Chapter VI: Part 6

In the Reptilia the medulla has a marked flexure with a ventral
convexity, and an undoubted cerebral cortex for the first time makes
its appearance. The mesial wall of the cerebral hemisphere is divided
into a large dorsal hippocampal area (fig. 18, _Hip._) and a smaller
ventral olfactory tubercle. Between these two a narrow area of
ganglionic matter runs forward from the side of the _lamina
terminalis_ and is known as the paraterminal or precommissural area
(Elliot Smith, _Journ. Anat. and Phys._ vol. xxxii. p. 411). To the
upper lateral part of the hemisphere Elliot Smith has given the name
of _neopallium_, while the lower lateral part, imperfectly separated
from it, is called the _pyriform lobe_. In the Lacertilia the pineal
eye, if it be an eye, is better developed than in any existing
vertebrate, though even in them there is no evidence of its being used
for sight. Behind the so-called pineal eye and its stalk is the
_epiphysis_ or pineal body, and sometimes there is a dorsal sac
between them (see fig. 18).[1] The middle or soft commissure appears
in certain reptiles (_Crocodilia_ and _Chelonia_), as does also the
_corpus mammillare_ (Edinger, Senckenberg, _Naturf. Gesell._ Bd. xix.,
1896, and Bd. xxii., 1899; Haller, _Morph. Jahrb._ Bd. xxviii., 1900,
p. 252). Among the birds there is great unity of type, the cerebellum
is large and, by its forward projection, presses the optic lobes down
toward the ventro-lateral part of the brain. The cerebral hemispheres
are also large, owing chiefly to the great size of the _corpora
striata_, which already show a differentiation into caudate nucleus,
putamen and globus pallidus. The pallium is reptilian in character,
though its cortical area is more extensive. The geniculate bodies are
very large (Bumm, _Zeits. wiss. Zool._ Bd. xxxviii., 1883, p. 430;
Brandis, _Arch. mikr. Anat._ Bd. xli., 1893, p. 623, and xliii., 1894,
p. 96, and xliv., 1895, p. 534; Boyce and Warrington, _Phil. Trans._
vol. cxci., 1899, p. 293).

Among the Mammalia the Monotremata have a cerebellum which shows, in
addition to the central lobe of the lower vertebrates, a flocculus on
each side, and the two halves of the cerebellum are united by a
ventral commissure, the _pons varolii_. The pallium is reptilian in
its arrangement, but that part of it which Elliot Smith has named the
neopallium is very large, both in the Ornithorynchus and Echidna, a
fact very difficult to account for. In the latter animal the cortical
area is so extensive as to be thrown into many and deep sulci, and yet
the Echidna is one of the lowliest of mammals in other respects. A
well-marked rhinal fissure separates the pyriform lobe from the
neopallium, while, on the mesial surface, the hippocampal fissure
separates the neopallium from the hippocampal area. Just below the
hippocampal fissure a specially coloured tract indicates the first
appearance of the fascia dentata (see fig. 20). The anterior
commissure is divided, as in reptiles, into dorsal and ventral parts,
of which the latter is the larger (fig. 20, _Comm. V. and D_.), while
just behind the dorsal part is the first appearance of the fimbria or
fornix. In addition to the two fissures already named, there is, in
the Echidna, one which in position and mode of formation corresponds
with the Sylvian fissure of higher mammals. Elliot Smith, however,
wisely refuses to homologize it absolutely with that fissure, and
proposes the name of pseudosylvian for it. The pineal body is
rudimentary, and the optic lobes are now, and throughout the Mammalia,
subdivided into four _corpora quadrigemina_.

FIG. 19.--Ventral and Dorsal Views of the Brain of Ornithorynchus.]

Among the Marsupialia the Tasmanian devil (Sarcophilus) gives a very
good idea of a generalized mammalian brain, and shows a large
development of the parts concerned in the sense of smell. The most
important advance on the monotreme brain is that the calcarine fissure
has now appeared on the posterior part of the mesial surface and
causes a bulging into the ventricle, called the _calcar avis_ or
hippocampus minor, just as the hippocampal fissure causes the
_hippocampus major_ (Gervais, _Nuov. Arch. Mus_. tom. v., 1869;
Ziehen, _Jenaische Denkschr_. Bd. vi., 1897).

FIG. 20.--Mesial and Lateral Views of the Brain of Ornithorynchus.]

FIG. 2l.--Mesial and Lateral Views of the Brain of the Tasmanian Devil
(_Sarcophilus_).]

