CHAPTER VIII.THE FUNCTIONS OF THE BRAIN.

Image unavailable: Fig. 28. Fig. 29. Fig. 30. (All after Huguenin.)Fig.28.Fig. 29.Fig. 30.(All after Huguenin.)

Embryological Sketch.—The brain is a sort ofpons asinorumin anatomy until one gets a certain general conception of it as a clue. Then it becomes a comparatively simple affair. The clue is given by comparative anatomy and especially by embryology. At a certain moment in the development of all the higher vertebrates the cerebro-spinal axis is formed by a hollow tube containing fluid and terminated in front by an enlargement separated by transverse constrictions into three 'cerebral vesicles,' so called (seeFig. 28). The walls of these vesicles thicken in mostplaces, change in others into a thin vascular tissue, and in others again send out processes which produce an appearance of farther subdivision. The middle vesicle or mid-brain (Mbin the figures) is the least affected by change. Its upper walls thicken into the optic lobes, orcorpora quadrigeminaas they are named in man; its lower walls become the so-called peduncles orcruraof the brain; and its cavity dwindles into the aqueduct of Silvius. A section through the adult human mid-brain is shown inFig. 31.

Fig. 31.—The 'nates' are the anterior corpora quadrigemina, the spot aboveaqis a section of the sylvian aqueduct, and the tegmentum and two 'feet' together make the Crura. These are markedC.C., and a cross (+) marks the aqueduct, inFig. 32.

Fig. 32(after Huxley).

The anterior and posterior vesicles undergo much more considerable change. The walls of the posterior vesicle thicken enormously in their foremost portion and form thecerebellumon top (Cbin all the figures) and thepons Varoliibelow (P.V.inFig. 33). In its hindmost portions the posterior vesicle thickens below into the medulla oblongata (Moin all the figures), whilst on top its walls thin out and melt, so that one can pass a probe into the cavity without breaking through any truly nervous tissue. The cavity which one thus enters from without is named the fourth ventricle (4 in Figs.32and33). One can run the probe forwardthrough it, passing first under the cerebellum and then under a thin sheet of nervous tissue (thevalve of Vieussens) just anterior thereto, as far as theaqueduct of Silvius. Passing through this, the probe emerges forward into what was once the cavity of the anterior vesicle. But the covering has melted away at this place, and the cavity now forms a deep compressed pit or groove between the two walls of the vesicle, and is called thethird ventricle(3 in Figs.32and33). The 'aqueduct of Sylvius' is in consequence of this connection often called theiter a tertio ad quartum ventriculum. The walls of the vesicle form theoptic thalami(Thin all the figures).

Fig. 33(after Huxley).

From the anterior vesicle just in front of the thalami there buds out on either side an enlargement, into which the cavity of the vesicle continues, and which becomes thehemisphereof that side. In man its walls thicken enormously and form folds, the so-calledconvolutions, on their surface. At the same time they grow backwards rather than forwards of their starting-point just in front of the thalamus, arching over the latter; and growing fastest along their top circumference, they end by bending downwards and forwards again when they have passed the rear end of the thalamus. When fully developed in man, they overlay and cover in all the other parts of the brain. Their cavities form thelateral ventricles, easier to understand by a dissection than by a description. A probe can be passed into either of them from the third ventricle at its anteriorend; and like the third ventricle, their wall is melted down along a certain line, forming a long cleft through which they can be entered without rupturing the nervous tissue. This cleft, on account of the growth of the hemisphere outwards, backwards, and then downwards from its starting point, has got rolled in and tucked away beneath the apparent surface.[29]

At first the two hemispheres are connected only with their respective thalami. But during the fourth and fifth months of embryonic life they become connected with each other above the thalami through the growth between them of a massive system of transverse fibres which crosses the median line like a great bridge and is called thecorpus callosum. These fibres radiate in the walls of both hemispheres and form a direct connection between the convolutions of the right and of the left side. Beneath the corpus callosum another system of fibres called thefornixis formed, between which and the corpus callosum there is a peculiar connection. Just in front of the thalami, where the hemispheres begin their growth, a ganglionic mass called thecorpus striatum(C.S., Figs.32and33) is formed in their wall. It is complex in structure, consisting of two main parts, callednucleus lenticularisandnucleus candatusrespectively. The figures, with their respective explanations, will give a better idea of the farther details of structure than any verbal description; so, after some practical directions for dissecting the organ, I will pass to a brief account of the physiological relations of its different parts to each other.

Dissection of Sheep's Brain.—The way really to understand the brain is to dissect it. The brains of mammals differ only in their proportions, and from the sheep's one can learn all that is essential in man's. The student is therefore strongly urged to dissect a sheep's brain. Full directions of the order of procedure are given in the human dissectingbooks, e.g. Holden's Practical Anatomy (Churchill), Morrell's Student's Manual of Comparative Anatomy and Guide to Dissection (Longmans), and Foster and Langley's Practical Physiology (Macmillan). For the use of classes who cannot procure these books I subjoin a few practical notes. The instruments needed are a small saw, a chisel with a shoulder, and a hammer with a hook on its handle, all three of which form part of the regular medical autopsy-kit and can be had of surgical-instrument-makers. In addition a scalpel, a pair of scissors, a pair of dissecting-forceps, and a silver probe are required. The solitary student can find home-made substitutes for all these things but the forceps, which he ought to buy.The first thing is to get off the skull-cap. Make two saw-cuts, through the prominent portion of each condyle (or articular surface bounding the hole at the back of the skull, where the spinal cord enters) and passing forwards to the temples of the animal. Then make two cuts, one on each side, which cross these and meet in an angle on the frontal bone. By actual trial, one will find the best direction for the saw-cuts. It is hard to saw entirely through the skull-bone without in some places also sawing into the brain. Here is where the chisel comes in—one can break by a smart blow on it with the hammer any parts of the skull not quite sawn through. When the skull-cap is ready to come off one will feel it 'wobble.' Insert then the hook under its forward end and pull firmly. The bony skull-cap alone will come away, leaving the periosteum of the inner surface adhering to that of the base of the skull, enveloping the brain, and forming the so-calleddura materor outer one of its 'meninges.' This dura mater should be slit open round the margins, when the brain will be exposed wrapped in its nearest membrane, thepia mater, full of blood-vessels whose branches penetrate the tissues.The brain in its pia mater should now be carefully 'shelled out.' Usually it is best to begin at the forward end, turning it up there and gradually working backwards. Theolfactory lobesare liable to be torn; they must be carefully scooped from the pits in the base of the skull to which they adhere by the branches which they send through the bone into the nose-cavity. It is well to have a little blunt curved instrument expressly for this purpose. Next theoptic nervestie the brain down, and must be cut through—close to the chiasma is easiest. After that comes thepituitary body, which has to be left behind. It is attached by a neck, the so-calledinfundibulum, into the upper part of which the cavity of the third ventricle is prolonged downwards for a short distance. It has no known function and is probably a 'rudimentary organ.' Other nerves, into the detail of which I shall not go, must be cut successively. Their places in the human brain are shown inFig. 34.When theyare divided, and the portion of dura mater (tentorium) which projects between the hemispheres and the cerebellum is cut through at its edges, the brain comes readily out.

