FOOTNOTES:

FOOTNOTES:[BR]Lay Sermons, chap, vii., p. 134.[BS]Darwin'sFertilization of Orchids, p. 2; Lubbock'sScientific Lectures, p. 8.[BT]Sir John Lubbock'sScientific Lectures, p. 3. Mr. Darwin refers to Fritz Müller's papers as reported inBotanische Zeitung, 1869-70.Appendix IX.[BU]The Various Contrivances by which Orchids are fertilized by Insects, 2d ed. p. 293.[BV]Fertilization, p. 5.[BW]Fertilization, p. 12.[BX]Fertilization, p. 102.[BY]Ib.p. 113.[BZ]Fertilization, p. 44.[CA]Scientific Lectures, p. 31.[CB]Fertilization, p. 2.[CC]Ib.p. 284.[CD]Fertilization, p. 285.[CE]Fertilization, p. 246.[CF]Botany, (Science Primers) by Dr. J. D. Hooker, C.B., P.R.S., p. 79.[CG]Scientific Lectures, p. 36.[CH]SeeAppendix X.[CI]SeeAppendix VIII.[CJ]Origin of Species, 6th ed. p. 230.[CK]Origin of Species, 6th ed. p. 233.[CL]Scient. Lects.p. 73.[CM]Ib.p. 78.[CN]Ib.p. 135.[CO]Scientific Lectures, p. 135.[CP]Origin of Species6th ed. p. 207.[CQ]Museum of Natural Historyedited by Richardson, Dallas, Cobbold, Baird, and White, vol. ii. p. 184.[CR]Scient. Lects.p. 68.

[BR]Lay Sermons, chap, vii., p. 134.

[BS]Darwin'sFertilization of Orchids, p. 2; Lubbock'sScientific Lectures, p. 8.

[BT]Sir John Lubbock'sScientific Lectures, p. 3. Mr. Darwin refers to Fritz Müller's papers as reported inBotanische Zeitung, 1869-70.Appendix IX.

[BU]The Various Contrivances by which Orchids are fertilized by Insects, 2d ed. p. 293.

[BV]Fertilization, p. 5.

[BW]Fertilization, p. 12.

[BX]Fertilization, p. 102.

[BY]Ib.p. 113.

[BZ]Fertilization, p. 44.

[CA]Scientific Lectures, p. 31.

[CB]Fertilization, p. 2.

[CC]Ib.p. 284.

[CD]Fertilization, p. 285.

[CE]Fertilization, p. 246.

[CF]Botany, (Science Primers) by Dr. J. D. Hooker, C.B., P.R.S., p. 79.

[CG]Scientific Lectures, p. 36.

[CH]SeeAppendix X.

[CI]SeeAppendix VIII.

[CJ]Origin of Species, 6th ed. p. 230.

[CK]Origin of Species, 6th ed. p. 233.

[CL]Scient. Lects.p. 73.

[CM]Ib.p. 78.

[CN]Ib.p. 135.

[CO]Scientific Lectures, p. 135.

[CP]Origin of Species6th ed. p. 207.

[CQ]Museum of Natural Historyedited by Richardson, Dallas, Cobbold, Baird, and White, vol. ii. p. 184.

[CR]Scient. Lects.p. 68.

HIGHER ORGANISMS.—RESEMBLANCES AND CONTRASTS.—BRAIN STRUCTURE.

The stage of investigation now reached requires us to consider recent advances in our knowledge of more complicated organisms. This leads into the line of observation disclosing steadily advancing complexity of structure, and brings us into contact with the claim that man be included within the area of scientific inquiry, and regarded as a more fully organized life to which lower orders are not only pointing, but actually tending.

As to this last claim, about which more must be said as we approach the close of these investigations, it may be remarked by way of preliminary, that as man belongs to nature, all the characteristics of his life must come within the area of scientific inquiry, and indeed the test of any theory of existence which may be offered, will be found in themeasure of success with which it explains our own nature. That man stands highest in the scale of organism belonging to this world admits of no doubt, therefore the explanation of human nature may be regarded as the supreme effort of science. Around this subject, however, serious differences have arisen among scientific men, but these differences do not concern the very simple question whether all that belongs to nature comes within the range of the science of nature. This is granted by all, whether there be a preference for including all such inquiry under the single name of science, or for distinguishing between physical science and mental philosophy. This is simply a matter of defining terms, and tracing the boundaries of recognized departments of inquiry. But whether a continued study of organism will conduct us to an adequate understanding of human nature, must be a matter of observation and inference. If it do, science has completed its work. If it do not, there remains a still higher question, how shall we account for features of life for which organism affords no scientific explanation? The whole field is certainly free to science, and the whole task which this immense fieldof research imposes must be undertaken, and persistently prosecuted to a rational issue.