In the Eutheria or mammals above the marsupials, the cerebellum
gradually becomes more complex, owing to the appearance of lateral
lobes between the flocculus and the vermis, as well as the
paraflocculus on the outer side of the flocculus. The corpus callosum
now first appears as a bridge between the neopallia, and its
development leads to the stretching of the hippocampal formation, so
that in the higher mammals the hippocampus is only found in the lower
and back part of the ventricle, while the rudiments of the dorsal part
remain as the _striae longitudinals_ on the corpus callosum. The
dorsal part of the original anterior commissure becomes the fornix,
and the paraterminal area is modified to form the septum lucidum. The
first appearance of the fissure of Rolando is probably in some of the
Carnivora, in which, as the _sulcus crucialis_, it forms the posterior
boundary of the "ursine lozenge" described by Mivart (_Journ. Linn.
Soc_. vol. xix., 1886) (see fig. 22, _Sulc. Cru_.). In the higher apes
or Anthropoidea the human fissures and sulci are largely recognizable,
so that a gibbon's brain, apart from all question of comparative
anatomy, forms a useful means of demonstrating to a junior class the
main gyri and sulci of Man in a simple and diagrammatic way. The main
points of difference, apart from greater simplicity, are that the
central lobe or island of Reil is exposed on the surface of the brain,
as it is in the human foetus, and that the anterior part of the
occipital lobe has a well-marked vertical sulcus, called the simian
sulcus or _Affenspalte_; this often has a semilunar shape with its
convexity forward, and is then called the _sulcus lunatus_. It is
usually concealed in European brains by the overgrowth of the
surrounding gyri, but it occasionally remains, though less frequently
than in the brains of Egyptian fellaheen. Its relation to the _white
stria of Gennari_ is especially interesting, and is recorded by Elliot
Smith in the _Anatomischer Anzeiger_, Bd. xxiv., 1904, p. 436. The
rhinal fissure, which is so characteristic a feature of the lower
mammals, almost disappears in Man, and is only represented by the
_incisura temporalis_ (see fig. 11, _i.t_). The hippocampal fissure
persists with little modification all through the mammalian class. The
calcarine fissure remains with many modifications from the marsupials
to man, and in view of the famous controversy of 1864, in which Owen,
Huxley and the then bishop of Oxford took part, it is interesting to
note that its hippocampus minor can now be clearly demonstrated, even
in the Marsupialia. Another very ancient and stable sulcus is the
_orbital_, which is a simple antero-posterior line until Man is
reached (see fig. 23, _Sulc. Orb._). The great point of importance,
however, in the evolution of the mammalian brain is the gradual
suppression of the olfactory region, and the development of the
neopallium, a development which takes a sudden stride between the
Anthropoid apes and Man. (For further particulars of this and other
points in the comparative anatomy of the brain, see _Catalogue of the
Physiological Series_ of the Museum of the Royal College of Surgeons
of England, vol. ii. 2nd ed., by R.H. Burne and G. Elliot Smith,
London, 1902.)

FIG. 22.--Dorsal and Lateral Views of the Brain of a Ratel (_Mellivora
indica_).]

_Embryology._

The brain, like the rest of the nervous system, is developed from the
ectoderm or outer layer of the embryo by the formation of a groove in
the mid-dorsal line. The lips of this _medullary groove_ unite to form
a canal beginning at the place where the neck of the embryo is to be.
The part of the neural canal in front of the earliest union forms the
brain and very early becomes constricted into three vesicles, to which
the names of _prosencephalon_, _mesencephalon_ and _rhombencephalon_
are now usually given. The simple tubular brain we have seen as a
permanent arrangement in Amphioxus, but the stage of the three
vesicles is a transitory one, and is not found in the adult of any
existing animal. From the sides of the prosencephalon, the optic
vesicles grow out before the neural tube is completely closed, and
eventually form the optic nerves and retinae, while, soon after this,
the cerebral hemispheres bulge from the antero-dorsal part of the
first primary vesicle, their points of evagination being the _foramina
of Munro_. From the ventral parts of these cerebral hemispheres the
olfactory lobes are constricted off, while just behind the openings
of the foramina of Munro a constriction occurs which divides the
prosencephalon into two secondary vesicles, the anterior of which,
containing the foramina of Munro, is called the _telencephalon_, while
the posterior is the _thalamencephalon_ or _diencephalon_. A
constriction also occurs in the hind vesicle or _rhombencephalon_,
dividing it into an anterior part, the _metencephalon_, from which the
cerebellum is developed, and a posterior or _myelencephalon_, the
primitive _medulla oblongata_. At this stage the general resemblance
of the brain to that of the lamprey is striking.

Before the secondary constrictions occur three vertical flexures begin
to form. The first is known as the _cephalic_, and is caused by the
prosencephalon bending sharply downward, below and in front of the
mesencephalon. The second is the _cervical_, and marks the place where
the brain ends and the spinal cord begins; the concavity of this
flexure is ventral. The third to appear has a ventral convexity and is
known as the _pontine_, since it marks the site of the future _pons
Varolii_; it resembles the permanent flexure in the reptilian brain.

FIG. 23.--Lateral view of cerebral hemisphere of Gorilla
(_Anthropopithecus gorilla_).]