The first thing is to get off the skull-cap. Make two saw-cuts, through the prominent portion of each condyle (or articular surface bounding the hole at the back of the skull, where the spinal cord enters) and passing forwards to the temples of the animal. Then make two cuts, one on each side, which cross these and meet in an angle on the frontal bone. By actual trial, one will find the best direction for the saw-cuts. It is hard to saw entirely through the skull-bone without in some places also sawing into the brain. Here is where the chisel comes in—one can break by a smart blow on it with the hammer any parts of the skull not quite sawn through. When the skull-cap is ready to come off one will feel it 'wobble.' Insert then the hook under its forward end and pull firmly. The bony skull-cap alone will come away, leaving the periosteum of the inner surface adhering to that of the base of the skull, enveloping the brain, and forming the so-calleddura materor outer one of its 'meninges.' This dura mater should be slit open round the margins, when the brain will be exposed wrapped in its nearest membrane, thepia mater, full of blood-vessels whose branches penetrate the tissues.

The brain in its pia mater should now be carefully 'shelled out.' Usually it is best to begin at the forward end, turning it up there and gradually working backwards. Theolfactory lobesare liable to be torn; they must be carefully scooped from the pits in the base of the skull to which they adhere by the branches which they send through the bone into the nose-cavity. It is well to have a little blunt curved instrument expressly for this purpose. Next theoptic nervestie the brain down, and must be cut through—close to the chiasma is easiest. After that comes thepituitary body, which has to be left behind. It is attached by a neck, the so-calledinfundibulum, into the upper part of which the cavity of the third ventricle is prolonged downwards for a short distance. It has no known function and is probably a 'rudimentary organ.' Other nerves, into the detail of which I shall not go, must be cut successively. Their places in the human brain are shown inFig. 34.When theyare divided, and the portion of dura mater (tentorium) which projects between the hemispheres and the cerebellum is cut through at its edges, the brain comes readily out.

Fig. 34.—The human brain from below, with its nerves numbered, after Henle I, olfactory; II, optic; III, oculo-motorius; IV, trochlearis; V, trifacial; VI, abducens oculi; VII, facial; VIII, auditory; IX, glosso-pharyngeal; X, pneumogastric; XI, spinal accessory; XII, hypoglossal;ncI, first cervical, etc.

It is best examined fresh. If numbers of brains have to be prepared and kept, I have found it a good plan to put them first in a solution of chloride of zinc, just dense enough at first to float them, and to leave them for a fortnight or less. This softens the pia mater, which can then be removed in large shreds, after which it is enough to place them in quite weak alcohol to preserve them indefinitely, tough, elastic, and in their natural shape, though bleached to a uniform white color. Before immersion in the chloride all the more superficial adhesions of the parts must be broken through, to bringthe fluid into contact with a maximum of surface. If the brain is used fresh, the pia mater had better be removed carefully in most places with the forceps, scalpel, and scissors. Over the grooves between the cerebellum and hemispheres, and between the cerebellum and medulla oblongata, thin cobwebby moist transparent vestiges of thearachnoidmembrane will be found.The subdivisions may now be examined in due order. For the convolutions, blood-vessels, and nerves the more special books must be consulted.First, looked at from above, with the deeplongitudinal fissurebetween them, the hemispheres are seen partly overlapping the intricately wrinkledcerebellum, which juts out behind, and covers in turn almost all the medulla oblongata. Drawing the hemispheres apart, the brilliant whitecorpus callosumis revealed, some half an inch below their surface. There is no median partition in the cerebellum, but a median elevation instead.Looking at the brain from below, one still sees the longitudinal fissure in the median line in front, and on either side of it theolfactory lobes, much larger than in man; theoptic tractsandcommissureor'chiasma'; theinfundibulumcut through just behind them; and behind that the singlecorpus albicansormamillare, whose function is unknown and which is double in man. Next thecruraappear, converging upon the pons as if carrying fibres back from either side. Theponsitself succeeds, much less prominent than in man; and finally behind it comes the medulla oblongata, broad and flat and relatively large. The pons looks like a sort of collar uniting the two halves of the cerebellum, and surrounding the medulla, whose fibres by the time they have emerged anteriorly from beneath the collar have divided into the two crura. The inner relations are, however, somewhat less simple than what this description may suggest.Now turn forward the cerebellum; pull out the vascularchoroid plexusesof the pia, which fill the fourth ventricle; and bring the upper surface of themedulla oblongatainto view. Thefourth ventricleis a triangular depression terminating in a posterior point called thecalamus scriptorius. (Here a very fine probe may pass into the central canal of the spinal cord.) The lateral boundary of the ventricle on either side is formed by therestiform bodyorcolumn, which runs into the cerebellum, forming itsinferiororposterior peduncleon that side. Including the calamus scriptorius by their divergence, the posterior columns of the spinal cord continue into the medulla as thefasciculi graciles. These are at first separated from the broad restiform bodies by a slight groove. But this disappears anteriorly, and the 'slender' and 'ropelike' strands soon become outwardly indistinguishable.Turn next to the ventral surface of the medulla, and note theanterior pyramids, two roundish cords, one on either side of the slightmedian groove. The pyramids are crossed and closed over anteriorly by thepons Varolii, a broad transverse band which surrounds them like a collar, and runs up into the cerebellum on either side, forming itsmiddle peduncles. The pons has a slight median depression and its posterior edge is formed by thetrapeziumon either side. The trapezium consists of fibres which, instead of surrounding the pyramid, seem to start from alongside of it. It is not visible in man. Theolivary bodiesare small eminences on the medulla lying just laterally of the pyramids and below the trapezium.