Entering now, therefore, on the contemplation of animal life, regarded as a higher order, distinguishable from vegetable life, we have the outstanding characteristics of sensibility and locomotion. Whether there is a distinct line of demarcation between vegetable and animal does not require special attention, for no matter of controversy on this point can delay procedure. There is, as already remarked, in the vegetable kingdom a singular approximation towards animal life, in so far as we have evidence of sensibility to touch among the plants, to a degree which appears wonderful chiefly by contrast with the common characteristics of the vegetable kingdom.

On the other hand, sensibility to influences operating from without is a common feature of animal life. Even the very lowest orders of animals are sensitive to touch, and as this form of experience is closely connected with power of locomotion, all animals have the conditions of their life largely affected by interference with their own movements, or resistance offered, whether by objects lying in their way, or by some force restraining theirprogress, or causing movement in an opposite direction. Now these two characteristics—sensibility to impression from without, and movement caused by an exercise of energy from within the organism itself—are both provided for by means of the nerve system belonging to the animal. This nerve system varies in the number and complexity of its arrangements, according to the complexity of the organism with which it is associated. As, therefore, we rise in the scale, passing from the soft pulpy form of the lowest orders, to those formed in segments or rings, next to those with distinct portions of organism fulfilling separate functions, as in insect life, with head, body, and legs; and next pass up to the vertebrates, with back-bone and skeleton, on which is built up a more or less complicated muscular system, we find a nerve system, growing in complexity along with the appearance of different organs of the body. And in all cases, this system fulfils these two functions—sensibility to touch, and movement of the body. These two are provided for by distinct lines or nerve fibres; and in all cases, these two sets are combined in a centre, thereby securing that the two setsbe coöperative, unitedly contributing to the management of the living organism. This appears even if we take for illustration an organism so low as theascidian mollusk, which floats in the water as if it were a sack drawn together towards the top, bulging out below; and which is nourished simply by the passing of a current of water in at the mouth, and out at a vent towards the lower end of the sac. A series of nerve lines comes from the mouth; a distinct ramification spreads over the lower portion of the sac; and these two are united in a single knot or ganglion, a little above the vent. By these contrivances, this little body, though for the most part stationary, is sensitive to the approach of any thing injurious, and by contraction of its mass expels the water with considerable force, driving the injurious matter to a distance. This combination of the two sets of nerves appears more strikingly in such an animal as thecentipede, along whose body are successive groups of nerves, combined in regular order in a series of knots, and united longitudinally by connecting threads, attaching the successive knots. The same plan is carried up into a more articulated form inthe case of thewinged insect, with head, antennæ or feelers projecting from the head, wings, and legs, leading to a more marked appearance of separate combinations, giving greater prominence to the head. When from this we rise to thefish, thence to thebird,thence to thequadruped, we find the head made conspicuously the central organ of the entire nerve system of the animal, while it occupies the front position in the body. It is no longer one of a set or series of knots; nor even the largest or more conspicuous in a graduated order of centres; but in the head of the animal is found that which is the true nerve centre for the whole nerve system, designated the brain. In the case of the vertebrates, not only does the skeleton afford the solid frame-work on which the muscular system is built, but the back-bone contains within it the main column of nerve fibres, which are given out at the several joints according to the requirements of the body.

If meanwhile we concentrate attention on our own bodies, we may by the aid of personal experience find easy illustration of the prominent features of the nerve system. We shall take first thetwo distinct linesof nervesalready mentioned, the one set concerned with sensibility, the other with movement of the muscles. From the tips of the fingers there run lines of nerve fibre, which are brought into combination at the wrist, and are carried up the arm, and onward by the shoulder and upper portion of the back-bone to the head. These are the nerves ofsensibility, by means of which, as by telegraph wires, the slightest impression made on the tips of the fingers is instantly conveyed to the great nerve centre in the brain. Distinct from these is another set of nerves issuing from the brain, and descending the arm, giving off its fibres as it passes to the several muscles above the elbow, next to those above the wrist, and next to the muscles of the hand and fingers. These are the nerves ofmovement, by means of which the whole arm may be brought into action at pleasure, or the hand may be set to work, while the arm is at rest.