It will now be seen that the original neural canal, which is lined by
ciliated epithelium, forms the ventricles of the brain, while
superficial to this epithelium (_ependyma_) the grey and white matter
is subsequently formed. It has been shown by His that the whole neural
tube may be divided into _dorsal_ or _alar_, and _ventral_ or _basal_
laminae, and, as the cerebral hemispheres bud out from the dorsal part
of the anterior primary vesicle, they consist entirely of alar
laminae. The most characteristic feature of the human and anthropoid
brain is the rapid and great expansion of these hemispheres,
especially in a backward direction, so that the mesencephalon and
metencephalon are hidden by them from above at the seventh month of
intra-uterine life. At first the foramina of Munro form a
communication not only between the third and lateral ventricles, but
between the two lateral ventricles, so that the cavity of each
hemisphere is continuous with that of the other; soon, however, a
median longitudinal fissure forms, into which the mesoderm grows to
form the falx, and so the foramina of Munro are constricted into a
V-shaped canal. In the floor of the hemispheres the corpora striata
are developed at an early date by a multiplication of nerve cells, and
on the external surface a depression, called the _Sylvian fossa_,
marks the position of the future central lobe, which is afterwards
hidden as the lips of the fossa (_opercula_) gradually close in on it
to form the Sylvian fissure. The real fissures are complete infoldings
of the whole thickness of the vesicular wall and produce swellings in
the cavity. Some of them, like the choroidal on the mesial surface,
are developed very early, while the vesicle is little more than
epithelial, and contain between their walls an inpushing of mesoderm
to form the choroid plexus. Others, like the hippocampal and
calcarine, appear in the second and third months and correspond to
invaginations of the nervous tissue, the hippocampus major and minor.
The sulci appear later than the fissures and do not affect the
internal cavity; they are due to the rapid growth of the cortex in
certain areas. The corpus callosum and fornix appear about the third
month and their development is somewhat doubtful; they are probably
modifications of the lamina terminalis, but they may be secondary
adhesions between the adjacent surfaces of the cerebral hemispheres
where the cortical grey matter has not covered the white. They begin
at their antero-ventral part near the genu of the corpus callosum and
the anterior pillars of the fornix, and these are the parts which
first appear in the lower mammals. The original anterior vesicle from
which the hemispheres evaginate is composed, as already shown, of an
anterior part or telencephalon and a posterior or thalamencephalon;
the whole forming the third ventricle in the adult. Here the alar and
basal laminae are both found, but the former is the more important;
from it the optic thalami are derived, and more posteriorly the
geniculate bodies. The anterior wall, of course, is the lamina
terminalis, and from it are formed the _lamina cinerea_, the _corpus
callosum_, _fornix_ and _septum lucidum_. The roof largely remains
epithelial and is invaginated into the ventricle by the mesoderm to
form the _choroid plexuses_ of the third ventricle, but at the
posterior part it develops the _ganglia habenulae_ and the pineal
body, from a structure just in front of which both a lens and retinal
elements are derived in the lower forms. This is one great difference
between the development of this organ and that of the true eyes;
indeed it has been suggested that the pineal is an organ of thermal
sense and not the remains of a median eye at all. The floor of the
third ventricle is developed from the basal laminae, which here are
not very important and from which the _tuber cinereum_ and, until the
fourth month, single _corpus mammillare_ are developed. The
_infundibulum_ or stalk of the posterior part of the pituitary body at
first grows down in front of the _tuber cinereum_ and, according to
Gaskel's theory, represents an ancestral mouth to which the ventricles
of the brain and the central canal of the cord acted as the stomach
and intestine (_Quart. Journ. of Mic. Sci._ 31, p. 379; and _Journ. of
Phys._ v. 10, p. 153). The reason why the basal lamina is here small
is because it contains the nuclei of no cranial nerves. The anterior
and posterior commissures appear before the middle and the middle
before the _corpus callosum_, as they do in phylogeny. In connexion
with the thalamencephalon, though not really belonging to it, may be
mentioned the anterior lobes of the pituitary body; these begin as an
upward _diverticulum_ from the posterior wall of the primitive pharynx
or _stomatodaeum_ about the fourth week. This _pouch of Rathke_, as it
is called, becomes nipped off by the developing base of the skull, and
its bifid blind end meets and becomes applied to the posterior part of
the body, which comes down from the brain. In the mesencephalon the
alar laminae form the _corpora quadrigemina_; these at first are
bigeminal and hollow as they are in the lower vertebrates. The basal
laminae thicken to form the _crura cerebri_. In the rhombencephalon
the division into basal and alar laminae is better marked than in any
other part; there is a definite groove inside the fourth ventricle,
which remains in the adult as the superior and inferior _fovea_ and
which marks the separation between the two laminae. In the basal
laminae are found the deep origins of most of the motor cranial
nerves, while those of the sensory are situated in the alar laminae.
The roof of the fourth ventricle widens out very much and remains
largely epithelial as the superior and inferior medullary vela. The
cerebellum develops in the anterior part of the roof of the
rhombencephalon as two lateral rudiments which unite in the mid line
and so form a transverse bar similar to that seen in the adult
lamprey; at the end of the second month the flocculus and
paraflocculus become marked, and later on a series of transverse
fissures occur dividing the various lobes. Of the cerebellar peduncles
the inferior develops first (third month), then the middle forming the
_pons_ (fourth month), and lastly the _superior_ (fifth month) (Elliot
Smith, _Review of Neurology and Psychiatry_, October 1903; W. Kuithan,
"Die Entwicklung des Kleinhirns bei Saugetieren," _Munchener Med.
Abhandl._, 1895; B. Stroud, "Mammalian cerebellum," _Journ. of Comp.
Neurology_, 1895). Much of our knowledge of the tracts of fibres in
the brain is due to the fact that they acquire their white sheaths at
different stages of development, some long after birth.

For further details and references see Quain's _Anat._ vol. i. (1908);
Minot's _Human Embryology_ (New York); W. His, _Anat. menschlicher
Embryonen_ (Leipzig, 1881); Marshall's _Vertebrate Embryology_;
Kolliker, _Grundriss der Entwickelungsgeschichte_ (Leipzig, 1880); A.
Keith, _Human Embryology and Morphology_ (London, 1904); O. Hertwig,
_Handbuch der vergleichenden und experimentellen Entwickelungslehre
der Wirbeltiere_, Bd. 2, part 3 (Jena, 1902-1906); _Development of the
Human Body_, J.P. McMurrich (1906). (F. G. P.)

