NERVOUS SYSTEM (of MAN)
### I. GENERALITY
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 performed by the single cell that constitutes the individual, which, however, may exhibit particular differentiations in this regard, such as cytoplasmic ones. In metazoa, or multicellular organisms, with the division of labor and the differentiation of various tissues, a characteristic tissue specializes for the nervous function, in which the specific functional element is the nerve cell. This cell is characterized by its extensions, which form the internal connections of the central nervous system and the fibers of the nerves in their peripheral course. The nerve cell, with its extensions, constitutes the anatomical and functional unit known as the "neuron." Nerve cells may be scattered among other tissues and connected by fibers that form plexuses, or they may be grouped into ganglia connected to each other and to receptors and effectors by extensions. In vertebrates and in man, in addition to the presence of scattered nerve cells and plexuses, ganglia, and nerves, most nerve cells are concentrated in an organ that constitutes the central nervous system or cerebrospinal axis.
### II. DEVELOPMENT OF THE NERVOUS SYSTEM IN VERTEBRATES AND MAN
The nervous system originates from a very early embryonic Anlage that becomes apparent at the time of gastrulation: the neural plate. With the folding of the plate, the neural tube is formed, which initially presents an anterior open end (anterior neuropore) and communicates posteriorly with the primitive intestine (neuroenteric canal).
Of great importance for the subsequent morphogenesis of the neural tube is the condition of the notochord, which forms beneath II. This supporting organ does not extend to its cephalic end but stops short, so that the neural tube is divided into a precordal part and a chordal part. The precordal part, in the absence of the dorsal obstacle, dilates into vesicles, as does, though more moderately, the most cephalic part of the chordal portion; the rest of the chordal part does not dilate and will give rise to the spinal cord, while the vesicles will give rise to the brain. After a two-vesicle stage and an intermediate three-vesicle stage, the five-vesicle stage is reached (telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon).
From this stage, characteristic of all vertebrates, the definitive differentiation proceeds through the development of two sac-like portions on the sides of the telencephalic vesicle, which in their greatest development represent the cerebral hemispheres, and through the inequality of the walls of the various vesicles, which also leads to a different width of the internal ventricular cavities. The spinal part of the neural tube, through the thickening of the lateral walls, is transformed into the spinal cord, and its internal cavity is reduced to a slender canal, 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, reaching the most conspicuous structural differentiations in man. The thickening of the walls of the neural tube occurs through the proliferation of the ependymal germinal layer, which produces two types of undifferentiated cells, the "neuroblasts" and the "spongioblasts," which migrate from this site to differentiate respectively into nerve cells and neuroglial cells. During their migration, directed by stimuli of various kinds, the neuroblasts gather into nuclei or nerve centers.
### III. MORPHOLOGY OF THE NERVOUS SYSTEM IN VERTEBRATES AND MAN
In the spinal cord, nerve cells form cellular columns (gray matter) along its length, which in cross-section take on the appearance of an H or 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 lies the somato-sensory area, where somatic sensations arrive; in the most apical part of the ventral horns lies the viscero-motor area, from which motor impulses depart for skeletal musculature; between these two areas, in an intermediate position but more dorsally, lies the viscero-sensory area, where visceral sensations arrive, and the somato-motor area, from which motor impulses depart for visceral musculature. The efferent, motor fibers exit from the ventral part and gather into bundles of fibers that form the ventral roots of the spinal nerves; the afferent, sensory fibers enter the spinal cord from the dorsal part (dorsal roots), and along their course lie the spinal ganglia. The two roots unite to form mixed spinal nerves. The sensory fibers that enter the spinal cord partly connect with cells of the dorsal horns and partly proceed cranially, reaching cell groups located in the medulla oblongata (nuclei of the gracile fasciculus and cuneate fasciculus). From the cells of the spinal sensory nuclei, fibers extend to various regions of the medulla oblongata (or bulb), the mesencephalon, the cerebellum, and the thalami of the diencephalon. The nervous element of the spinal ganglia, a typical bipolar element in the embryo (later transforming into a T-shaped cell), thus provided with two extensions, connects on one side with the spinal cord as described above, and on the other with the periphery through various peripheral devices (expansions, diffuse networks) and sensory corpuscles (of Meissner, Pacini, Golgi-Mazzoni, etc.).