The subdivisions may now be examined in due order. For the convolutions, blood-vessels, and nerves the more special books must be consulted.

First, looked at from above, with the deeplongitudinal fissurebetween them, the hemispheres are seen partly overlapping the intricately wrinkledcerebellum, which juts out behind, and covers in turn almost all the medulla oblongata. Drawing the hemispheres apart, the brilliant whitecorpus callosumis revealed, some half an inch below their surface. There is no median partition in the cerebellum, but a median elevation instead.

Looking at the brain from below, one still sees the longitudinal fissure in the median line in front, and on either side of it theolfactory lobes, much larger than in man; theoptic tractsandcommissureor'chiasma'; theinfundibulumcut through just behind them; and behind that the singlecorpus albicansormamillare, whose function is unknown and which is double in man. Next thecruraappear, converging upon the pons as if carrying fibres back from either side. Theponsitself succeeds, much less prominent than in man; and finally behind it comes the medulla oblongata, broad and flat and relatively large. The pons looks like a sort of collar uniting the two halves of the cerebellum, and surrounding the medulla, whose fibres by the time they have emerged anteriorly from beneath the collar have divided into the two crura. The inner relations are, however, somewhat less simple than what this description may suggest.

Now turn forward the cerebellum; pull out the vascularchoroid plexusesof the pia, which fill the fourth ventricle; and bring the upper surface of themedulla oblongatainto view. Thefourth ventricleis a triangular depression terminating in a posterior point called thecalamus scriptorius. (Here a very fine probe may pass into the central canal of the spinal cord.) The lateral boundary of the ventricle on either side is formed by therestiform bodyorcolumn, which runs into the cerebellum, forming itsinferiororposterior peduncleon that side. Including the calamus scriptorius by their divergence, the posterior columns of the spinal cord continue into the medulla as thefasciculi graciles. These are at first separated from the broad restiform bodies by a slight groove. But this disappears anteriorly, and the 'slender' and 'ropelike' strands soon become outwardly indistinguishable.

Turn next to the ventral surface of the medulla, and note theanterior pyramids, two roundish cords, one on either side of the slightmedian groove. The pyramids are crossed and closed over anteriorly by thepons Varolii, a broad transverse band which surrounds them like a collar, and runs up into the cerebellum on either side, forming itsmiddle peduncles. The pons has a slight median depression and its posterior edge is formed by thetrapeziumon either side. The trapezium consists of fibres which, instead of surrounding the pyramid, seem to start from alongside of it. It is not visible in man. Theolivary bodiesare small eminences on the medulla lying just laterally of the pyramids and below the trapezium.

Fig. 35.—Fourth ventricle, etc. (Henle).III, third ventricle;IV, fourth ventricle;P, anterior, middle, and posterior peduncles of cerebellum cut through;Cr, restiform body;Fg, funiculus gracilis;Cq, corpora quadrigemina.

Now cut through the peduncles of the cerebellum, close to their entrance into that organ. They give one surface of section on each side, though they receive contributions from three directions. Theposterior and middle portions we have seen: theanterior pedunclespass forward to thecorpora quadrigemina. The thin white layer of nerve-tissue between them and continuous with them is called thevalve of Vieussens. It covers part of the canal from the fourth ventricle to the third. The cerebellum being removed, examine it, and cut sections to show the peculiar distribution of white and gray matter, forming an appearance called thearbor vitæin the books.Now bend up the posterior edge of the hemispheres, exposing the corpora quadrigemina (of which the anterior pair are dubbed thenatesand the posterior thetestes), and noticing thepineal gland, a small median organ situated just in front of them and probably, like the pituitary body, a vestige of something useful in premammalian times. The rounded posterior edge of the corpus callosum is visible now passing from one hemisphere to the other. Turn it still farther up, letting the medulla, etc., hang down as much as possible and trace the under surface from this edge forward. It is broad behind but narrows forward, becoming continuous with thefornix. The anterior stem, so to speak, of this organ plunges down just in front of theoptic thalami, which now appear with the fornix arching over them, and the medianthird ventriclebetween them. The margins of the fornix, as they pass backwards, diverge laterally farther than the margins of the corpus callosum, and under the name ofcorpora fimbriataare carried into the lateral ventricles, as will be seen again.It takes a good topographical mind to understand these ventricles clearly, even when they are followed with eye and hand. A verbal description is absolutely useless. The essential thing to remember is that they are offshoots from the original cavity (now the third ventricle) of the anterior vesicle, and that a great split has occurred in the walls of the hemispheres so that they (the lateral ventricles) now communicate with the exterior along a cleft which appears sickle shaped, as it were, and folded in.The student will probably examine the relations of the parts in various ways. But he will do well to begin in any case by cutting horizontal slices off the hemispheres almost down to the level of the corpus callosum, and examining the distribution of gray and white matter on the surfaces of section, any one of which is the so-calledcentrum ovale. Then let him cut down in a fore-and-aft direction along the edge of the corpus callosum, till he comes 'through' and draw the hemispherical margin of the cut outwards—he will see a space which is the ventricle, and which farther cutting along the side and removing of its hemisphere-roof will lay more bare. The most conspicuous object on its floor is thenucleus caudatusof thecorpus striatum.