These two sets of nerves—the sensory and motor—are exactlysimilar in structure, consisting of an outer covering, within which floating in a white fluid is a thread which constitutes the nerve proper. The outer covering provides forisolationof the fibre, fromother fibres laid alongside of it, just as copper wire is isolated by a gutta-percha covering when the two connecting lines from an electric battery are laid down in close proximity as in the arrangement for electric bells. By this provision the nerve fibres are completely isolated making it possible to distinguish sensory impressions so as to tell which finger has been touched. The similarity of structure in the two lines of nerves is a striking fact in view of the completely distinct functions fulfilled. This leads to a special explanation of the provision for different modes of action. This is secured bydiversity in the terminal arrangementsfor the two classes of nerves. The nerves of sensibility have a peculiarly sensitive arrangement spread under the skin, constituting an end-bulb or touch organ. In certain parts of the body more sensitive than others, such as the tips of the fingers, there are additional minute corpuscles, grouped alongside of the nerve, liable to contract under the slightest pressure, and which add greatly to the sensitiveness of the particular parts about which they cluster. The terminal arrangements of the motor nerves are quite different. The nerve fibres pass intothe substance of the muscle to be moved by them, and the nerve fibre is subdivided and distributed, so as to bring the several parts of the muscle under control. These fibres are so laid and connected, that a whole set of muscles can be moved simultaneously, being made to work in perfect harmony.

The vital activityof this whole arrangement of nerve fibres, including sensory and motor in one system, depends upon living connection of all with the great nerve centre in the brain, where the nerve energy is provided which keeps all in functional activity. Only, there is this striking difference with the two sets of fibres, that in the case of the sensory nerve the pulsation of energy is upwards to the brain, in the case of the motor nerve it is downwards towards the muscle. There is no scientific explanation yet reached of this contrast of molecular action. But by means of it the one order of nerves plays the part of a vehicle of impression providing for knowledge of what is without, the other order fulfils the part of an instrument for moving the muscular system which is part of the organism itself.

Diagram of Cerebro-Spinal Nerve Centres.Diagram of Cerebro-Spinal Nerve Centres.DARK REPRESENTING SENSORY; THE LIGHT, MOTOR CENTRES. THE ARROWS INDICATE THE DIRECTION OF THECURRENTOF INFLUENCE.

Diagram of Cerebro-Spinal Nerve Centres.DARK REPRESENTING SENSORY; THE LIGHT, MOTOR CENTRES. THE ARROWS INDICATE THE DIRECTION OF THECURRENTOF INFLUENCE.

Nerve System of the Insect, showing distinct centres.Nerve System of the Insect, showing distinct centres.

Nerve System of the Insect, showing distinct centres.

These two orders are not, however, to beregarded as separate systems quite apart from each other, but as two sides of one system, which are essentially and closely related to each other. There is a provision forcombined actionof the two sets, so that an impulse communicated along a sensory nerve or set of nerves, may pass over to the motor system and terminate in muscular activity. This is most simply illustrated by the circumstance that the nerves of sensibility become instruments ofpain, when a severe shock or blow is given, or some injury is inflicted. Suffering becomes a signal of risk and instantly the injured part shrinks or starts away from the source of suffering. This is a phase of sensori-motor activity illustrating a law which has a wide range of application in animal life. This sketch of the arrangements and functions of the two sides of the nerve system though traced in view of its application to human nature, will suffice to indicate the general plan in accordance with which sensibility and muscular activity are provided for in the animal kingdom generally. The ramification of the nerve lines will in each case be according to the simplicity or complexity of structure belonging to the animal; but theprovisions for sensitiveness to touch, and power of movement are in all cases the same. Fish, bird, and quadruped are alike sensitive to touch, and they are alike capable of movement, though the mechanical contrivances by which locomotion is secured vary greatly; but a double distribution of nerve fibres in all cases provides for these two characteristics of animal life.

From this, we advance to the nerve centre,—the brain,—to which the nerves of sensibility run up, and from which the nerves of motion come forth. Here also there is identity in the nature of the organ, while there is variety in its size, with more or less complicated plans of arrangement, according to the extent of the nerve system of which it is the central organ. Still keeping to the human body for illustration, we may find in the most complex organism known to us illustration of what holds good in the main so far as essential structure is concerned.