Anterior to the spinal cord is the brain with its various subdivisions. The encephalic portion that is directly connected to the spinal cord is the medulla oblongata or bulb, which still closely resembles the spinal cord in morphology. The medulla oblongata presents lateral walls and a base, while the roof remains thin and forms the choroid tela of the fourth ventricle, represented by a wide dilation of the ependymal cavity derived from the myelencephalic vesicle.
In addition to the named centers for the connection of ascending long tracts from the spinal cord, the bulb contains motor and sensory centers of cranial nerves and centers for motor coordination (cutaneous centers, inferior olives). It contains the motor nuclei of somatic nerves such as the hypoglossal (XII pair) and the abducens nerve of the eye (VI pair), and the nuclei of visceral nerves that innervate musculature of branchial origin, salivary glands, and smooth musculature of the visceral organs of the body, such as the glossopharyngeal (IX pair), the vagus (X pair), and the accessory (XI pair), as well as the somato-sensory nuclei of the trigeminal (V pair), the cochlear and vestibular nerves, which constitute the acoustic complex (VIII pair), and the viscero-sensory nuclei of the VII, IX, and X pairs, which constitute the so-called "solitary fasciculus." In the white matter of the bulb, the passage of long tracts that go from the spinal cord to higher centers and of those descending from these to the spinal cord is found. Anterior to the medulla oblongata are the differentiations of the metencephalic vesicle, which, with the
The development of the dorsal part gives rise to the "cerebellum." Within it, centers differentiate that connect on one side with general prospective sensitivity (joint, tendon, and muscular sensitivity) and special sensitivity (semicircular canals), and on the other side with all motor impulses from the cortical, pyramidal, and extrapyramidal pathways, becoming the central organ for motor coordination and synergies, as well as the integration of all movement and balance activities. It operates autonomously, outside of voluntary control, though it maintains connections with the cerebral cortex via the centers of the pons of Varolius. Cerebellar responses occur through the cerebellar olives or dentate nuclei and the reflex motor centers of the tectum and the red nucleus of the midbrain.
Anterior to the metencephalon, the midbrain thickens in all its walls so that the ventricular cavity is reduced to a slender canal known as the "aqueduct of Sylvius." In its basal part lie other motor centers for the eyes (the oculomotor nerve, III pair, and the trochlear nerve, IV pair) and certain centers of the extrapyramidal motor pathway (which will be discussed shortly), such as the red nucleus and the substantia nigra. Additionally, the midbrain contains long ascending and descending pathways that do not have a relay station in this region. The dorsal part consists of the quadrigeminal plate, formed by a pair of superior colliculi (visual reflex centers) and a pair of inferior colliculi (auditory reflex centers). The diencephalic vesicle, situated anterior to the midbrain, differentiates during development into the thalami (thickenings of the lateral walls), the hypothalamus (thickening of the base), and the epithalamus (thickening of the roof). The cavity is reduced to a broad vertical fissure, which, together with the unpaired cavity of the subsequent telencephalic vesicle, forms the third ventricle.
The thalami are primarily sensory centers receiving long pathways from the spinal cord and other posterior regions (medulla oblongata, midbrain); a ventral portion (subthalamus) is motor. Within the sensory part of the thalami, the following regions are distinguished: an optic zone (pulvinar), connected with a differentiation of the thalamus itself, the lateral geniculate body; an auditory zone, connected with the medial geniculate body; and a zone for cutaneous sensitivity, derived from the long exteroceptive pathways of the spinal cord and medulla oblongata. The thalamus also contains association centers that, in lower vertebrates lacking a cerebral cortex, represent the highest centers where more complex, albeit primitive, nervous activities—such as instincts—are elaborated, without any manifestation of conscious will.