Now bend up the posterior edge of the hemispheres, exposing the corpora quadrigemina (of which the anterior pair are dubbed thenatesand the posterior thetestes), and noticing thepineal gland, a small median organ situated just in front of them and probably, like the pituitary body, a vestige of something useful in premammalian times. The rounded posterior edge of the corpus callosum is visible now passing from one hemisphere to the other. Turn it still farther up, letting the medulla, etc., hang down as much as possible and trace the under surface from this edge forward. It is broad behind but narrows forward, becoming continuous with thefornix. The anterior stem, so to speak, of this organ plunges down just in front of theoptic thalami, which now appear with the fornix arching over them, and the medianthird ventriclebetween them. The margins of the fornix, as they pass backwards, diverge laterally farther than the margins of the corpus callosum, and under the name ofcorpora fimbriataare carried into the lateral ventricles, as will be seen again.

It takes a good topographical mind to understand these ventricles clearly, even when they are followed with eye and hand. A verbal description is absolutely useless. The essential thing to remember is that they are offshoots from the original cavity (now the third ventricle) of the anterior vesicle, and that a great split has occurred in the walls of the hemispheres so that they (the lateral ventricles) now communicate with the exterior along a cleft which appears sickle shaped, as it were, and folded in.

The student will probably examine the relations of the parts in various ways. But he will do well to begin in any case by cutting horizontal slices off the hemispheres almost down to the level of the corpus callosum, and examining the distribution of gray and white matter on the surfaces of section, any one of which is the so-calledcentrum ovale. Then let him cut down in a fore-and-aft direction along the edge of the corpus callosum, till he comes 'through' and draw the hemispherical margin of the cut outwards—he will see a space which is the ventricle, and which farther cutting along the side and removing of its hemisphere-roof will lay more bare. The most conspicuous object on its floor is thenucleus caudatusof thecorpus striatum.

Fig. 36.—Horizontal section of human brain just above the thalami.—Ccl, corpus callosum in section;Cs, corpus striatum;Sl, septum lucidum;Cf, columns of the fornix;Tho, optic thalami;Cn, pineal gland. (After Henle.)

Cut the corpus callosum transversely through near its posterior edge and bend the anterior portion of it forwards and sideways. The rear edge (splenium) leftin situbends round and downwards and becomes continuous with thefornix. The anterior part is also continuous with the fornix, but more along the median line, where a thinnish membrane, theseptum lucidum, triangular in shape, reaching from the one body to the other, practically forms a sort of partition between thecontiguous portion of the lateral ventricles on the two sides. Break through theseptumif need be and expose the upper surface of the fornix, broad behind and narrow in front where itsanterior pillarsplunge down in front of the third ventricle (from a thickening in whose anterior walls they were originally formed), and finally penetrate the corpus albicans. Cut these pillars through and fold them back, exposing the thalamic portion of the brain, and noting the under surface of the fornix. Its divergingposterior pillarsrun backwards, downwards, and then forwards again, forming with their sharp edges thecorpora fimbriata, which bound the cleft by which the ventricle lies open. The semi-cylindrical welts behind thecorpora fimbriataand parallel thereto in the wall of the ventricle are thehippocampi. Imagine the fornix and corpus callosum shortened in the fore-and-aft direction to a transverse cord; imagine the hemispheres not having grown backwards and downwards round the thalamus; and the corpus fimbriatum on either side would then be the upper or anterior margin of a split in the wall of the hemispheric ventricle of which the lower and posterior margin would be the posterior border of the corpus striatum where it grows out of the thalamus.The little notches just behind the anterior pillar of the fornix and between them and the thalami are the so-calledforamina of Monrothrough which the plexus of vessels, etc., passes from the median to the lateral ventricles.See the thickmiddle commissurejoining the two thalami, just as the corpus callosum and fornix join the hemispheres. These are all embryological aftergrowths. Seek also theanterior commissurecrossing just in front of the anterior pillars of the fornix, as well as theposterior commissurewith its lateral prolongations along the thalami, just below the pineal gland.On a median section, note the thinnishanterior wallof the third ventricle and its prolongation downwards into theinfundibulum.Turn up or cut off the rear end of one hemisphere so as to see clearly the optic tracts turning upwards towards the rear corner of the thalamus. Thecorpora geniculatato which they also go, distinct in man, are less so in the sheep. The lower ones are visible between the optic-tract band and the 'testes,' however.The brain's principal parts are thus passed in review. A longitudinal section of the whole organ through the median line will be found most instructive (Fig. 37). The student should also (on afreshbrain, or one hardened in bichromate of potash or ammonia to save the contrast of color between white and gray matter) make transverse sections through thenatesandcrura, and through the

The little notches just behind the anterior pillar of the fornix and between them and the thalami are the so-calledforamina of Monrothrough which the plexus of vessels, etc., passes from the median to the lateral ventricles.

See the thickmiddle commissurejoining the two thalami, just as the corpus callosum and fornix join the hemispheres. These are all embryological aftergrowths. Seek also theanterior commissurecrossing just in front of the anterior pillars of the fornix, as well as theposterior commissurewith its lateral prolongations along the thalami, just below the pineal gland.

On a median section, note the thinnishanterior wallof the third ventricle and its prolongation downwards into theinfundibulum.

Turn up or cut off the rear end of one hemisphere so as to see clearly the optic tracts turning upwards towards the rear corner of the thalamus. Thecorpora geniculatato which they also go, distinct in man, are less so in the sheep. The lower ones are visible between the optic-tract band and the 'testes,' however.

The brain's principal parts are thus passed in review. A longitudinal section of the whole organ through the median line will be found most instructive (Fig. 37). The student should also (on afreshbrain, or one hardened in bichromate of potash or ammonia to save the contrast of color between white and gray matter) make transverse sections through thenatesandcrura, and through the

Fig. 37.—Median section of human brain below the hemispheres.Th, thalamus;Cg, corpora quadrigemina;VIII, third ventricle;Com, middle commissure;F, columns of fornix;Inf, infundibulum;Op.n, optic nerve;Pit, pituitary body;Av, arbor vitæ. (After Obersteiner).

hemispheres just in front of the corpus albicans. The latter section shows on each side thenucleus lenticularisof the corpus striatum, and also theinner capsule(seeFig. 38,Nl, andIc).