The brain is made up of two entirely distinct substances. In the interior of the organ, and altogether concealed from view when a drawing of it is made, or the organ itself is exposed to observation, isa white massconsisting of a multitude of fibres. These are simply crowds of nerve lines gathered together, led up from the extremities and trunk, or provided for intercommunication with the several parts of this central organ. Gathered all round about this, and constituting the external mass, on the summit, sides, and base of the brain, is a completely distinct substance known asthe grey matter, folded up in wavings, twistings, or convolutions, enclosing myriads of cells from which nerve energy is discharged. These cells differ considerably in form and size, suggesting the possibility of distinct functions being assigned to cells of different structure, some being smaller and less intimately connected with those around, others so much larger and more important as to have suggested the name of pyramidal cells, and also having lines of connection between themselves and other parts much more numerous than in the case of the smaller cells. Every cell has a nucleus or central point, which is the centre of vitality, while the fibres which they send out, varying in number from one to four or five, establish connection between cells, or pass into the nerves proper. These cells are packed together in a soft glutinous substance, in the outer layer of which they are fewer in number; approaching the interior, they become more numerous; and they are both more abundant, larger in size, and more distinguished by the number of their protoplasmic[CS]fibres as they lie nearer to the mass of nerve fibres. In this crowd of nerve cells are the stores of nerve energy supplied to the nerve system, with every exercise of which molecular changes in the brain are believed to take place. On this account there must be regular and ample supply of nourishment for the brain, for which such provision has been made that, according to Haller's computation, one fifth part of the whole blood supply goes to the brain.

Human Brain, with centres of electric excitation.Human Brain, with centres of electric excitation.

Human Brain, with centres of electric excitation.

Regarded as the great central organ, the brain is divided into two halves or hemispheres, from each one of which goes forth supply of nerve fibres and nerve energy for the opposite side of the body. Its greatest depth is in the central part, the front and back being rounded down, the frontal region being, however, considerably more massive than the rear. Besides this great central body, there are several dependent subordinate bodies,placed underneath, and directly above the upper part of the spine. Most important of these is thecerebellum, or little brain, whose functions are now generally believed to be closely connected with the equilibrium of the body when moving. Somewhat nearer the centre, and quite under the brain proper is theponsor bridge, providing for the interlacing of the fibres on their way out from the the central organ, and just below that are certain elongated bodies (medulla oblongata), consisting of masses of fibre just above the spinal cord.

Before closing this very brief and hasty description of the nerve system, there is one peculiarly striking arrangement to which special reference may be made. The mass of nerve fibre which passes down within the back-bone constituting the spinal column, which is formed in two divisions equivalent to the hemispheres of the brain, gives out at each of the vertebræ or spinal joints a supply of nerve for the portion of the body contiguous. This supply is sent out from each side of the column, and issues in two roots, a posterior and anterior; the posterior root being a body of sensory nerves, the anterior root ofmotor nerves. Shortly after passing out, these two form into one, uniting to constitute a nerve trunk. Just after they have thus united, the trunk again opens up into two, and in each one of these two a share of the sensory and motor roots finds a place, and thus preparation is made for sending out towards both the front and back of the body suitable proportion of both sets of nerves. The two roots drawn together as if to bind them into one, are by some inexplicable process subdivided, and the two bands issuing from the united band are found to have each a share of the contents of each root. Of all the singular occurrences coming under scientific observation there is nothing more surprising. The fact is certain, but there is no scientific explanation of the contrivance by which such a singular result is secured.

Brain of the Crab.Brain of the Crab.

Brain of the Crab.

Brain of the Cod, the two larger lobes being those of vision, the brain being in front of these.Brain of the Cod, the two larger lobes being those of vision, the brain being in front of these.

Brain of the Cod, the two larger lobes being those of vision, the brain being in front of these.

Having now before us in outline a representation of the nervous system of man, and having in this a guide to the understanding of the prominent features involved in the distribution of two orders of nerves over the body, and their concentration in a central organ, we are prepared for considering the comparative brain development presented toview as we ascend the scale of animal life. The main features of gradation may be shortly stated. In all cases, the brain is a soft pulpy body, composed as described, the exterior portion being cellular tissue, the interior fibrous, from the gathering of nerve lines. In the lowest orders of animals, the brain is of very small size. In theinsects, such as the ant, bee, and wasp, it is only a slight band stretching from eye to eye. In the whole order offishesan advance in organization appears, though the brain is small relatively to the size of the body, a fact which seems readily explained by the fact that there is little articulation in the structure of the fish, the whole body moving in one mass, by simple management of the fins and tail. The brain as a rule is simply two small round lobes of smooth surface laid together; and what is most to be remarked is that the brain proper is quite inferior in size to lobes of vision.[CT]In front of the brain are slight strands connected with the organ of smell; and behind it are the two large lobes known as optic lobes, before whichthe brain appears comparatively insignificant. This is the ordinary arrangement, but in the case of the shark the brain extends to much larger proportions, greatly surpassing the optic lobes, and having in front of it unusually ample provision for the organ of smell.[CU]