The most anterior vesicle, the telencephalon, undergoes the greatest differentiation and exhibits the highest degree of development among vertebrates. Initially unpaired and median like the others, this vesicle evaginates into two lateral vesicles, forming the two lateral ventricles (I and II). In lower vertebrates (fishes, amphibians, and lower reptiles), these hemispheres are exclusively related to olfactory sensitivity (paleopallium and archipallium); at the base, thickenings develop, known as the basal ganglia or corpus striatum, which represent the most cephalic motor relay and, even in higher forms, constitute a center of the extrapyramidal pathway.
In higher reptiles and mammals, in addition to the olfactory part, the other walls of the hemispheres thicken with the formation of a neopallium or cerebral cortex, which reaches its maximum development in humans. The neocortex is distinguished from other cerebral areas by the presence of typical pyramidal cells.
In studying the complexity of encephalic structures, the issue of fiber decussation must be considered first. Indeed, the vast majority of nerve fibers that have a peripheral relationship with one side connect with centers on the opposite side; thus, along the medial sagittal zone of the neuroaxis, decussations, chiasms, or commissures are observed, which sometimes constitute very prominent anatomical entities. In the telencephalon, three main commissures contribute significantly to its morphology: the "anterior commissure" and the "hippocampal commissure," composed exclusively of olfactory pathways (and thus the most primitive), and the "corpus callosum," through which the decussation pathways of the neocortex pass (making it the most evolved commissure). The corpus callosum reaches its greatest development in humans, where the neocortex is also maximally developed. With the partial development of the corpus callosum, the olfactory cortex is pushed far back from its initial morphological position, so that the fibers forming the hippocampal commissure must travel a long distance along the roof of the third ventricle, forming the anatomical structure known as the "three-pillar vault" or "fornix." The development of the cerebral cortex is primarily superficial and, in humans, presents a very large number of folds (convolutions). In the human species, the complexity is greatest, far exceeding that of even the most advanced mammals, such as anthropoid apes (v. cervello). The deepest sulci divide the cerebral cortex into five lobes: occipital, temporal, frontal, parietal, and the Insula, the latter situated deeply and operculated by the temporal lobe, which has grown over II. The sulci delimit the gyri.
IV. PHYSIOLOGY
A fundamental subdivision of the nervous system is that into two subsystems, distinct both morphologically and physiologically, though in intimate anatomical and functional connection: a "somatic" nervous system, which participates in the activities of relational life, and a "visceral" or "neurovegetative" nervous system (see below, § V), which participates in the physiological activities of the viscera. While the somatic nervous system operates through involuntary and voluntary mechanisms, the visceral nervous system functions exclusively autonomously or involuntarily.The somatic nervous system is represented by its peripheral part (peripheral sensory organs, motor end plates, peripheral nerves, spinal ganglia) and a central part (somatic portions of the spinal cord and brain); the visceral nervous system consists of peripheral sensory organs, nerves, visceral and spinal ganglia, and the central visceral portions of the spinal cord and brain (the presence of visceral centers in the telencephalic part is still debated). The viscera receive a dual visceral innervation, sympathetic and parasympathetic, with antagonistic actions. The adequate function for the physiological moment arises from the balance of these two activities.
V. RECEPTORS AND EFFECTORS
Nervous activity is expressed through the excitability of receptors and the activity of effectors. Receptors consist either of sensory cells located in specific peripheral epithelia and the sensory ganglionic neuron that transmits excitation to the central nervous system, or of ganglionic cells themselves, whose peripheral prolongation is complicated by microscopic peripheral devices (corpuscles, networks, plexuses, arborizations, etc.) situated in epithelia or connective tissues. Effectors consist of the motor nerve cell (spinal, bulbar, mesencephalic, or ganglionic), whose peripheral prolongation is complicated by terminal apparatuses adhering to striated muscle fibers (motor end plates) or smooth muscle fibers (with various arborizations).Receptors have been classified according to the type of stimulus.
NERVOUS SYSTEM (OF MAN) — Pyramidal cells of the cerebral cortex.