Fig. 38.—Transverse section through right hemisphere (after Gegenbaur).Cc, corpus callosum;Pf, pillars of fornix;Ic, internal capsule;V, third ventricle;Nl, nucleus lenticularis.

When all is said and done, the fact remains that, for the beginner, the understanding of the brain's structure is not an easy thing. It must be gone over and forgotten and learned again many times before it is definitively assimilated by the mind. But patience and repetition, here as elsewhere, will bear their perfect fruit.

General Idea of Nervous Function.—If I begin chopping the foot of a tree, its branches are unmoved by my act, and its leaves murmur as peacefully as ever in the wind. If, on the contrary, I do violence to the foot of a fellow-man, the rest of his body instantly responds to the aggression by movements of alarm or defence. The reason of this difference is that the man has a nervous system, whilst the tree has none; and the function of the nervous system is to bring each part into harmonious coöperation with every other. The afferent nerves, when excited by some physical irritant, be this as gross in its mode of operation as a chopping axe or as subtle as the waves of light, conveys the excitement to the nervous centres. The commotion set up in the centres does not stop there, but discharges through the efferent nerves, exciting movements which vary with the animal and with the irritant applied. These acts of response have usually the common character of being of service. They ward off the noxious stimulus and support the beneficial one; whilst if, in itself indifferent, the stimulus be a sign of some distant circumstance of practical importance, the animal's acts are addressed to this circumstance so as to avoid its perils or secure its benefits, as the case may be. To take a common example, if I hear the conductor calling 'All aboard!' as I enter the station, my heart first stops, then palpitates, and my legs respond to the air-waves falling on my tympanum by quickening their movements. If I stumble as I run, the sensation of falling provokes a movement of the hands towards the direction of the fall, the effect of which is to shield thebody from too sudden a shock. If a cinder enter my eye, its lids close forcibly and a copious flow of tears tends to wash it out.

These three responses to a sensational stimulus differ, however, in many respects. The closure of the eye and the lachrymation are quite involuntary, and so is the disturbance of the heart. Such involuntary responses we know as 'reflex' acts. The motion of the arms to break the shock of falling may also be called reflex, since it occurs too quickly to be deliberately intended. It is, at any rate, less automatic than the previous acts, for a man might by conscious effort learn to perform it more skilfully, or even to suppress it altogether. Actions of this kind, into which instinct and volition enter upon equal terms, have been called 'semi-reflex.' The act of running towards the train, on the other hand, has no instinctive element about it. It is purely the result of education, and is preceded by a consciousness of the purpose to be attained and a distinct mandate of the will. It is a 'voluntary act.' Thus the animal's reflex and voluntary performances shade into each other gradually, being connected by acts which may often occur automatically, but may also be modified by conscious intelligence.

The Frog's Nerve-centres.—Let us now look a little more closely at what goes on.

The best way to enter the subject will be to take a lower creature, like a frog, and study by the vivisectional method the functions of his different nerve-centres. The frog's nerve-centres are figured in the diagram over the page, which needs no further explanation. I shall first proceed to state what happens when various amounts of the anterior parts are removed, in different frogs, in the way in which an ordinary student removes them—that is, with no extreme precautions as to the purity of the operation.

If, then, we reduce the frog's nervous system to the spinal cord alone, by making a section behind the base of the skull, between the spinal cord and the medulla oblongata,thereby cutting off the brain from all connection with the rest of the body, the frog will still continue to live, but with a very peculiarly modified activity. It ceases to breathe or swallow; it lies flat on its belly, and does not, like a normal frog, sit up on its forepaws, though its hind-legs are kept, as usual, folded against its body and immediately resume this position if drawn out. If thrown on its back it lies there quietly, without turning over like a normal frog. Locomotion and voice seem entirely abolished. If we suspend it by the nose, and irritate different portions of its skin by acid, it performs a set of remarkable 'defensive' movements calculated to wipe away the irritant. Thus, if the breast be touched, both fore-paws will rub it vigorously; if we touch the outer side of the elbow, the hind-foot of the same side will rise directly to the spot and wipe it. The back of the foot will rub the knee if that be attacked, whilst if the foot be cut away, the stump will make ineffectual movements, and then, in many frogs, a pause will come, as if for deliberation, succeeded by a rapid passage of the opposite unmutilated foot to the acidulated spot.

Fig. 39.—C,H, cerebral hemispheres;O Th, optic thalami;O L, optic lobes;Cb, cerebellum;M O, medulla oblongata;S C, spinal cord.

The most striking character of all these movements, after their teleological appropriateness, is their precision. They vary, in sensitive frogs and with a proper amount of irritation, so little as almost to resemble in their machine-like regularity the performances of a jumping-jack, whose legs must twitch whenever you pull the string. The spinal cord of the frog thus contains arrangements of cells and fibres fitted to convert skin-irritations into movements of defence. We may call it thecentre for defensive movementsin this animal. We may indeed go farther than this, and by cutting the spinal cord in various places find that itsseparate segments are independent mechanisms, for appropriate activities of the head and of the arms and legs respectively. The segment governing the arms is especially active, in male frogs, in the breeding season; and these members alone, with the breast and back appertaining to them, and everything else cut away, will actively grasp a finger placed between them and remain hanging to it for a considerable time.

Similarly of the medulla oblongata, optic lobes, and other centres between the spinal cord and the hemispheres of the frog. Each of them is proved by experiment to contain a mechanism for the accurate execution, in response to definite stimuli, of certain special acts. Thus with the medulla the animal swallows; with the medulla and cerebellum together he jumps, swims, and turns over from his back; with his optic lobes he croaks when pinched; etc.A frog which has lost his cerebral hemispheres alone is by an unpractised observer indistinguishable from a normal animal.