2. PHYSIOLOGY

The nervous system has as its function the co-ordinating of the activities of the organs one with another. It puts the organs into such mutual relation that the animal reacts as a whole with speed, accuracy and self-advantage, in response to the environmental agencies which stimulate it. For this office of the nervous system there are two fundamental conditions. The system must be thrown into action by agencies at work in the environment. Light, gravity, mechanical impacts, and so on, which are conditions significant for animal existence, must find the system responsive and through it evoke appropriate activity in the animal organs. And in fact there have been evolved in the animal a number of structures called receptive organs which are selectively excitable by different environmental agencies. Connected with these receptive organs lies that division of the nervous system which is termed _afferent_ because it conducts impulses inwards towards the nervous centres. This division consists of elongated nerve-cells, in man some two million in number for each half of the body. These are living threads of microscopic tenuity, each extending from a receptive organ to a central nervous mass. These central nervous masses are in vertebrates all fused into one, of which the part which lies in the head is especially large and complex, because directly connected with particularly important and delicate receptive organs. The part of the central nervous organ which lies in the head has, in consequence of its connexion with the most important receptive organs, evolved a dominant importance in the nervous system, and this is especially true of the higher animal forms. This head part of the central nervous organ is sufficiently different from the rest, even to anatomical examination, to have received a separate name, the _brain_. But the fact of its having received a separate name ought not to obscure the singleness and solidarity of the whole central nervous organ as one entity. The functions of the whole central nervous organ from region to region are essentially similar throughout. One of its essential functions is reception, via afferent nerves, of nervous impulses generated in the receptive organs by environmental agents as stimuli. In other words, whatever the nature of the agent, its result on the receptive organs enters the central nervous organ as a nervous impulse, and all segments of the central nervous organ receive impulses so generated. Further, it is not known that nervous impulses present qualitative differences among themselves. It is with these impulses that the central nervous organ whether spinal cord or brain has to deal.

_Material and Psychical Signs of Cerebral Activity._--In the central nervous organ the action resulting from entrant impulses has issue in three kinds of ways. The reaction may die out, be suppressed, and so far as discoverable lead to nothing; or the impulses may evoke effect in either or both of two forms. Just as from the receptive organs, nerves lead into the central nervous organ, so conversely from the central organ other nerves, termed _efferent_, lead to various organs of the body, especially glands and muscles. The reaction of the central nervous organ to impulses poured into it commonly leads to a discharge of impulses from it into glands and muscles. These centrifugal impulses are, so far as is known, qualitatively like the centripetal impulses. On reaching the glands and muscles they influence the activity of those organs. Since those organs are therefore the mechanisms in which the ultimate effect of the nervous reaction takes place, they are often termed from this point of view _effector organs_. A change ensuing in effector organs is often the only sign an observer has that a nervous reaction has occurred, unless the nervous system under observation be the observer's own.

If the observer turns to his own nervous system for evidence of reaction, he meets at once in numberless instances with _sensation_ as an outcome or sign of its reaction. This effect he cannot show to any being beside himself. He can only describe it, and in describing it he cannot strictly translate it into any term of material existence. The unbridged gulf between sensation and the changes produced in effector organs necessitates a separate handling of the functions of the nervous system according as their office under consideration is sensation or material effect. This holds especially in the case of the brain, and for the following reasons.

_Psychosis and the Fore-Brain._--Hippocrates wrote, "It is through the brain that we become mad, that delirium seizes us, that fears and terrors assail us." "We know that pleasure and joy on the one hand and pain and grief on the other are referable to the brain. It is in virtue of it that we think, understand, see, hear, know ugliness and beauty, evil and good, the agreeable and the disagreeable." Similarly and more precisely Descartes indicated the brain, and the brain alone, as the seat of consciousness. Finally, it was Flourens who perhaps first definitely insisted on the restriction of the seat of consciousness in higher animals to that part of the brain which is the fore-brain. A functional distinction between the fore-brain and the remainder of the nervous system seems, in fact, that consciousness and physical reactions are adjunct to the fore-brain in a way in which they are not to the rest of the system. After transection of the spinal cord, or of the brain behind the fore-brain, psychical phenomena do not belong to the reactions of the nervous arcs posterior to the transection, whereas they do still accompany reactions of the nervous arcs in front and still connected with the fore-brain. A man after severance of the spinal cord does not possess in the strict sense consciousness of the limbs whose afferent nerves lie behind the place of spinal severance. He can see them with his eyes, and if the severance lie between the arms and the legs, can feel the latter with his hands. He knows them to be a part of his body. But they are detached from his consciousness. Sensations derived from them through all other channels of sense than their own do not suffice to restore them in any adequate measure to his consciousness. He must have the sensations so called "resident" in them, that is, referred to them, without need of any logical inference. These can be yielded only by the receptive organs resident in the part itself, its skin, its joints, its muscles, &c., and can only be yielded by those receptive organs so long as the nerve impulses from them have access to the fore-brain. Consciousness, therefore, does not seem to attach to any portion of the nervous system of higher animals from which the fore-brain has been cut off. In the dog it has been found that no sign of memory, let alone intelligence, has been forthcoming after removal of the greater part of the fore-brain.

In lower vertebrates it is not clear that consciousness in primitive form requires always the co-operation of the fore-brain. In them the fore-brain does not seem a _conditio sine qua non_ for psychosis--so far as we may trust the rather hazardous inferences which study of the behaviour of fish, &c., allows. And the difference between higher and lowlier animal forms in respect of the fore-brain as a condition for psychosis becomes more marked when the Arthropoda are examined. The behaviour of some Insecta points strongly to their possessing memory, rudimentary in kind though it may be. But in them no homologue of the fore-brain of vertebrates can be indisputably made out. The head ganglia in these Invertebrates may, it is true, be analogous in function in certain ways to the brain of vertebrates. Some experiments, not plentiful, indicate that destruction of these head ganglia induces deterioration of behaviour such as follows loss of psychical functions in cases of destruction of the fore-brain in vertebrates. Though, therefore, we cannot be clear that the head ganglia of these Invertebrates are the same structure morphologically as the brain of vertebrates, they seem to hold a similar office, exercising analogous functions, including psychosis of a rudimentary kind. We can, therefore, speak of the head ganglia of Arthropods as a brain, and in doing so must remember that we define by physiological evidence rather than by morphological.

