NERVOUS SYSTEM (of Man). - I. GENERALITIES. - It is a general property of protoplasm to be excitable, that is, to perceive stimuli of various kinds and to react to them with appropriate responses. In unicellular animals, the protozoa, this function is carried out by the single cell that constitutes the individual, which, however, may present particular differentiations in this regard with the cytoplasmic ones. In protozoa, or multicellular organisms, with the division of labor and the differentiation of the various tissues, a tissue characteristic of the nervous function comes to be specialized, in which the specific functional element is the nerve cell. This cell is characteristic for its processes, which come to constitute the internal connections of the central nervous system and the fibers of the nerves along their peripheral course. The nerve cell with its processes constitutes the anatomical and functional unit that is called the "neuron." Nerve cells may be scattered among the other tissues and connected to one another by fibers that come to constitute plexuses, or be gathered into groups or ganglia connected to one another and to the receptors and effectors by means of their processes. In vertebrates and in man, in addition to the presence of scattered nerve cells and of plexuses, ganglia and nerves, there is the centralization of the majority of the nerve cells in an organ that constitutes the central nervous system or cerebrospinal axis.
II. DEVELOPMENT OF THE NERVOUS SYSTEM OF VERTEBRATES AND MAN
The nervous system originates from a very early embryonic rudiment that takes shape at the moment of gastrulation: the neural plate. Through the folding of the plate, the neural tube is formed, which initially appears open at the anterior end (anterior neuropore) and posteriorly communicates with the primitive intestine (neurenteric canal).
Very important for the subsequent morphogenesis of the neural tube is the condition of the notochord, which forms beneath it. This supporting organ, in fact, does not extend as far as its cephalic extremity, but stops short of it, so that the nervous tube comes to be distinguished into a prechordal part and a chordal part. The prechordal part, in relation to the absence of the chordal obstacle, dilates into vesicles, and so too, though much more moderately, does the most cephalic portion of the chordal part; the rest of the chordal part does not dilate and will give rise to the spinal cord; the vesicles will give rise to the encephalon. After a two-vesicle stage and an intermediate three-vesicle stage, one reaches the five-vesicle stage (the telencephalon, the diencephalon, the mesencephalon, the metencephalon and the myelencephalon).
From this stage, characteristic in all vertebrates, one passes to definitive differentiation through a process of development of two sacciform portions at the sides of the telencephalic vesicle which, at their maximum development, will represent the cerebral hemispheres, and through the unequal thickening of the walls of the various vesicles, which also leads to a different width of the internal ventricular cavities. The spinal part of the nervous tube, through thickening of the lateral walls, is transformed into the spinal cord, and the internal cavity is reduced to a thin little tube, the ependymal canal. The development of the cerebral walls varies in the different classes of vertebrates in the sense that it becomes more complex as one ascends the zoological scale up to man, in whom the most conspicuous structural differentiations are attained. The thickening of the walls of the neuraxis is brought about by the proliferation of the ependymal germinal layer, which produces two types of undifferentiated cells, the "neuroblasts" and the "spongioblasts", which migrate from this site more externally, where they differentiate respectively into nerve cells and into neuroglial cells. The neuroblasts in their migration, directed by stimuli of various kinds, gather into nuclei and nerve centers.
III. MORPHOLOGY OF THE NERVOUS SYSTEM OF VERTEBRATES AND MAN
In the spinal cord the nerve cells come to form cell columns (gray matter) along the length of the cord which, in section, assume the appearance of an H or of a butterfly with open wings; thus two dorsal horns and two ventral horns are identified.