Not only is he capable, on proper instigation, of all the acts already mentioned, but he guides himself by sight, so that if an obstacle be set up between him and the light, and he be forced to move forward, he either jumps over it or swerves to one side. He manifests the sexual instinct at the proper seasons, and discriminates between male and female individuals of his own species. He is, in short, so similar in every respect to a normal frog that it would take a person very familiar with these animals to suspect anything wrong or wanting about him; but even then such a person would soon remark the almost entire absence of spontaneous motion—that is, motion unprovoked by any present incitation of sense. The continued movements of swimming, performed by the creature in the water, seem to be the fatal result of the contact of that fluid with its skin. They cease when a stick, for example, touches his hands. This is a sensible irritant towards which the feet are automatically drawn by reflex action, and on which the animal remains sitting. He manifests no hunger, and willsuffer a fly to crawl over his nose unsnapped at. Fear, too, seems to have deserted him. In a word, he is an extremely complex machine whose actions, so far as they go, tend to self-preservation; but still amachine, in this sense—that it seems to contain no incalculable element. By applying the right sensory stimulus to him we are almost as certain of getting a fixed response as an organist is of hearing a certain tone when he pulls out a certain stop.

But now if to the lower centres we add the cerebral hemispheres, or if, in other words, we make an intact animal the subject of our observations, all this is changed. In addition to the previous responses to present incitements of sense, our frog now goes through long and complex acts of locomotionspontaneously, or as if moved by what in ourselves we should call an idea. His reactions to outward stimuli vary their form, too. Instead of making simple defensive movements with his hind-legs, like a headless frog, if touched; or of giving one or two leaps and then sitting still like a hemisphereless one, he makes persistent and varied efforts of escape, as if, not the mere contact of the physiologist's hand, but the notion of danger suggested by it were now his spur. Led by the feeling of hunger, too, he goes in search of insects, fish, or smaller frogs, and varies his procedure with each species of victim. The physiologist cannot by manipulating him elicit croaking, crawling up a board, swimming or stopping, at will. His conduct has become incalculable—we can no longer foretell it exactly. Effort to escape is his dominant reaction, but hemaydo anything else, even swell up and become perfectly passive in our hands.

Such are the phenomena commonly observed, and such the impressions which one naturally receives. Certain general conclusions follow irresistibly. First of all the following:

The acts of all the centres involve the use of the same muscles.When a brainless frog's hind-leg wipes the acid,he calls into play all the leg-muscles which a frog with his full medulla oblongata and cerebellum uses when he turns from his back to his belly. Their contractions are, however,combineddifferently in the two cases, so that the results vary widely. We must consequently conclude that specific arrangements of cells and fibres exist in the cord for wiping, in the medulla for turning over, etc. Similarly they exist in the thalami for jumping over seen obstacles and for balancing the moved body; in the optic lobes for creeping backwards, or what not. But in the hemispheres, since the presence of these organsbrings no new elementary form of movementwith it, but onlydetermines differently the occasionson which the movements shall occur, making the usual stimuli less fatal and machine-like, we need suppose no such machinerydirectlycoördinative of muscular contractions to exist. We may rather assume, when the mandate for a wiping-movement is sent forth by the hemispheres, that a current goes straight to the wiping-arrangement in the spinal cord, exciting this arrangement as a whole. Similarly, if an intact frog wishes to jump, all he need do is to excite from the hemispheres the jumping-centre in the thalami or wherever it may be, and the latter will provide for the details of the execution. It is like a general ordering a colonel to make a certain movement, but not telling him how it shall be done.

The same muscle, then, is repeatedly represented at different heights; and at each it enters into a different combination with other muscles to coöperate in some special form of concerted movement. At each height the movement is discharged by some particular form of sensorial stimulus, whilst the stimuli which discharge the hemispheres would seem not so much to be elementary sorts of sensation, as groups of sensations forming determinateobjectsorthings.

The Pigeon's Lower Centres.—The results are just the same if, instead of a frog, we take a pigeon, cut out his hemispheres carefully and wait till he recovers from theoperation. There is not a movement natural to him which this brainless bird cannot execute; he seems, too, after some days to execute movements from some inner irritation, for he moves spontaneously. But his emotions and instincts exist no longer. In Schrader's striking words:

"The hemisphereless animal moves in a world of bodies which ... are all of equal value for him.... He is, to use Goltz's apt expression,impersonal.... Every object is for him only a space-occupying mass, he turns out of his path for an ordinary pigeon no otherwise than for a stone. He may try to climb over both. All authors agree that they never found any difference, whether it was an inanimate body, a cat, a dog, or a bird of prey which came in their pigeon's way. The creature knows neither friends nor enemies, in the thickest company it lives like a hermit. The languishing cooing of the male awakens no more impression than the rattling of the peas, or the call-whistle which in the days before the injury used to make the birds hasten to be fed. Quite as little as the earlier observers have I seen hemisphereless she-birds answer the courting of the male. A hemisphereless male will coo all day long and show distinct signs of sexual excitement, but his activity is without any object, it is entirely indifferent to him whether the she-bird be there or not. If one is placed near him, he leaves her unnoticed.... As the male pays no attention to the female, so she pays none to her young. The brood may follow the mother ceaselessly calling for food, but they might as well ask it from a stone.... The hemisphereless pigeon is in the highest degree tame, and fears man as little as cat or bird of prey."

General Notion of Hemispheres.—All these facts lead us, when we try to formulate them broadly, to some such conception as this:The lower centres act from present sensational stimuli alone; the hemispheres act from considerations, the sensations which they may receive serving only as suggesters of these. But what are considerations but expectations, in the fancy, of sensations which will be felt one way or another according as action takes this course orthat? If I step aside on seeing a rattlesnake, from considering how dangerous an animal he is, the mental materials which constitute my prudential reflection are images more or less vivid of the movement of his head, of a sudden pain in my leg, of a state of terror, a swelling of the limb, a chill, delirium, death, etc., etc., and the ruin of my hopes. But all these images are constructed out of my past experiences. They arereproductionsof what I have felt or witnessed. They are, in short,remotesensations; and the main difference between the hemisphereless animal and the whole one may be concisely expressed by saying thatthe one obeys absent, the other only present, objects.