_Cerebral Control over Lower Nervous Centres._--There accrues to the brain, especially to the fore-brain of higher Vertebrates, another function besides that of grafting psychical qualities upon the reactions of the nervous system. This function is exhibited as power to control in greater or less measure the pure reflexes enacted by the system. These pure reflexes have the character of fatality, in the sense that, given a particular stimulus, a particular reaction unvaryingly follows; the same group of muscles or the same gland is invariably thrown into action in the same way. Removal of the fore-brain, i.e. of that portion of the central nervous organ to which psychosis is adjunct, renders the nervous reactions of the animal more predictable and less variable. The animal, for instance, a dog, is given over more completely to simple reflexes. Its skin is touched and it scratches the spot, its jaw is stroked and it yawns, its rump is rubbed and it shakes itself, like a dog coming out of water; and these reactions occur fatally and inopportunely, for instance, when food is being offered to it, when the dog normally would allow no such insignificant skin stimuli as the above to defer his appropriate reaction. Goltz relates the behaviour of a dog from which almost the whole fore-brain had been removed. The animal lived healthily under the careful treatment accorded it. At feeding time a little quinine (bitter) added to its sop of meat and milk led to the morsels, after being taken into the mouth, being at once and regularly rejected. None was ever swallowed, nor was the slightest hesitation in their rejection ever obtained by any coaxing or command, or encouragement of the animal by the attendant who constantly had charge of it. On the other hand, directly an undoctored piece had entered the mouth it was swallowed at once. Goltz threw to his own house-dog a piece of the same doctored meat. The creature wagged its tail and took it eagerly, then after receiving it into its mouth pulled a wry face and hesitated, astonished. But on encouragement to go on eating it the dog did so. Perhaps it deemed it unseemly to appear ungrateful to the giver and reject the gift. It overcame its reflex of rejection, and by its self-control gave proof of the intact cerebrum it possessed.

There seems a connexion between consciousness and the power to modify reflex action to meet the exigencies of the occasion. Pure reflexes are admirably adapted to certain ends. They are reactions which have long proved advantageous to the phylum of which the existent animal is the representative embodiment. But the reflexes have a machine-like fatality, and conscious aim does not forerun their execution. The subject as active agent does not direct them. Yet they lie under the control of higher centres. The cough, the eye-closure, the impulse to smile, all these can be suppressed. The innate respiratory rhythm can be modified to meet the requirements of vocal utterance. In other words, the reaction of reflex arcs is controllable by the mechanism to whose activity consciousness is adjunct. The reflexes controlled are often reactions but slightly affecting consciousness, but consciousness is very distinctly operative with the centres which exert the control. It may be that the primary aim, object and purpose of consciousness is control. "Consciousness in a mere automaton," writes Professor Lloyd Morgan, "is a useless and unnecessary epiphenomenon." As to _how_ this conscious control is operative on reflexes, how it intrudes its influence on the running of the reflex machinery, little is known.

_The Cerebrum an Organ giving Adaptation and Readjustment of Motor Acts._--The exercise of this control and the acquirement of skilled actions have obviously elements in common. By skilled actions, we understand actions not innately given, actions acquired by training in individual experience. The controlling centres pick out from an ancestral motor action some part, and isolate and enhance that until it becomes a skilled act. The motor co-ordination ancestrally provided for the ring finger gives an extending of it only in company with extension of the fingers on either side of it. The isolated lifting of the ring finger can, however, soon be acquired by training. In such cases the higher centre with conscious effort is able to dissociate a part from an ancestral co-ordination, and in that way to add a skilled adapted act to the powers of the individual.

The nervous organs of control form, therefore, a special instrument of adaptation and of readjustment of reaction, for better accommodation to requirements which may be new. The attainment of more precision and speed in the use of a tool, or the handling of a weapon, means a process in which nervous organs of control modify activities of reflex centres themselves already perfected ancestrally for other though kindred actions. This process of learning is accompanied by conscious effort. The effort consists not so much in any course of reasoning but rather in the acquiring of new sensorimotor experience. To learn swimming or skating by simple cogitation or mere visual observation is of course impossible. The new ideas requisite cannot be constructed without motor experience, and the training must include that motor experience. Hence the training for a new skilled motor manoeuvre must be simply _ad hoc_, and is of itself no training for another motor co-ordination.

The more complex an organism the more points of contact does it have with its environment, and the more does it need readjustment amid an environment of shifting relationships. Hence the organs of consciousness and control, being organs of adaptation and readjustment of reaction, will be more pronounced the farther the animal scale is followed upward to its crowning species, man. The cerebrum and especially the cerebral cortex may be regarded as the highest expression of the nervous organ of individual adaptation of reactions. Its high development in man makes him the most successful animal on earth's surface at the present epoch. The most important part of all this adjustment in his case, as he stands now, consists doubtless in that nervous activity which is intellectual. The mentality attached to his cerebrum includes reason in higher measure than is possessed by the mentality of other animals. He, therefore, more than they, can profitably forecast the future and act suitably to meet it from memory of the past. The cerebrum has proved itself by his case the most potent weapon existent for extending animal dominance over the environment.

_Means and Present Aims of Physiological Study of the Brain._--The aspects of cerebral activity are therefore twofold. There is the contribution which it makes to the behaviour of the animal as seen in the creature's doings. On the other hand there is its product in the psychical life of the animal. The former of these is subject matter for physiology; the latter is especially the province of psychology. Physiology does, however, concern itself with the psychical aspect of cerebral functions. Its scope, embracing the study of the bodily organs in regard to function, includes the psychic as well as the material, because as just shown the former inextricably interlace with the latter. But the relation between the psychic phenomena and the working of the brain in regard to any data of fundamental or intimate character connecting the two remains practically as unknown to us as to the Greek philosophers. What physiology has at present to be content with in this respect is the mere assigning of certain kinds of psychic events to certain local regions of the cerebrum. This primitive quest constitutes the greater part of the "neurology" of our day, and some advance has been made along its lines. Yet how meagre are really significant facts will be clear from the brief survey that follows. Before passing finally from these general considerations, we may note that it becomes more and more clear that the brain, although an organ than can be treated as a whole, is complex in the sense that separable functions belong in some measure to its several parts.