When we reach thereptilesthe normal order appears which continues thereafter up the whole range of animal life. The brain takes precedence of the lobes of special sense, and is the most important organ. This appears quite decisively in the brain of the frog. On account of the possession of four limbs, and its power of locomotion by forward leaps, provided for by the superior size and strength of the hind legs, there is much greater need for distribution of nerve lines, to place distinct muscles under control, and as a consequence the brain or central organ assumes a position of greater importance.

Brain of the Bird.Brain of the Bird.

Brain of the Bird.

Brain of the Cat, with bulb of smell in front, and little brain behind.Brain of the Cat, with bulb of smell in front, and little brain behind.

Brain of the Cat, with bulb of smell in front, and little brain behind.

Passing next tobirds, we find a marked advance in the structure of the brain. The two hemispheres are considerably extended towards the rear, and the two optic lobes underneath the back part of the brain are separated from each other, being placed somewhat to the side. The cerebellum, or little brain, regulating equilibrium becomes more important in size and form, being laid up in transverse furrows. These important advances indicate a life of much more varied activity than in the lower orders. This animal walks, hops, perches on branches by the clutching of its claws, and flies from place to place. To provide for these varied forms of activity, there must be a more detailed arrangement of nerve system, which is clearly indicated in the complexity of the central organ.

The next advance introduces to notice thesmaller quadrupeds, known as the rodents, of which the rat, rabbit, and hare may be taken as the most familiar examples. Here we still have the smooth surface of the brain, without any subdivision and twining into folds such as afterwards appears, but it is somewhat elongated in shape. An additional element here comes into view, that is, extra provision for acuteness of smell, in accordance with the well-known characteristics of the class of animals. Set out in front of the brain are two distinct lobes, which are the olfactory lobes. Wherever these are so placed in front of thebrain, it is a clear proof that the life of the animal is largely directed by smell, that is, in a relatively greater degree than by sight, though constantly using the organs of vision with rapidity and acuteness. The cerebellum is in all cases prominent to the rear, presenting the laminated appearance always distinctive of the organ.

We now make a very marked transition in the development of brain, introducing to view the doubled or convoluted form occasioned by the folding of the material in a series of windings,—a form which is in complete contrast from the smooth surface characteristic of the brain in all lower orders. This series of windings or convolutions appears quite decidedly in the brain of thecat, in a manner very similar in the brain of thedog, and with still greater beauty and amplitude of fold in the brain of thehorse. This folding process which is resorted to in the case of all the higher quadrupeds, seems a contrivance by which it is possible to pack a greater amount of material in such a way as to expose a greater degree of surface, within the narrow space at command inside the cranium. In all the three examples named, great prominenceis given to the bulbs of smell, which are spread out quite conspicuously in front of the brain,—implying, as in lower examples, a life largely governed by sense of smell.

Brain of Horse, with bulb of smell in front, little brain in rear.Brain of Horse, with bulb of smell in front, little brain in rear.

Brain of Horse, with bulb of smell in front, little brain in rear.

Diagram of Sensory and Motor Apparatus.Diagram of Sensory and Motor Apparatus.THE UPPER IS THE SENSORY, WITH BULB, NERVE LINE, AND NERVE CELL. THE UNDER IS THE MOTOR, WITH MUSCLE, NERVE LINE, AND NERVE CELL.

Diagram of Sensory and Motor Apparatus.THE UPPER IS THE SENSORY, WITH BULB, NERVE LINE, AND NERVE CELL. THE UNDER IS THE MOTOR, WITH MUSCLE, NERVE LINE, AND NERVE CELL.