The hemispheres would then seem to be the chief seat of memory.Vestiges of past experience must in some way be stored up in them, and must, when aroused by present stimuli, first appear as representations of distant goods and evils; and then must discharge into the appropriate motor channels for warding off the evil and securing the benefits of the good. If we liken the nervous currents to electric currents, we can compare the nervous system,C, below the hemispheres to a direct circuit from sense-organ to muscle along the lineS ...C ...MofFig. 40.The hemisphere,H, adds the long circuit or loop-line through which the current may pass when for any reason the direct line is not used.

Fig. 40.

Thus, a tired wayfarer on a hot day throws himself on the damp earth beneath a maple-tree. The sensations of delicious rest and coolness pouring themselves through the direct line would naturally discharge into the muscles of complete extension: he would abandon himself to the dangerous repose. But the loop-line being open, part of the current is drafted along it, and awakens rheumatic or catarrhal reminiscences, which prevail over the instigations of sense, and make the man arise and pursue his wayto where he may enjoy his rest more safely. Presently we shall examine the manner in which the hemispheric loop-line may be supposed to serve as a reservoir for such reminiscences as these. Meanwhile I will ask the reader to notice some corollaries of its being such a reservoir.

First, no animal without it can deliberate, pause, postpone, nicely weigh one motive against another, or compare. Prudence, in a word, is for such a creature an impossible virtue. Accordingly we see that nature removes those functions in the exercise of which prudence is a virtue from the lower centres and hands them over to the cerebrum. Wherever a creature has to deal with complex features of the environment, prudence is a virtue. The higher animals have so to deal; and the more complex the features, the higher we call the animals. The fewer of his acts, then, cansuchan animal perform without the help of the organs in question. In the frog many acts devolve wholly on the lower centres; in the bird fewer; in the rodent fewer still; in the dog very few indeed; and in apes and men hardly any at all.

The advantages of this are obvious. Take the prehension of food as an example and suppose it to be a reflex performance of the lower centres. The animal will be condemned fatally and irresistibly to snap at it whenever presented, no matter what the circumstances may be; he can no more disobey this prompting than water can refuse to boil when a fire is kindled under the pot. His life will again and again pay the forfeit of his gluttony. Exposure to retaliation, to other enemies, to traps, to poisons, to the dangers of repletion, must be regular parts of his existence. His lack of all thought by which to weigh the danger against the attractiveness of the bait, and of all volition to remain hungry a little while longer, is the direct measure of his lowness in the mental scale. And those fishes which, like our cunners and sculpins, are no sooner thrown back from the hook into the water than they automatically seize the hook again, would soon expiate the degradation of theirintelligence by the extinction of their type, did not their extraordinary fecundity atone for their imprudence. Appetite and the acts it prompts have consequently become in all higher vertebrates functions of the cerebrum. They disappear when the physiologist's knife has left the subordinate centres alone in place. The brainless pigeon will starve though left on a corn-heap.

Take again the sexual function. In birds this devolves exclusively upon the hemispheres. When these are shorn away the pigeon pays no attention to the billings and cooings of its mate. It is the same, according to Goltz, with male dogs who have suffered large losses of cerebral tissue. Those who have read Darwin's Descent of Man will recollect what an importance this author ascribes to the agency of sexual selection in the amelioration of the breeds of birds. The females are naturally coy, and their coyness must be overcome by the exhibition of the gorgeous plumage, and various accomplishments in the way of strutting and fighting, of the males. In frogs and toads, on the other hand, where (as we saw onpage 94) the sexual instinct devolves upon the lower centres, we find a machine-like obedience to the present incitements of sense, and an almost total exclusion of the power of choice. The consequence is that every spring an immense waste of batrachian life, involving numbers of adult animals and innumerable eggs, takes place from no other cause than the blind character of the sexual impulse in these creatures.

No one need be told how dependent all human social elevation is upon the prevalence of chastity. Hardly any factor measures more than this the difference between civilization and barbarism. Physiologically interpreted, chastity means nothing more than the fact that present solicitations of sense are overpowered by suggestions of æsthetic and moral fitness which the circumstances awaken in the cerebrum; and that upon the inhibitory or permissive influence of these alone action directly depends.

Within the psychic life due to the cerebrum itself thesame general distinction obtains, between considerations of the more immediate and considerations of the more remote. In all ages the man whose determinations are swayed by reference to the most distant ends has been held to possess the highest intelligence. The tramp who lives from hour to hour; the bohemian whose engagements are from day to day; the bachelor who builds but for a single life; the father who acts for another generation; the patriot who thinks of a whole community and many generations; and, finally, the philosopher and saint whose cares are for humanity and for eternity,—these range themselves in an unbroken hierarchy, wherein each successive grade results from an increased manifestation of the special form of action by which the cerebral centres are distinguished from all below them.

The Automaton-Theory.—In the 'loop-line' along which the memories and ideas of the distant are supposed to lie, the action, so far as it is a physical process, must be interpreted after the type of the action in the lower centres. If regarded here as a reflex process, it must be reflex there as well. The current in both places runs out into the muscles only after it has first run in; but whilst the path by which it runs out is determined in the lower centres by reflections few and fixed amongst the cell-arrangements, in the hemispheres the reflections are many and instable. This, it will be seen, is only a difference of degree and not of kind, and does not change the reflex type. The conception ofallaction as conforming to this type is the fundamental conception of modern nerve-physiology. This conception, now, has led to two quite opposite theories about the relation to consciousness of the nervous functions. Some authors, finding that the higher voluntary functions seem to require the guidance of feeling, conclude that over the lowest reflexes some such feeling also presides, though it may be a feeling connected with the spinal cord, of which the higher conscious self connected with the hemispheres remains unconscious. Others, finding thatreflex and semi-automatic acts may, notwithstanding their appropriateness, take place with an unconsciousness apparently complete, fly to the opposite extreme and maintain that the appropriateness even of the higher voluntary actions connected with the hemispheres owes nothing to the fact that consciousness attends them. They are, according to these writers, results of physiological mechanism pure and simple.