Specific mention is made of those capable of perceiving stimuli, which can be of various kinds, in receptors of physical and chemical stimuli. Physical receptors are distinguished according to the number of vibrations per square meter to which they are excitable. Tactile receptors are capable of receiving very slow vibrations up to 155 per second; acoustic receptors perceive vibrations from 30 to 30,000. Ether vibrations are stimuli for cutaneous thermoreceptors when their number is between 3,000 and 4,000.
At 30,000 vibrations are visual stimuli for the photoreceptors of the retina, and those from 400,000 billion to 800,000 billion per square meter. These values apply to the human species and vary in other species. Thus, in other animals, other ranges of vibrations can be perceived due to the presence of suitable receptors, just as they can be insensitive to others due to the lack of specific receptors. Chemoreceptors are specialized to perceive stimuli of a chemical nature: these are the olfactory and gustatory receptors. Among the gustatory receptors located in the taste buds of the tongue (and in other animals, of the palate or skin), there are those specialized for sweet, salty, bitter substances, etc. For physical or chemical conditions of the environment to be stimuli, it is necessary not only that the soul possess the appropriate sense organ, or receptor, but also that the stimulus action exceed a certain threshold of intensity.
Receptors can be somatic or visceral. Somatic receptors are subdivided, according to Sherrington, into exteroceptors and proprioceptors: the former are deputed to perceive stimuli from the external world directly (tactile, thermal, visual, acoustic, olfactory stimuli), while the latter are indirectly so because the site of stimulation is in the joints, tendons, and muscles, in relation to the stance on the limbs and muscular efforts, activities that are indirectly related to 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 relative to the environment, which represents the perception necessary for the function of equilibrium. Visceral receptors are called visceroreceptors and are represented by neurons of the visceral nervous system and their peripheral expansions (recti, plexuses, and corpuscles). The effectors, consisting of the various motor units, are also somatic, represented by the somatic motor neuron and the striated muscle fibers connected to it by terminal motor plates, and visceral, represented by the ganglionic visceral motor neuron and the expansions on the smooth or striated muscles (heart) of the viscera connected to II.
Of particular physiological importance are the contact zones between the peripheral sensory cell (where it exists) and the sensory fiber and that between the motor plate and the muscle fiber, called "synapses," and the relationships between neuron and neuron in the constitution of chains (interneuron synapses). The synapse allows the transmission of excitation while preserving the integrity of the anatomical unit (neuron theory), allows, with its multiplicity, the "spatial probing" of the excitation, explains the latency period in reflex conduction, and explains the polarization of conduction in a chain of neurons. The mechanism by which excitation crosses the synapse has been the subject 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 for the production of "chemical mediators" (acetylcholine, inactivated after the action of cholinesterase).
VI. REFLEXES
Reference has already been made in the study of the evolution of the nervous system to how the simplest nervous mechanism is expressed through the intervention of only two neurons: the sensory neuron and the motor neuron. This is called a simple reflex arc. The spinal cord is the site of these reflexes through 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, peripherally connected with the fibers of a muscle. This spinal reflex arc, like all reflexes, is involuntary. In the spinal cord, extensor reflexes (such as the well-known knee-jerk reflex, elicited by tapping below the patella) and flexor reflexes (such as those due to noxious stimuli to the extremities) are manifested. The two associated reflexes are deputed to maintain the animal on its limbs in opposition to gravity. When a muscle contracts, the antagonistic apparatus relaxes according to a fundamental principle of reciprocal innervation and inhibition. More complex spinal reflex mechanisms are those intersegmental, by which the reflex movement of a pair of limbs will involve an "integration" with movements of other muscles of the trunk or the other pair of limbs and even with effects on the visceral sphere (genitalia). This is well demonstrated in spinal animals, i.e., with the spinal cord resected and separated from the brain.The physiological value of the higher brain centers on spinal activities can be studied by shifting the plane of resection more and more cephalad, or by removing the brain with resections at increasingly higher levels.