The means principally adopted in studying the functions of the brain--and it must be remembered that this study in its present phase is almost exclusively a mere search for localization--are four. These are the physiological, the clinico-pathological, the histological and the zoological. The first named proceeds by observing the effects of artificial excitation, chiefly electric, of various parts of the brain, and the defects produced by destruction or removal of circumscribed portions. The clinico-pathological proceeds by observing the disturbances of body and mind occurring in disease or injury, and ascertaining the extent of the disease or injury, for the most part _post mortem_. The histological method examines the microscopic structure of the various regions of the brain and the characters and arrangement of the nerve-cells composing it. The zoological follows and compares the general features of the brain, as represented in the various types of animal creation.

It is on the functions of the fore-brain that interest now mainly focuses, for the reasons mentioned above. And the interest in the fore-brain itself chiefly attaches to the functions of its cortex. This is due to several causes. In man and the animals nearest him the cortex forms by far the larger part of the whole cerebral hemisphere. More than any other part it constitutes the distinctively human feature. It lies accessible to various experimental observations, as also to traumatic lesions and to the surgeon's art. It is composed of a great unbroken sheet of grey matter; for that reason it is a structure wherein processes of peculiar interest for the investigation in view are likely to occur. To make this last inference more clear a reference to the histology of nervous tissue must be made. The whole physiological function of the nervous system may be summed up in the one word "conduction." This "conduction" may be defined as the transmission of states of excitement (nerve-impulses) along the neural arcs composing the system. The whole nervous system is built up of chains of nerve-cells (neurones) which are nervous conductors, the chains often being termed arcs. Each neurone is an elongated cell which transmits nerve-impulses from its one end to its other, without so far as is known modifying the impulses in transit, unless in that part of the nerve-cell where the nucleus lies. That part of the neurone or nerve-cell is called the perikaryon or cell-body, and from that part usually many branches of the cell (each branch being a nerve-fibre) ramify. There is no evidence that impulses are modified in transit along a branch of a nerve-cell, but there is clear evidence of manifold modification of nerve-impulses in transit along the nerve-arcs of the nervous system. These nerve-arcs are neurone-chains. In them one neurone continues the line of conduction where the immediately foregoing neurone left it. That is, the neurones are laid in conductive series, the far end of one apposed to the near end of its precursor. The place of juxtaposition of the end of one neurone against the beginning of another is called the _synapse_. At it the conduction which has so far been wholly intra-neuronic is replaced by an inter-neuronic process, in which the nerve impulse passes from one neurone to the next. The process there, it is natural to think, must be physiologically different from that conductive process that serves for transmission merely within the neurone itself. It may be that to this inter-neuronic conduction are due the differences between conduction in nerve-_arcs_ and nerve-_trunks_ (nerve-fibres) respectively. Significant of the former are changes in rhythm, intensity, excitability and modifications by summation and inhibition; in fact a number of the main features of nervous reaction. These characters impressed upon conduction in nerve arcs (neurone-chains) would therefore be traceable to the intercalation of perikarya and synapses, for both these structures are absent from nerve-trunks. It is therefore probably to perikarya and synapses that the greater part of the co-ordination, elaboration and differentiation of nervous reactions is due. Now, perikarya and synapses are not present in the _white_ matter of the central nervous organ, any more than they are in nerve-trunks. They are confined exclusively to those portions of the central organ which consist of _grey_ matter (so called from its naked-eye appearance). Hence it is to the great sheet of grey matter which enfolds the cerebrum that the physiologist turns, as to a field where he would expect to find evidences of the processes of cerebral co-ordination at work. It is therefore to items regarding the functions of the great sheet of cerebral cortex that we may now pass.

_The Cerebral Cortex and its Functions._--The main question which vexed the study of the physiology of the cerebral hemispheres in the 19th century was whether differences of function are detectible in the different regions of the hemisphere and especially in those of its cortex. One camp of experimenters and observers held that the cortex was identical in function throughout its extent. These authorities taught that the various faculties and senses suffer damage in proportion to the amount of cortex removed or injured, and that it is a matter of indifference what may be the particular region wherein the destruction takes place. Against this an opposed set of observers held that different regions perform different functions, and this latter "differential" view was raised in two wholly dissimilar forms in the first and last quarters of the 19th century respectively. In the first quarter of the century, a school, with which the name of Gall is prominently associated, held that each faculty of a set of particular so-called "faculties," which it assumed constituted intelligence, has in the brain a spatially separate organ proper to itself. Gall's doctrine had two fundamental propositions. The first was that intelligence resides exclusively in the brain: the second, that intelligence consists of twenty-seven "faculties," each with a separate local seat in the brain. The first proposition was not new. It is met with in Hippocrates, and it had been elaborated by Descartes and others. But Bichat in his _Anatomie generale_ had partly wandered from the gradually established truth and referred the emotions to the visceral organs, returning to a naive view popularly prevalent. Gall's first proposition was probably raised especially in reaction against Bichat. But Gall's proposition was retrograde from the true position of the science of his time. Flourens and others of his contemporaries had already shown not only that intelligence was resident exclusively in the brain, but that it was resident exclusively in that part of the brain which is the fore-brain. Now Gall placed certain of his twenty-seven intellectual faculties in the cerebellum, which is part of the hind-brain.