To comprehend completely this latter doctrine one should apply it to examples. The movements of our tongues and pens, the flashings of our eyes in conversation, are of course events of a physiological order, and as such their causal antecedents may be exclusively mechanical. If we knew thoroughly the nervous system of Shakespeare, and as thoroughly all his environing conditions, we should be able, according to the theory of automatism, to show why at a given period of his life his hand came to trace on certain sheets of paper those crabbed little black marks which we for shortness' sake call the manuscript of Hamlet. We should understand the rationale of every erasure and alteration therein, and we should understand all this without in the slightest degree acknowledging the existence of the thoughts in Shakespeare's mind. The words and sentences would be taken, not as signs of anything beyond themselves, but as little outward facts, pure and simple. In like manner, the automaton-theory affirms, we might exhaustively write the biography of those two hundred pounds, more or less, of warmish albuminoid matter called Martin Luther, without ever implying that it felt.

But, on the other hand, nothing in all this could prevent us from giving an equally complete account of either Luther's or Shakespeare's spiritual history, an account in which every gleam of thought and emotion should find its place. The mind-history would run alongside of the body-history of each man, and each point in the one would correspond to, but not react upon, a point in the other. So the melody floats from the harp-string, but neither checksnor quickens its vibrations; so the shadow runs alongside the pedestrian, but in no way influences his steps.

As a mereconception, and so long as we confine our view to the nervous centres themselves, few things are more seductive than this radically mechanical theory of their action. And yet our consciousnessis there, and has in all probability been evolved, like all other functions, for a use—it is to the highest degree improbablea priorithat it should have no use. Its useseemsto be that ofselection; but to select, it must be efficacious. States of consciousness which feel right are held fast to; those which feel wrong are checked. If the 'holding' and the 'checking' of the conscious states severally mean also the efficacious reinforcing or inhibiting of the correlated neural processes, then it would seem as if the presence of the states of mind might help to steer the nervous system and keep it in the path which to the consciousness seemed best. Now on the average what seems best to consciousness is really best for the creature. It is a well-known fact that pleasures are generally associated with beneficial, pains with detrimental, experiences. All the fundamental vital processes illustrate this law. Starvation; suffocation; privation of food, drink, and sleep; work when exhausted; burns, wounds, inflammation; the effects of poison, are as disagreeable as filling the hungry stomach, enjoying rest and sleep after fatigue, exercise after rest, and a sound skin and unbroken bones at all times, are pleasant. Mr. Spencer and others have suggested that these coincidences are due, not to any preëstablished harmony, but to the mere action of natural selection, which would certainly kill off in the long-run any breed of creatures to whom the fundamentally noxious experience seemed enjoyable. An animal that should take pleasure in a feeling of suffocation would, if that pleasure were efficacious enough to make him keep his head under water, enjoy a longevity of four or five minutes. But if conscious pleasure does not reinforce, and conscious pain does notinhibit, anything, one does not see (without some sucha priorirational harmony as would be scouted by the 'scientific' champions of the automaton-theory) why the most noxious acts, such as burning, might not with perfect impunity give thrills of delight, and the most necessary ones, such as breathing, cause agony. The only considerable attempt that has been made to explain thedistributionof our feelings is that of Mr. Grant Allen in his suggestive little work,Physiological Æsthetics; and his reasoning is based exclusively on that causal efficacy of pleasures and pains which the partisans of pure automatism so strenuously deny.

Probability and circumstantial evidence thus run dead against the theory that our actions arepurelymechanical in their causation. From the point of view of descriptive Psychology (even though we be bound to assume, as onp. 6, that all our feelings have brain-processes for their condition of existence, and can be remotely traced in every instance to currents coming from the outer world) we have no clear reason to doubt that the feelings may react so as to further or to dampen the processes to which they are due. I shall therefore not hesitate in the course of this book to use the language of common-sense. I shall talk as if consciousness kept actively pressing the nerve-centres in the direction of its own ends, and was no mere impotent and paralytic spectator of life's game.

The Localization of Functions in the Hemispheres.—The hemispheres, we lately said, must be the organ of memory, and in some way retain vestiges of former currents, by means of which mental considerations drawn from the past may be aroused before action takes place. The vivisections of physiologists and the observations of physicians have of late years given a concrete confirmation to this notion which the first rough appearances suggest. The various convolutions have had special functions assigned to them in relation to this and that sense-organ, as well as to this or that portion of the muscular system. This book isno place for going over the evidence in detail, so I will simply indicate the conclusions which are most probable at the date of writing.

Mental and Cerebral Elements.—In the first place, there is a very neat parallelism between the analysis of brain-functions by the physiologists and that of mental functions by the 'analytic' psychologists.

The phrenological brain-doctrine divided the brain into 'organs,' each of which stood for the man in a certain partial attitude. The organ of 'Philoprogenitiveness,' with its concomitant consciousness, is an entire man so far as he loves children, that of 'Reverence' is an entire man worshipping, etc. The spiritualistic psychology, in turn, divided the Mind into 'faculties,' which were also entire mental men in certain limited attitudes. But 'faculties' are not mentalelementsany more than 'organs' are brain-elements. Analysis breaks both into more elementary constituents.

Brain and mind alike consist of simple elements, sensory and motor. "All nervous centres," says Dr. Hughlings Jackson, "from the lowest to the very highest (the substrata of consciousness), are made up of nothing else than nervous arrangements, representing impressions and movements.... I do not see of what other materials the braincanbe made." Meynert represents the matter similarly when he calls the cortex of the hemispheres the surface of projection for every muscle and every sensitive point of the body. The muscles and the sensitive points arerepresentedeach by a cortical point, and the Brain is little more than the sum of all these cortical points, to which, on the mental side, as many sensations andideascorrespond. The sensations and ideas of sensation and of motion are, in turn, the elements out of which the Mind is built according to the analytic school of psychology. The relations between objects are explained by 'associations' between the ideas; and the emotional and instinctive tendencies, by associations between ideas and movements.The same diagram can symbolize both the inner and the outer world; dots or circles standing indifferently for cells or ideas, and lines joining them, for fibres or associations. The associationist doctrine of 'ideas' may be doubted to be a literal expression of the truth, but it probably will always retain a didactic usefulness. At all events, it is interesting to see how well physiological analysis plays into its hands. To proceed to details.

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