If a resection is performed before the pons Varolii, the characteristic decerebrate rigidity manifests itself, which is caused by a continuous contraction of the muscles, predominantly extensors. This occurs due to the continuous excitation of the vestibular centers, which remain connected to the spinal cord and are not dominated by the higher centers and particularly by the extrapyramidal system. Postural, static movements are expressed through five main reflex mechanisms: 1) labyrinthine reflexes; 2) reflexes of righting the body in relation to the position of the head; 3) reflexes of righting the body independently of the head (which are expressed through the bulbar centers connected to the red nucleus of the midbrain); 4) reflexes of righting the neck (integrated in the spinal cord); 5) reflexes of righting the eyes (integrated in the cerebral cortex). Other reflex mechanisms are those of the eyes (in the midbrain, as demonstrated by faradic stimulation in this region). The spinal cord is also the site of visceral reflexes, such as vasomotor reflexes, sweating, defecation, and urination. Thus, in connection with the visceral pathways, abnormal impulses can be determined which, via reflex pathways, can cause visceral pain, cutaneous hypersensitivity, muscular contracture, secretory activity, etc. In the brainstem there are somatic and visceral centers related to other characteristic reflexes, such as those of swallowing, mastication, salivary secretion, coughing, sneezing, vomiting, dilation or constriction of the pupils, etc. Thus, centers are found that regulate automatic activities such as respiration, heartbeat, etc. The spinal cord and brainstem (medulla oblongata and midbrain) are traversed by long ascending pathways that carry "excitations" to the higher diencephalic and telencephalic centers, and by long descending pathways that, originating from these higher sectors, reach the same motor cells of the spinal cord and brainstem, which come into action for the various local reflexes and are therefore also defined as "final common pathways."
VII. THE THALAMUS
The ascending fibers that come fromFrom the spinal medulla and carrying sensations from exteroceptors have a station in the medulla oblongata and from there reach the thalami of the diencephalon, while the active proper tracts stop at the cerebellum; other exteroceptive tracts of the head have their centers in the medulla oblongata and from there, like the spinal tracts, reach the thalami. Thus acoustic and optic sensations reach the thalami with a first station in the medial and lateral geniculate bodies. Acoustic, optic, and olfactory sensations, in addition to reaching the thalamus and then being projected into the cerebral cortex, have some reflex motor centers: these are the posterior colliculi of the midbrain for acoustic reflexes, the anterior colliculi for optic reflexes, and the diencephalic mammillary bodies for olfactory reflexes.
In the diencephalon, besides the development of the two thalami, the sensory areas of the lateral walls, which receive all these sensory tracts, there are olfactory centers in the epithalamus (habenular ganglia) and in the hypothalamus (the mammillary bodies). In the hypothalamus there are also 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 tracts, are in relation with the hypophysis, the directing gland of the vast majority of the organism’s hormonal activities.
VIII. The Cerebral Hemispheres
The telencephalic vesicle, which is the seat of the greatest cerebral complications, gives rise, 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 psyche are elaborated. The telencephalic centers are: the centers and tracts of olfactory sensitivity, the basal ganglia or striate bodies, and the cerebral cortex. Indeed, one can 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 elasmobranchs); only to the striate bodies, in addition to olfactory fibers, do visceral and somatic fibers arrive. Subsequently, to these parts are added new, non-olfactory parts, both to the basal ganglia and to the cortex: the neostriatum and the neocortex. These centers appear primitively in higher reptiles (crocodiles); their maximum development is attained in man, and here they surpass by far all the other centers of the neuron hitherto considered.In vertebrates lacking a neocortex, the striate bodies represent the higher centers where the response impulses to various external and internal stimuli are elaborated; they are therefore the highest reflex centers. Voluntary motor centers develop in the neocortex and only in certain areas of II. The motor tracts that arise in the striate bodies are associated with tracts from other motor centers of the brainstem, such as the red nucleus, tegmental centers, reflex, visual, acoustic, vestibular, and olfactory motor centers; they are connected with the cerebellum, which has the task of reinforcing and coordinating these impulses into an integrated motor activity with general proprioceptive sensations so as to effect motor responses adequate to the static conditions of the animal. The motor tracts that derive from these involuntary and automatic centers, together with the cortical motor tracts that are not part of the pyramidal tracts (of which more will be said), constitute the extrapyramidal involuntary motor system. Lesions of this system cause conditions of spastic contracture and rigidity. According to some authors, including Kappers, all acts, even those that begin voluntarily but become automatic and involuntary through habit or education, are carried out by this system, which is the only one in lower vertebrates lacking pyramidal tracts. The highest sensory integration centers in 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 included affective sensations, devoid of critical judgment. The cerebral cortex represents the most developed part of the entire human encephalon (v. CERVELLO) with a surface area of 200,000 mm². This large surface is achieved through folding into convolutions (sulci and gyri).