According to the research of Johnston, four functional areas can be distinguished in the spinal cord: in the most apical part of the dorsal horns there is the somato-sensory area, to which somatic sensations arrive; in the most apical part of the ventral horns there is the viscero-motor area, from which motor impulses for the skeletal musculature depart; between these two areas, in an intermediate position, there is more dorsally the viscero-sensory area, to which sensations of a visceral nature come, and the somato-sensory area, from which motor impulses for the visceral musculature depart. The motor, efferent fibers emerge from the ventral part and gather into bundles of fibers that come to constitute the ventral roots of the spinal nerves; the afferent sensory fibers reach the cord from the dorsal part (dorsal roots) and along their course the spinal ganglia are situated. The two roots unite into mixed spinal nerves. The sensory fibers that penetrate the spinal cord partly enter into relation with cells of the dorsal horns and partly proceed cranially, reaching cell groups situated in the medulla oblongata (nuclei of the fasciculus gracilis and of the fasciculus cuneatus). From the cells of the spinal sensory nuclei depart fibers that reach various zones of the medulla oblongata (or bulb), of the midbrain, of the cerebellum and of the thalami of the diencephalon. The nervous element of the spinal ganglia, a typical bipolar element in the embryo (later it transforms into a T-shaped cell), is therefore provided with two processes, while on one side it connects in the manner described above with the cord, on the other it enters into relation with the periphery by means of various peripheral devices (expansions, diffuse networks) and sensory corpuscles (of Meissner, Pacini, Golgi-Mazzoni, etc.).
Anterior to the spinal cord is the encephalon with its various subdivisions. The encephalic portion that connects directly with the spinal cord is the medulla oblongata or bulb, which still greatly recalls the spinal cord itself in its morphology. The medulla oblongata presents thickened lateral walls and base, while the roof remains sterile and constitutes the choroid tela of the IV ventricle, represented by a broad dilation of the ependymal cavity deriving from the myelencephalic vesicle.
Besides the aforementioned centers of connection of the long ascending pathways from the spinal cord, in the bulb are found motor and sensory centers of cranial nerves and centers of motor coordination (tegmental centers, inferior olives). There are found the motor nuclei of the somatic nerves such as the hypoglossal (XII pair) and the abducent nerve of the eye (VI pair), and the nuclei of the visceral nerves that innervate musculature of branchial origin, the salivary glands and the smooth musculature of the viscera of the body, such as the glossopharyngeal (IX pair), the vagus (X pair) and the accessory (XI pair), and the somato-sensory nuclei of the trigeminal (V pair), of the cochlear and of the vestibular which constitute the acoustic complex (VIII pair), and the viscero-sensory nuclei of the VII, IX and X which constitute the so-called "solitary fasciculus". In the white matter of the bulb is found the passage of the long pathways that go from the cord to the higher centers and of those descending that from these descend to the spinal cord. Anterior to the medulla oblongata are found the differentiations of the metencephalic vesicle which, with the development of the dorsal part, give origin to the "cerebellum". In this are differentiated centers that enter into relation on one side with general proprioceptive sensibility (articular, tendinous, muscular sensibility) and special sensibility (semicircular canals) and on the other with all the motor impulses of the cortical, pyramidal and extrapyramidal pathways, becoming the center of coordination and motor synergy and of integration of all the activities of movement and equilibrium. It acts autonomically, outside the will, although being in relation with the cerebral cortex by way of the centers of the pons of Varolius. The cerebellar response takes place by way of the cerebellar olives or dentate nuclei and the reflex motor centers of the tectum and the red nucleus of the midbrain.
Anterior to the midbrain, the midbrain thickens in all its walls so that the ventricular cavity is reduced to a thin little canal known as the "aqueduct of Sylvius". In its basal part are found other motor centers of the eyes (common oculomotor, III pair, and the trochlear, IV pair) and some centers of the extrapyramidal motor pathway (of which mention will be made shortly) such as the red nucleus and the substantia nigra. Through the midbrain there also pass the long ascending and descending pathways that have no station in this sector. The dorsal part is constituted by the quadrigeminal lamina formed by a pair of anterior colliculi, reflex visual centers, and by a pair of posterior colliculi, reflex acoustic centers. The diencephalic vesicle, anterior to the midbrain, with development comes to be distinguished into the thalami, thickenings of the lateral walls, into the hypothalamus, thickening of the base, and into the epithalamus, thickening of the roof. The cavity is reduced to a broad vertical fissure that associates with the unpaired cavity of the subsequent telencephalic vesicle to constitute the III ventricle.
