefore the process of development of the nervous system of man; such are the powers which consequently he successively displays.

His reason at last is paramount. No longer are his actions exclusively prompted by sensations; they are determined much more by ideas that have resulted from his former experiences. While animals which approach him most closely in construction require an external stimulus to commence a train of thought, he can direct his mental operations, and in this respect is parted from them by a vast interval. The states through which he has pa.s.sed are the automatic, the instinctive, the intellectual; each has its own apparatus, and all at last work harmoniously together.

[Sidenote: Every person consists of two lateral individuals.] But besides this superposition of an instinctive apparatus upon an automatic one, and an intellectual upon an instinctive, the nervous system consists of two equal and symmetrical lateral portions, a right half and a left. Each person may be considered as consisting in reality of two individuals. The right half may be stricken with palsy, the left be unimpaired; one may lose its sight or hearing, the other may retain them. These lateral halves lead independent lives. Yet, though independent in this sense, they are closely connected in another. The brain of the right side rules over the left half of the body, that of the left side rules over the right of the body. [Sidenote: Consequences of this doubleness of construction.] On the relations.h.i.+ps and antagonisms of the two halves of the cerebro-spinal system must be founded our explanations of the otherwise mysterious phenomena of double and alternate life; of the sentiment of pre-existence; of trains of thought, often double, but never triple; of the wilful delusions of castle-building, in which one hemisphere of the brain listens to the romance suggestions of the other, though both well know that the subject they are entertaining themselves with is a mere fiction. The strength and precision of mental operations depend as much upon the complete equivalency of the two lateral halves as upon their absolute development. It is scarcely to be expected that great intellectual indications will be given by him, one of whose cerebral hemispheres is unequal to the other. But for the detailed consideration of these topics I may refer the reader to my work on Physiology. He will there find the explanation of the nature of registering ganglia; the physical theory of memory; the causes of our variable psychical powers at different times; the description of the ear as the organ of time; the eye as the organ of s.p.a.ce; the touch as that of pressures and temperatures; the smell and taste as those for the chemical determination of gases and liquids.

[Sidenote: Conclusions from the foregoing anatomical facts.] From a consideration of the construction, development, and action of the nervous system of man, we may gain correct views of his relations to other organic beings, and obtain true psychical and metaphysical theories. There is not that h.o.m.ogeneousness in his intellectual structure which writers on those topics so long supposed. It is a triple mechanism. [Sidenote: Man a member of the animal series.] A gentle, a gradual, a definite development reaches its maximum in him without a breach of continuity. Parts which, because of their completion, are capable of yielding in him such splendid results, are seen in a rudimentary and useless condition in organisms very far down below. On the clear recognition of this rudimentary, this useless state, very much depends. It indicates the master-fact of psychology--the fact that Averroes overlooked--that, while man agrees with inferior beings in the type of his construction, and pa.s.ses in his development through transformations a.n.a.logous to theirs, he differs from them all in this, that he alone possesses an accountable, an immortal soul. It is true that there are some which closely approach him in structure, but the existence of structure by no means implies the exercise of functions. In the still-born infant, the mechanism for respiration, the lungs, is completed; but the air may never enter, and the intention for which they were formed never be carried out.