_Phrenology._--As to Gall's second proposition, the set of faculties into which he analysed intelligence shows his power of psychological analysis to have been so weak that it is matter of surprise his doctrine could obtain even the ephemeral vogue it actually did. Among his twenty-seven faculties are, for instance, "_l'amour de la progeniture, l'instinct carnassier, l'amitie, la ruse, la sagacite comparative, l'esprit metaphysique, le talent poetique, la mimique_," &c. Such crudity of speculation is remarkable in one who had undoubtedly considerable insight into human character. Each of the twenty-seven faculties had its seat in a part of the brain, and that part of the brain was called its "organ." The mere spatial juxtaposition or remoteness of these organs one from another in the brain had, according to Gall, an influence on the constitution of the mind. "_Comme l'organe des arts est place loin de l'organe du sens des couleurs, cette circonstance explique pourquoi les peintres d'histoire ont ete rarement coloristes_." All these "faculty-organs" were placed by Gall at the surface of the brain. "This explains the correspondence which exists between craniology and the doctrine of the functions of the brain (cerebral physiology), the single aim of my researches." Gall wrote that he found the bump of pride (_la bosse de l'orgueil_) as far down in the animal series as the goat. Broussais traced the "organ" of veneration as far down as the sheep. Gall found the bump of murder (_bosse du meurtre_) in the carnivora. Later it was traced also in herbivora. Broussais added apologetically that "the herbivora cause a real destruction of plants."

Gall's doctrine enjoyed enormous vogue. He himself had the gifts and the demerits of quackery. His doctrine possessed, apart from its falsity, certain other mischievous qualities. "_Que ces hommes si glorieux, qui font egorger les nations par millions, sachent qu'ils n'agissent point de leur propre chef, que c'est la nature qui a place dans leur coeur la rage de la destruction_." One of his scientific opponents rejoined, "Nay, it is not that which they should know. What they should know is that if providence has allowed to man the possibility of doing evil, it has also endowed him with the power to do good." The main cause of the success of phrenology (q.v.) has been no doubt the common desire of men to read the characters and hidden thoughts of others by external signs. Each bump or "bosse" on the cranium was supposed to indicate the existence and degree of development of one or other of the twenty-seven "faculties." One such "bosse" showed the development of the organ of "goodness," and another the development of the organ of "murder." Such an easy means to arrive at information so curious delighted many persons, and they were not willingly undeceived.

_Modern Localization Doctrines._--The crude localization of the phrenologists is therefore too clumsy to possess an interest it might otherwise have had as an early expression of belief in cerebral localization, a belief which other labours have subsequently justified, although on facts and lines quite different from these imagined by Gall and his followers. Patient scientific toil by the hands of E. Hitzig and D. Ferrier and their followers has slowly succeeded in obtaining certain facts about the _cortex cerebri_ which not only show that different regions of it are concerned with different functions, but, for some regions at least, outline to some extent the kind of function exercised. It is true that the greater part of the cortex remains still _terra incognita_ unless we are content with mere descriptive features concerning its coarse anatomy. For several scattered regions some knowledge of their function has been gained by physiological investigation. These scattered regions are the _visual_, the _auditory_, the _olfactory_ and the _precentral_.

The grey matter of the cerebral cortex is broadly characterized histologically by the perikarya (nerve-cells bodies) which lie in it possessing a special shape; they are pyramidal. The dendrite fibres of these cells--that is, their fibres which conduct _towards_ the perikarya--are branches from the apex and corners of the pyramid. From the base often near its middle arises one large fibre--the axone fibre, which conducts impulses away from the perikaryon. The general appearance and arrangement of the neurones in a particle of cortical grey matter are shown in fig. 15, above. The apices of the pyramidal perikarya are turned towards the free surface of the cortex. The figure as interpreted in terms of functional conduction means that the cortex is beset with conductors, each of which collects nerve-impulses, from a minute but relatively wide field by its branched dendrites, and that these nerve-impulses converge through its perikaryon, issue by its axone, and are carried whithersoever the axone runs. In some few cells the axone breaks up into branches in the immediate neighbourhood of its own perikaryon in the cortex. In most cases, however, the axone runs off into the subjacent white matter, leaving the cortex altogether. On reaching the subjacent white matter it mingles with other fibres and takes one of the following courses:--(1) to the grey matter of the cortex of the same hemisphere, (2) to the grey matter of the cortex of the opposite hemisphere, (3) to the grey matter of the pons, (4) to the grey matter of the bulb or spinal cord. It is noteworthy that the dendrite fibres of these cortical neurones do not transgress the limits of the grey cortex and the immediate neighbourhood of the perikaryon to which they belong; whereas the discharging or axone fibre does in the vast majority of cases transgress the limits of the grey matter wherein its perikaryon lies. The cortical neurone therefore collects impulses in the region of cortex just about its perikaryon and discharges them to other regions, some not cortical or even cerebral, but spinal, &c. One question which naturally arises is, do these cells spontaneously generate their impulses or are they stirred to activity by impulses which reach them from without? The tendency of physiology is to regard the actions of the cortex as reactions to impulses communicated to the cortical cells by nerve-channels reaching them from the sense organs. The neurone conductors in the cortex are in so far considered to resemble those of reflex centres, though their reactions are more variable and complex than in the use of the spinal. The chains of neurones passing through the cortex are more complex and connected with greater numbers of associate complex chains than are those of the spinal centres. But just as the reflex centres of the cord are each attached to afferent channels arriving from this or that receptive-organ, for instance, tactile-organs of the skin, or spindles of muscle-sense, &c., so the regions of cortex whose function is to-day with some certainty localized seem to be severally related each to some particular sense-organ. The localization, so far as ascertained, is a localization which attaches separate areas of cortex to the several species of sense, namely the visual, the auditory, the olfactory, and so on. This being so, we should expect to find the sensual representation in the cortex especially marked for the organs of the great distance-receptors, the organs which--considered as _sense_ organs--initiate sensations having the quality of projicience into the sensible environment. The organs of distance-receptors are the olfactory, the visual and the auditory. The environmental agent which acts as stimulus in the case of the first named is chemical, in the second is radiant, and in the last is mechanical.