The thalami are for the greatest part sensory centers to which the long pathways coming from the spinal cord and from the other posterior sectors (bulb, midbrain) arrive; a ventral portion (subthalamus) is motor. In the sensory part of the thalami are distinguished: an optic zone (pulvinar) that connects with a differentiation of the thalamus itself, the lateral geniculate body; an acoustic zone, connected with the medial geniculate body; a zone of cutaneous sensibility, coming from the long exteroceptive pathways of the spinal cord and of the bulb. In the thalamus there are furthermore association centers which in the lower vertebrates, devoid of cerebral cortex, represent the highest centers in which the most complex nervous activities are elaborated, of a primitive nature however, such as the instincts, in which no expression of conscious will is manifested.
The most anterior vesicle, the telencephalon, is the one that undergoes the greatest differentiative complication and is the one that presents the widest degrees of development in the series of vertebrates. This vesicle, initially unpaired and median like the others, everts into two lateral vesicles so that two lateral ventricles (I and II) come to be formed. In the lower vertebrates (fish, amphibians and lower reptiles) these hemispheres are exclusively in relation with olfactory sensibility (paleopallium and archipallium); at the base develop thickenings, called ganglia of the base or corpora striata, which represent the most cephalic motor station and which even in the higher forms represent a center of the extrapyramidal pathway.
In the higher reptiles and in mammals, besides the olfactory part, the other walls of the hemispheres also thicken with the constitution of a neopallium or cerebral cortex. It reaches its maximum development in man. The neocortex is distinguished from the other cerebral areas by the presence of the typical pyramidal cells.
In the study of the complexity of the encephalic structures, the question of the crossing of the fibers must be considered first of all. In fact, the great prevalence of nerve fibers that have a peripheral relationship with one side enter into relation with centers of the opposite side; thus along the medial sagittal zone of the neuraxis one observes decussations, chiasmas or commissures that at times constitute very conspicuous anatomical entities. In the telencephalon three principal commissures are observed that contribute very greatly to its morphology: the "commissura anterior" and the "commissure of the hippocampus," exclusively made up of olfactory pathways (and which are therefore the most primitive), and the "corpus callosum," through which pass the decussation pathways of the neocortex (which is therefore the most evolved commissure). The corpus callosum reaches its greatest development in man, since in him the neocortex also reaches its greatest development. With the development of the corpus callosum the olfactory cortex is displaced much farther back from its initial morphological position, so that the fibers that make up the commissure of the hippocampus must travel a long course over the roof of the third ventricle, coming to constitute that anatomical structure known as the "fornix" or "vault with three pillars." The development of the cerebral cortex is chiefly in surface area and presents in man with a very large number of foldings (convolutions). In the human species is found the greatest complexity, by far greater even than in the most evolved mammals, such as the anthropomorphs (v. CERVELLO). The deepest sulci divide the cerebral cortex into five lobes: occipital, temporal, frontal, parietal, and insular, the latter situated deeply and operculated by the temporal lobe that has overlapped it. The sulci delimit the gyri.
IV. PHYSIOLOGY
A fundamental subdivision of the nervous system is that into two subsystems, distinct morphologically as well as physiologically, but in intimate reciprocal anatomical-functional connection: a "somatic" nervous system that participates in the activities of the life of relation and a "visceral" or "neurovegetative" nervous system (see below § V) that participates in the physiological activities of the viscera. While the somatic nervous system acts through involuntary and voluntary mechanisms, the visceral nervous system acts in an exclusively autonomous or involuntary manner.