Omitting special reference to animals of great bulk, and possessing enormous muscular power, such as the elephant and the whale, both of which have singularly complicated and beautiful brains, I pass to the races ofmonkeysandapes, which are nearest in structure to man. In these animals the configuration of body is certainly the nearest approach to the human figure which is to be found anywhere in the animal kingdom. They can not, indeed, assume the perfectly erect posture of man, but they come very near to it; and though they move on all four limbs, feeling themselves more secure in that mode of advance, they have a formation of hand analogous to that of man, with a distinctly formed thumb, enabling them to grasp an object in a manner closely resembling the human grasp. The apes have even an advantage over the human race, for they have a thumb on the foot, as well as on the hand; which may also have its own disadvantages, for it might prove no convenience to us if we were so endowed.But the presence of the thumb on the lower extremities suggests the use which it serves in the animal's ordinary life, in grasping the branches along which it moves. If from the similarity of outward configuration, we pass to contemplate the brain, we find here also great similarity of structure. And indeed if the relations of muscle, nerve, and brain be as already indicated, it follows from the resemblances of outward form that there must be a greater resemblance between the brain of man and the brain of the monkey and of the ape, than between the human brain and that of any other animal known to us. And so it proves to be. The brain of the monkey has its subdivisions and convolutions very similar to those of the human brain, only the convolutions are simpler in arrangement. In outline it is deficient only in the diminished bulk of the front part, and also the back part of the organ; but in its expansion it resembles the human brain in this, that to the rear it spreads back over the cerebellum, so as to cover it. The brain of the ape, including under this designation the orang, gorilla, and chimpanzee, is in still closer resemblance to the human, being still, however, somewhatsimpler in the arrangement of its convolutions, but so closely approximating that the exact state of the case is as nearly as possible described, if we say that the brain of the ape, while it is decidedly smaller, appears like a miniature of the human brain in a slightly undeveloped state.[CV]

Human BrainHuman Brain

Human Brain

Brain of Monkey, with cerebellum beneathBrain of Monkey, with cerebellum beneath

Brain of Monkey, with cerebellum beneath

The human brain is an elaborate organ, exceedingly complicated in its convolutions. We can not, indeed, describe it as the most convoluted, for the brain of the elephant is at least as distinguished for the beauty and complication of its folding, and the brain of the whale is far more minute and detailed, presenting quite a multitude of minute convolutions. For descriptive purposes, the human brain is divided into four superficial areas, known as lobes, and pretty clearly defined by certain natural boundaries. From the lower part of the organ, entering at a point scarcely half way back is a fissure or cutting running up into the mass in a direction uniformly inclining towards the rear, known as the Sylvian fissure; while coming over the summit, at a point near the middle, and inclining down towards that just described, is another fissure, known as the fissure of Rolando. By thesetwo deeply cut hollows, the brain is marked off into four separate areas superficially, a front and a rear lobe; and two central lobes, the one upper and the other under. Besides this there is a concealed and isolated lobe, described on account of its situation as an island, which is covered from view by the overlapping of the two sides of the Sylvian fissure. Such is a description in outline of the configuration of the human brain, to which must be added the statement that each lobe is filled in with its own special arrangement of convolutions, each one having at least three well defined lines of convolution. Each of the hemispheres is similarly arranged, though not by any means quite identical in disposal of convolutions, yet the general description now given is strictly applicable to both. The two hemispheres, connected mainly with the ramification of nerve fibre running to the opposite sides of the body, are united together a considerable way down by a transverse band of nerve fibres, which at once unite the two into one organ, and make the union so effected a living efficient union by carrying a multitude of lines of communication from the one side to the other. Just below this, in the interiorof the organ are two great central bodies, known as the basal ganglia, and consisting of nerve fibres massed together with grey matter around them, that in front being chiefly motor nerves brought to a junction, the latter sensory nerves combined in like manner. The same arrangement holds in both hemispheres, thereby providing that the respective masses of motor, and of sensory nerves lie exactly opposite each other. Behind these in the centre, lying in a position under both hemispheres are four small bulbs connected with the nerves of vision, and also with the cerebellum; and behind them, covered by the posterior lobe of the brain is the cerebellum itself, or little brain, largely concerned with coördination of movements, or equilibrium of the two sides of the system. Just below these arrangements the two great cords of nerve fibre descend towards the body, which are covered by a transverse mass, known as the bridge, appearing complete as a crossing, and containing transverse fibres from the cerebellum, as well as a series of longitudinal fibres. Immediately underneath the bridge are pillars or masses of nerve, constituting the crowning portion ofthe spinal system, and formed in eight distinct bodies, the two in front and the two in rear being elongated and known as pyramids, those in the centre being rounded in figure. From the elongated bodies, the nerve fibres pass across to the opposite sides of the body. This gathering is known as themedulla oblongata.Just beneath comes the spinal canal, from which at the different joints of the spine are given out a suitable supply of sensory and motor nerves as previously described.