_Olfactory Region of Cortex._--There is phylogenetic evidence that the development of the _cortex cerebri_ first occurred in connexion with the distance-receptors for chemical stimuli--that is, expressed with reference to psychosis, in connexion with olfaction. The olfactory apparatus even in mammals still exhibits a neural architecture of primitive pattern. The cell which conducts impulses to the brain from the olfactory membrane in the nose resembles cells in the skin of the earthworm, in that its cell-body lies actually amid the epithelium of the skin-surface and is not deeply buried near or in the central nervous organ. Further, it has at its external end tiny hairlets such as occur in specially receptive-cells but not usually in purely nervous cells. Hence we must think that one and the same cell by its external end receives the environmental stimulus and by its deep end excites the central nervous organ. The cell under the stimulation of the environmental agent will therefore generate in itself a nervous impulse. This is the clearest instance we have of a neurone being actually excited under natural circumstances by an agent of the environment _directly_, not indirectly. The deep ends of these olfactory neurones having entered the central nervous organ come into contact with the dendrites of large neurones, called, from their shape, mitral. In the dog, an animal with high olfactory sense, the axone of each olfactory neurone is connected with five or six mitral cells. In man each olfactory neurone is connected with a single mitral cell only. We may suppose that the former arrangement conduces to intensification of the central reaction by summation. At the same time it is an arrangement which could tend to smother sharp differentiation of the central reaction in respect to locality of stimulus at the receptive surface. Considering the diffuse way in which olfactory stimuli are applied in comparison, for instance, with visual, the exact localization of the former can obviously yield little information of use for locating the exact position of their source. On the other hand, in the case of visual stimuli the locus of incidence, owing to the rectilinear propagation of light, can serve with extraordinary exactitude for inferences as to the position of their source. The adaptation of the neural connexions of the two organs in this respect is therefore in accord with expectation.

The earliest cerebral cortex is formed in connexion with the neurone-chains coming into the central nervous organ from the patch of olfactory cells on the surface of the head. The region of cerebrum thus developed is the so-called olfactory lobe and hippocampal formation. The greater part of the cerebral hemisphere is often termed the _pallium_, because as its development extends it folds cloak-wise over the older structures at the base of the brain. The olfactory lobe, from its position, is sometimes called the _pallium basale_, and the hippocampal formation the _pallium marginale_; and these two parts of the pallium form what, on account of their phylogenetic history, Elliott Smith well terms the _archipallium_. A fissure, the limbic fissure, marks off more or less distinctly this archipallium from the rest of the pallium, a remainder which is of later development and therefore designated by Elliott Smith the _neopallium_. Of the archipallium, the portion which constitutes the olfactory lobe is well formed in the selachian fish. In the reptilian cerebrum the hippocampal region, the pallium marginale, coexists in addition. These are both of them olfactory in function. Even so high up in the animal scale as the lowest mammals they still form one half of the entire pallium. But in the higher apes and in man the olfactory portion of the pallium is but a small fraction of the pallium as a whole. It is indeed so relatively dwarfed and obscured as to be invisible when the brain is regarded from the side or above. The olfactory part of the pallium exhibits little variation in form as traced up through the higher animals. It is of course small in such animals as Cetaceans, which are _anosmatic_. In highly osmatic such as the dog it is large. The _uncus_, and _subiculum cornu ammonis_ of the human brain, belong to it. Disease of these parts has been accompanied by disturbance of the sense of smell. When stimulated electrically (in the rabbit) the olfactory pallium occasions peculiar torsion of the nose and lips (Ferrier), and change, often slowing or arrested, of the respiratory rhythm. P.E. Flechsig has shown that the nerve-fibres of this part of the pallium attain the final stage of their growth, that is to say, acquire their sheaths of myelin, early in the ontogenetic development of the brain. In the human brain they are myelinate before birth. This is significant from the point of view of function, for reasons which have been made clear especially by the researches of Flechsig himself.

The completion of the growth of the nerve-fibres entering and leaving the cortex occurs at very various periods in the growth of the brain. Study of the development of the fibres entering and leaving the various regions of the pallium in the human brain, discovers that the regions may be conveniently grouped into those whose fibres are perfected before birth and those whose fibres are perfected during the first post-natal month, and those whose fibres are perfected after the first but before the end of the fourth post-natal month. The regions thus marked out by completion before birth are five in number, and are each connected, as also shown by collateral evidence, with one or other particular species of sense-organ. And these regions have another character in common recognizable in the nerve-fibres entering and leaving them, namely, they possess fibres projected to or from parts of the nervous system altogether outside the cortex itself. These fibres are termed "projection" fibres. Other regions of the cortex possess fibres coming from or going to various regions of the cortex itself, but do not possess in addition, as do the five primitive cortical fields, the fibres of projection. So that the facts established by Flechsig for the regions of pallium, which other evidence already indicated as connected with the sense-organ of smell, support that evidence and bring the olfactory region of cortex into line with certain other regions of cortex similarly primarily connected with organs of sense.