The somatic nervous system is represented by the peripheral part (peripheral sense organs, motor end-plates, peripheral nerves, spinal ganglia) and by a central part (somatic parts of the spinal cord and of the encephalon); the visceral nervous system is formed by peripheral sensory organs, nerves, visceral and spinal ganglia and by the visceral central parts of the spinal cord and of the encephalon (only in the telencephalic part is the presence of visceral centers debated). The viscera have a double visceral innervation, sympathetic and parasympathetic, with antagonistic action. From the equilibrium of the two activities comes the function appropriate to the physiological moment.
V. RECEPTORS AND EFFECTORS
Nervous activity is carried out through the excitability of receptors and the activity of effectors. The receptors are constituted either by the sensory cells situated in the specific peripheral epithelia and by the sensory ganglion neuron that transmits the excitation to the central nervous system, or by the ganglion cells themselves which present the peripheral prolongation complicated into microscopic peripheral devices (corpuscles, networks, plexuses, arborizations, etc.) situated in the epithelia or in the connective tissues. The effectors are constituted by the motor nerve cell (spinal, bulbar, mesencephalic or ganglionic), by its peripheral prolongation which becomes complicated into terminal apparatuses adhering to striated muscle fibers (the motor end-plates) or smooth ones (with various arborizations).
The receptors have been subdivided in relation to the stimulusspecific that are suited to perceiving, and which may be of various nature, into receptors of physical stimuli and of chemical stimuli. Physical receptors are distinguished according to the number of vibrations per m." to which they are excitable. Tactile receptors are suited to receiving very slow vibrations up to 1552 per second; acoustic receptors perceive vibrations from 30 to 30,000. Heterogeneous vibrations are stimuli for the cutaneous thermoreceptors when their number is between 3000 billion and 800,000 bi-
lions per m."; visual stimuli for the photoreceptors of the retina are vibrations from 400,000 billion to 800,000 billion per m." These values hold for the human species and vary in other species. Thus in other animals other ranges of vibrations may be perceived through the presence of suitable receptors, just as they may be insensitive to others through the lack of specific receptors. Chemoreceptors are specialized to perceive stimuli of a chemical nature: these are the olfactory and gustatory ones. Among the gustatory ones situated in the taste buds of the tongue (and, in other animals, of the palate or of the skin), there are specializations for sweet, salty, bitter substances, etc. For the physical or chemical conditions of the environment to be stimuli, it is necessary not only that the animal possess the appropriate sense organ, that is, the appropriate receptor, but also that the action of the stimulus exceed a certain threshold of intensity.
Receptors can be somatic or visceral. Somatic receptors are divisible, according to Sherrington, into exteroceptors and proprioceptors: the former are deputed to perceive directly the stimuli of the external world (tactile, thermal, visual, acoustic, olfactory stimuli), while the latter do so indirectly, inasmuch as the site of the stimulus is in the articular joints, in the tendons and in the muscles, in relation to the position of the limbs and to muscular efforts, activities that are in indirect relation with the environment; to these must be added the excitability of the receptors of the semicircular canals which, together with the aforementioned muscular receptors, give the sensation of the position of the body with respect to the environment, which represents the perception necessary for the function of equilibrium. Visceral receptors are called visceroceptors and are represented by the neurons of the visceral nervous system and by their peripheral expansions (recta, plexuses and corpuscles). The effectors too, constituted by the various motor units, are somatic, represented by the somatic motor neuron and by the striated muscle fibers connected to it with the terminal motor end-plates, and visceral, represented by the ganglionic visceral motor neuron and by the expansions on the smooth or striated muscles (heart) of the viscera connected to it.