Having thus given a general account of the central arrangements of the nerve system of the human body, it is important to state that an order of things closely analogous obtains in other and lower orders of organism, in respect of interior plan, so that if the interior of the brain of the dog were laid open to view it would present a plan of distribution very similar to that now described.

To complete the view of the functions of the brain as indicated by recent research, I have next to give a brief account of an extended course of experiments of great delicacy designed to ascertain whether it may be possible to localize certain functions within a definite area of the brain. All are familiarwith the fanciful subdivisions of the outer surface of the human skull, under the name of phrenology, represented on moulds of the head, all marked with dividing lines and figures. This pretentious and unscientific assumption of knowledge which no one possessed, has had its time of popularity, aided by a general recognition of comparative superiority in head formation in persons of known ability. Any thing equivalent to an exact partition of the bony covering protecting the brain, has not been favored by scientific observations; but these fanciful maps of the head, which have been sold cheap, and fully certified, may serve as a guide to a general notion of what has been attempted on the surface of the brain itself, after removal of the skull. The illustrative aid, however, consists in nothing more than the suggestion of distinct areas, for there is no analogy between what has been discovered by the observations now to be described, and the "bumps" alleged to be found on the cranium.

The conjecture which may be said to have originated experiments as to localization was that there was a close resemblance between the action of nerve energy, and an electriccurrent. The attempt made was to similate the action of the nerve cells, by discharging a current of electricity upon the grey matter of the brain, and recording the results which came under observation. Experiments were begun in 1870 in Germany by Fritsch and Hitzig, the dog being the animal experimented upon. The investigation was undertaken also by Dr. Ferrier of King's College London, and much more extended and varied results were published by him in 1873. Confirmatory work, executed with many precautions, was undertaken on the subject in 1874 by a committee of the New York Society of Neurology and Electrology,—a committee which included Drs. Dalton, Arnold, Beard, Flint, and Masson,—testing results by frequent renewal of the experiments; and at the same time, a similar course of inquiry was being conducted in Paris by Carville and Duret.[CW]

By these investigations, the possibility of electric stimulation of the cortical or grey matter of the brain, and consequent activity of the nerve system has been fully established; and though there is still considerable diversityof opinion concerning the interpretation of the facts, it can not be disputed that by directing the electrode on certain well defined areas of the surface of the brain, it is possible to bring into natural activity certain portions of the muscular system, as controlled by the motor nerves.

The plan adopted is, after putting the animal into an insensible state by use of chloroform, and removing the cranium so as to expose the brain, to apply the electrode connected with an electric battery to a given point on the surface, record the result, and gradually shift the needle round the original spot until a new result is obtained, in which the spot previously tested becomes an index for the boundary of one circle, and this marks the fact that a new circle has been entered.

By this process of investigation a series of centres for active stimulation have been discovered. These number, in the brain of the rat, six; in the brain of the rabbit, seven; of the cat, eleven; of the dog, thirteen; and of the monkey, at least, seventeen. A curious limitation to the area of experiment has been encountered here, for all the centres identified are found to cluster over the central region of thebrain, and both the front and rear parts of the organ are silent, offering no response however greatly stimulated. The explanation of this silence remains a matter of doubt. It may be that these portions of the brain are concerned with movements which do not come under the observation of the operator, or that they are centres of sensibility from which no movement can naturally follow, or that they fulfil functions which can not be recognized by this mode of experiment. Uncertainty hangs over this department in the investigation.

The actual results may be indicated by a few examples. At a point well forward in the brain of the dog, marked number one by Ferrier, is a centre which when stimulated leads to movement of the hind leg on the opposite side; and by exciting another portion of the brain quite contiguous, marked number four, movement of the opposite fore leg is produced. By exciting a point situated over these two and on a distinct convolution, wagging of the tail is induced. By transferring the needle to a point much lower down, towards the base of the brain, but still well forward, marked by Ferrier nine, the mouth is opened and the tongue moved, while inmany cases a decided bark is emitted. These examples may suffice to indicate the class of results obtained; and similar results have been seen in all animals subjected to this test, with such variations as may be considered inevitable in view of the configuration of the animal.