The fibers that reach the neocortex from the underlying regions and those that leave it constitute the afferent and efferent projection fibers; afferent are the fibers that come from the thalamus and the geniculate bodies: general somatic sensations go to the postcentral gyrus, optic sensations to the occipital lobe, and acoustic sensations to the temporal lobe; afferent are the fibers that go directly to the bulbospinal motor centers and that leave the precentral motor area, forming the pyramidal tract (so called because of their passage through the pyramids of the medulla oblongata), those that go to the motor centers of the extrapyramidal tract, and those that go to the centers of the pons for connection with the cerebellum. Moreover, in the cortex there are association fibers that establish connections between various cerebral areas and the commissural fibers that establish such relationships with areas of the two hemispheres, fibers that come to form 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 unconscious nervous mechanisms are carried out, such as conditioned reflexes, but because it is the seat of the highest psychic activities that transcend the strictly physiological field with the manifestations of will, intelligence, and consciousness. These nervous phenomena are distinctly 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 elevated one, of a general phenomenon that already manifests itself in simple form even in other animals. It must be borne in mind, however, that in evaluating intelligence, manifestations are included that, like memory, are expressions of a general phenomenon of living matter, as well as the capacity for perception and the execution of acts that can be resolved by conditioned reflexes. Thus many phenomena considered to be intelligence in animals are entirely unconscious, as are the countless 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 constructive will! But they are perfect builders, to limit ourselves to this aspect of their activities, as, for example, our bone cells are in building 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 manner. There is no error in the constructions of animal architects: whereas error is human and demonstrates the completely different way in which the *homo faber* makes his constructions; with training and effort; with intelligence.
Conditioned reflexes are reflexes that are established
Combining an unconditioned stimulus with a conditioned one. Pavlov’s famous experiment showed that by repeatedly feeding a dog while simultaneously ringing a bell, he could later induce gastric secretion with the bell alone; the sound thus became a conditioned stimulus, and the secretion of gastric juice a conditioned reflex. Conditioned reflexes are innumerable and may be excitatory or inhibitory in type; they can also become complicated through a series of secondary and tertiary reflexes, and so on. In man such chain reflexes are very common, and even speech itself is a succession of such phenomena. Many acts that appear voluntary are actually carried out through the agency of conditioned reflexes. In man these are mediated by associations formed in the cerebral cortex, which Pavlov considered indispensable; yet reflexes of this kind, though simpler, can also occur in animals after removal of the cerebral cortex.
It has been observed that the degree of intelligence in men is not related to the volume or weight of the brain but is proportional to the complexity of its convolutions—that is, to its surface area and to the number of pyramidal cells.
The cerebral surface is divided into areas with characteristic functions; anatomically it is subdivided into lobes, and these into gyri delimited by sulci.
In addition to the efferent or motor projection areas located in the precentral or pre-Rolandic gyrus of the frontal lobe and to the extrapyramidal motor areas situated in various points of the occipital, parietal, and temporal lobes, the cerebral cortex also contains sensory projection areas: in the postcentral or post-Rolandic gyrus of the parietal lobe the fibers of general cutaneous sensitivity are projected; in the occipital lobe, those of visual sensitivity; in the temporal lobe, those of auditory sensitivity; and throughout the cortex derived from the olfactory pallium, the fibers of olfactory sensitivity. The olfactory cortex forms in man the cornu Ammonis, visible on the surface only in the piriform lobe.
Of particular importance in the human species is the motor zone for articulated speech, located 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 expressed.