Of particular physiological importance are the zones of contact between the peripheral sensory cell (in the cases in which it exists) and the sensory fiber, and that between the motor end-plate and the muscle fiber, called "synapses," and the relations between neuron and neuron in the constitution of chains (interneuronal synapses). The synapse permits the transmission of excitation while preserving the integrity of the anatomical unit (neuron theory), permits through its multiplicity the "spatial summation" of excitation, explains the latency period in reflex conduction, explains the polarization of conduction in a chain of neurons. The mechanism by which excitation crosses the synapse has been the object of much research and today it is considered from a physical point of view (production of action potentials, depolarizing action on the membrane of the neuron in which the excitation is transmitted, etc.) and from a chemical point of view, on account of the production of "chemical mediators" (acetylcholine, inactivated after the action of cholinesterase).
VI. REFLEXES
It has already been mentioned in the study of the evolution of the nervous system how the simplest nervous mechanism is manifested through the involvement of only two neurons: the sensory one and the motor one. This is called the simple reflex arc. The spinal cord is the site of these reflexes by means of an arc comprising the sensory neuron, whose cell body is in the spinal ganglion, and the motor neuron whose cell is in the anterior horns, connected peripherally with the fibers of a muscle. This spinal reflex arc, like all reflexes, is involuntary. In the spinal cord, extensor reflexes manifest themselves (such as, for example, the well-known knee reflex, elicited by tapping below the kneecap) and flexor reflexes (such as those due to noxious stimuli, at the extremities). The two associated reflexes are responsible for maintaining the animal on its limbs in opposition to gravity. When a muscle contracts, the antagonistic apparatus is released according to a fundamental principle of reciprocal innervation and inhibition. More complex spinal reflex mechanisms are the intersegmental ones, whereby the reflex movement of a pair of limbs involves an "integration" with movements also of other muscles of the trunk or of the other pair of limbs and also with effects on the visceral sphere (genitals). This is well demonstrated in spinal animals, that is, with the spinal cord resected and separated from the encephalon.
The physiological role of the higher encephalic centers on spinal activities can be studied by moving the plane of resection progressively more cephalically, that is, by removing the encephalon with resections at progressively more cephalic levels.
If a resection is performed before the pons of Varolius, the characteristic decerebration rigidity manifests itself, which is caused by a continuous contracture of the muscles, predominantly extensors. This occurs because of the continuous excitation of the vestibular centers, which remain connected to the spinal cord and which are not dominated by the higher centers and particularly by the extrapyramidal system. Postural, static movements are manifested through five principal reflex mechanisms: 1) labyrinthine reflexes; 2) righting reflexes of the body in relation to the position of the head; 3) righting reflexes of the body independently of the head (which are manifested through the bulbar centers connected to the red nucleus of the midbrain); 4) righting reflexes of the neck (integrated in the spinal cord); 5) righting reflexes of the eyes (integrated in the cerebral cortex). Other reflex mechanisms are those of the eyes (in the midbrain, as demonstrated by faradic excitation in this site). The spinal cord is also the site of visceral reflexes, such as the vasomotor ones, sweat secretory ones, those of defecation and micturition. Thus, in connection with the visceral pathways, abnormal impulses can be determined which can, by reflex route, provoke visceral pains, cutaneous hypersensitivity, muscular contracture, secretory activity, etc. In the brainstem there are somatic and visceral centers relating to other characteristic reflexes, such as those of swallowing, mastication, salivary secretion, coughing, sneezing, vomiting, dilation or closure of the pupils, etc. Thus are found the centers that regulate automatic activities such as respiration, heartbeat, etc. The spinal cord and the brainstem (medulla oblongata and midbrain) are traversed by long ascending pathways, carrying excitations to the higher encephalic and telencephalic centers, and by long descending pathways which, coming from these higher sectors, reach the same motor cells, spinal and brainstem, which enter into action for the various local reflexes and which are therefore also defined as "final common pathways."