While distinct areas or circles of the brain have thus been marked, warranting localizing of certain functions, the facts connected with these experiments do not favor the view that each area is to be taken as so rigidly distinct that it may be supposed to operate separately in a quite isolated manner. On the contrary, a conjoint action of several centres seems more commonly implied when the natural activity of the brain is contemplated in line of these results. Additional weight must be given to this consideration, when it is noticed that the centres are nominallymotor centres,—movement and not sensibility being the result most patent to the observer,—nevertheless on closer scrutiny it proves true, that many of the movements occasioned by electric stimulation are those induced naturally as the result of sensation. Such for example are the movements of the eyelids consequent upon a dazzling of the eyes, or movement of the ears becauseof a startling sound. In this way it becomes clear that within a given area a centre of sensibility is in communication with a motor centre close by, or it may be even at some little distance. Thus this most delicate and difficult course of investigation supports the view that much of the activity of the animal organism is provided for by an established connection between nerve cells respectively presenting the terminus in the brain for a sensory nerve, and the starting point for a motor nerve, or point of communication with such a nerve. From this conclusion, it follows that a very large amount of the activity which we witness in the case of animals, often attributed to instinct, or even to voluntary determination, is to be described assensori-motor activity. That is to say, the action is brought about by a contrivance which may be described as partly mechanical, partly chemical. Its history may be sketched in this way: an impression is made on one of the nerves of sensibility, or on one of the organs of special sense, such as the eye or ear; a wave of impulse passes along the incarrying nerve fibre, leading to molecular change in the nerve cell, and to sensibility in some way unknown; theexcitation occasioned there is extended along a connecting fibre to a second nerve cell, which is the starting point for a motor nerve; along that line the impulse is instantly and inevitably continued; and as an almost instantaneous result, without any form of sensibility to indicate what is taking place, the muscular energy is liberated, and action is the direct consequence. The problem which immediately arises is this,—How far may the activity of all living organism be accounted for in this way, including even the activity of man? This is a problem which will present an interesting subject for discussion in the next stage of this inquiry, the import of which must now be made apparent by the sketch of the structure of the nerve system, and the results of the experiments as to localization.

Nothing more is now required to complete this narrative leading up to this problem, and discovering its proportions, than a brief account of correlative inquiry which has afforded strong confirmatory evidence as to the truth of the conclusions favoring localization, and coördinate action of different portions of the brain as the central organ governing the whole nerve system. Thecorroborativeevidence at once supporting the conclusions as to localization and favoring their extension to human nature is obtained by reference to the results of injury to the nerve system at various parts of the body, and injury to the brain as ascertained after death. Continuing experiments on the animals, it has been shown that even if a portion of the brain be cut away, it is still possible to operate on the nerve lines in the usual manner by means of electricity. Pushing experiment in this direction still further it has been found that more serious injury permanently destroys the centre, and entails paralysis of the muscles controlled by it when in a healthy state. In like manner it has been proved that if the nerve itself be cut, the communication is at an end, and movement by stimulation has become impossible.

By perpetually occurring cases of paralysis in human experience, and careful examination after death of the exact situation and extent of disease in the brain, it has been shown by accumulation of evidence, that the laws which provide for sensibility and for muscular activity in the history of the lower animals, do also hold in the case of man. While the brain continues in full vigor, all the usual forms of sensibility, and modes of action are simple; where these have become disturbed, restricted or impossible, some injury has been accidentally inflicted on the brain of the sufferer, or disease has begun in the organ, and has gained a hold exactly proportionate to the forms of restraint and disturbance which have become outwardly manifest. These are results which show how much is due by way of sympathy, and patience, and encouragement to those who suffer under any degree of brain injury or disease, due from all around them whose conduct may have any part in determining their experience. These results testify how closely the human organism stands allied to lower orders of organism around; how many homologies of structure there are, and how many analogies in experience. These things declare that science has a clear andunchallengeablefield of inquiry in seeking an explanation of human nature on the same lines of procedure as those which have been followed in ascending the scale of living organism. The nature and extent of materials at its disposal as the result of the most recent investigations have now been indicated. The problem is, How far canthe anatomy and physiology of the human frame account for the facts of human life? The strength and practical power of religious thought in the world will depend upon the answer, for science must here carry some test of religion. On the other hand, the problem which human life presents is by far the most severe test which science has to encounter. In facing the facts, science is engaged with the settlement of its own boundaries,—the demonstration of its own limits. In facing this highest problem which human observation encounters,—man's explanation of himself,—let us cease from comparisons between scientific claims and religious, and let us face with patience and resolution the question—What is the exact place, and what the destiny of man, who has piled up the sciences, and midst the turmoil and conflict of life, has found his most elevating exercise, and most profound calm, in worship of "the King eternal, immortal, invisible, the only wise God"?

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