VII. THE THALAMI
The ascending fibers that come from the spinal cord and that carry the sensations of the exteroceptors have a station in the medulla and from here reach the thalami of the diencephalon, while the proprioceptive pathways stop at the cerebellum; other exteroceptive pathways of the head have their centers in the medulla and from here reach the thalami, as do the spinal ones. Thus the acoustic and optic sensations reach the thalami with a first station in the medial and lateral geniculate bodies. The acoustic, optic and olfactory sensations, besides reaching the thalamus and then being projected into the cerebral cortex, have certain reflex motor centers: these are the posterior colliculi of the midbrain for the acoustic reflexes, the anterior colliculi for the optic ones, and the diencephalic mammillary bodies for the olfactory reflexes.
In the diencephalon, besides the development of the two thalami, thickenings of the lateral walls, which receive all these sensory pathways, there are olfactory centers in the epithalamus (ganglia of the habenulae) and in the hypothalamus (the mammillary bodies). In the hypothalamus there are furthermore the most cephalic centers of visceral regulation which, connected on one side with the thalami, with the olfactory centers, with the cerebral cortex and with the extrapyramidal motor pathways, are in relation with the hypophysis, the governing gland of the great majority of the hormonal activities of the organism.
VIII. THE CEREBRAL HEMISPHERES
The telencephalic vesicle, which is the seat of the major cerebral complications, gives origin, besides other centers, to the centers in which the most complex involuntary and voluntary nervous phenomena are elaborated and in which, in man, the highest expressions of knowledge and of the psyche are elaborated. The telencephalic centers are: the centers and pathways of olfactory sensibility, the basal ganglia or corpora striata, and the cerebral cortex. One can indeed observe how in phylogenetic development the lateral outgrowths of the telencephalon, which will constitute the cerebral hemispheres, arise as purely olfactory centers (e.g., in the dogfish); only to the corpora striata, besides olfactory fibers, do visceral and somatic fibers arrive. Subsequently new, non-olfactory parts are added to these parts, both to the basal ganglia and to the cortex: the neostriatum and the neocortex. These centers appear in a primitive form in the higher reptiles (crocodiles); their maximum development is reached in man, and here they exceed in size all the other centers of the neuraxis considered thus far.
The corpora striata represent, in vertebrates lacking the neocortex, the higher centers where the impulses of response to various external and internal stimuli are elaborated; they are therefore the highest reflex centers. The voluntary motor centers develop in the neocortex and only in certain areas of it. The motor pathways that arise in the striata are associated with the pathways of other motor centers of the brainstem, such as the red nucleus, the tegmental centers, the reflex motor, visual, acoustic, vestibular, and olfactory centers; they are connected with the cerebellum, which has the task of reinforcing and coordinating these impulses into a motor activity integrated with general proprioceptive sensations so as to effect motor responses adequate to the static conditions of the animal. The motor pathways that derive from these involuntary and automatic centers, together with the cortical motor pathways that are not part of the pyramidal pathways (of which we shall now speak), constitute the involuntary extrapyramidal motor system. Lesions of this system provoke conditions of spastic contracture and rigidity. According to some authors, among them Kappers, all acts, even those that are initiated voluntarily but that become automatic and involuntary through habit or education, are carried out by this system, which is the only one in the lower vertebrates lacking pyramidal pathways. The higher sensory centers of integration of the lower vertebrates are instead represented, as has been said, by the thalami; in man many primitive thalamic functions are taken over by the neocortex, but some primitive forms, even at the threshold of consciousness, are still carried out by these centers; among these activities are to be ascribed the affective sensations, devoid of criticism. The cerebral cortex represents the most developed part of the entire human encephalon (v. CERVELLO) with a surface of 200,000 mm². This great surface is obtained by foldings into convolutions (sulci and gyri).
The fibers that reach the neocortex from the underlying regions and those that depart from it constitute the afferent and efferent projection fibers; afferent are the fibers that come from the thalamus and from the geniculate bodies: general somatic sensations go to the postcentral gyrus, those optical to the occipital lobe and those acoustic to the temporal lobe; afferent are the fibers that go directly to the bulbo-spinal motor centers and that depart from the precentral motor area constituting the pyramidal pathway (so called because of their passage through the pyramids of the medulla), those that go to the motor centers of the extrapyramidal pathway and those that go to the centers of the pons of Varolius for the connection with the cerebellum. Moreover, in the cortex there are association fibers that establish connections of various cerebral areas with one another and the commissural fibers that establish such a relationship with areas of the two hemispheres, fibers that come to constitute the corpus callosum (for the neocortex), the anterior commissure and the hippocampal commissure (for the olfactory cortex).
The cerebral cortex represents the highest nervous center, not only because in it the most complex nervous mechanisms of an unconscious nature are carried out, such as conditioned reflexes, but because it is the seat of the highest psychic activities that go beyond the strictly physiological field with the manifestations of will, intelligence and consciousness. These nervous phenomena are purely human, inasmuch as only man possesses that structural complexity capable of expressing them; according to some authors, however, human psychic phenomena would be only a degree, albeit an extremely high one, of a general phenomenon that already manifests itself in a simple form also in other animals. One must nevertheless take into account how in the evaluation of intelligence there are included manifestations that, like memory, are the expression of a general phenomenon of living matter, and likewise the capacity for perceptions and the execution of acts that are resolvable by conditioned reflexes. Thus many phenomena considered to be intelligence in animals are entirely unconscious, as are the innumerable instincts that guide them. Truly sublime intelligence, superior to human intelligence, would be that of many animals, even invertebrates, if their complex behavior were conscious and if their work were the fruit of training and of a constructive will! But they are perfect builders, to limit ourselves to this aspect of their activities, as are, for example, our bone cells in the building of bones according to the most precise laws of graphic statics. It is not a question of intelligence, but of intrinsic aptitude: the newborn bee already knows how to construct its hexagonal cells in the most perfect way. There is no error in the construction of the animal architects: whereas error is human and demonstrates the completely different way in which homo faber makes his constructions; with training and toil; with intelligence.
Conditioned reflexes are reflexes that are carried out asso-
by associating an unconditioned stimulus with a conditioned stimulus. Famous is Pavlov's experiment in which, by repeatedly giving a dog food while simultaneously ringing a bell, he could then provoke gastric secretion with the sound of the bell alone; the sound thus constituted a conditioned stimulus and the secretion of gastric juice a conditioned reflex. Conditioned reflexes are innumerable and of both excitatory and inhibitory type, and can become complicated into a series of secondary and tertiary reflexes, etc. In man such chain reflexes are very common, and the word itself is a succession of such phenomena. Very many acts, apparently voluntary, are carried out by means of conditioned reflexes. In man they are effected through associations that take place in the cerebral cortex, which, according to Pavlov, is indispensable, but reflexes of this nature, though simpler, are also effected in animals after removal of the cerebral cortex.
It has been possible to establish that the degree of intelligence in men is not related to cerebral volume or its weight, but is proportional to the complexity of the convolutions, that is, to its surface and to the number of pyramidal cells.
The cerebral surface is divided into areas with characteristic functions; anatomically it is divided into lobes, and these into gyri delimited by sulci.
Besides the efferent or motor projection areas localized in the precentral or prerolandic gyrus of the frontal lobe, and the extrapyramidal motor areas, situated at various points of the occipital, parietal and temporal lobes, there are localized in the cerebral cortex the sensory projection areas: in the postrolandic or postcentral gyrus of the parietal lobe are projected the fibers of general cutaneous sensibility, in the occipital lobe the fibers of visual sensibility, in the temporal lobe those of acoustic sensibility, and in the whole cortex deriving from the olfactory pallium the fibers of olfactory sensibility; the olfactory cortex constitutes in man the horn of Ammon, visible on the surface only in the piriform lobe.
Particularly important in the human species is the motor zone of articulate speech localized in Broca's gyrus, which is an extension of the precentral gyrus, and the frontal association area in which the higher activities of intelligence are carried out.
Bibliography6 works
Alberto Stefanelli