GERVELLO. - The encephalon (from Gr. ε_{γ} γκ ε ́ φα λ 0 ς) constitutes the most important center of the entire nervous system and is located within the cranial cavity, while the spinal cord occupies the vertebral canal. Within it are primary centers that have direct connections with the periphery (e.g., for the transmission of sensory impulses), along with correlation, coordination, and association centers that are particularly well developed in humans.
In its overall shape, the brain resembles an ovoid oriented in an anteroposterior direction, though its form naturally varies with the shape of the enclosing cranial cavity. In dolichocephalic individuals (v. cranio), the brain is relatively narrower and longer, while in brachycephalic individuals it is relatively wider and shorter.
The volume of the brain is particularly large in humans compared to all other mammals. It may be said that this is one of the principal characteristics of human physical organization. The same can be said for weight, though one must take into account significant racial and individual variations. According to absolute values, the weight of the brain is lower in women than in men (in women, according to Broca, the average is 995 g, while in men it is 1157 g; though other authors have recorded even higher values; according to data collected by Chiarugi for a group of 663 Italian males, the average is 1133 g, while for a group of 428 Italian females it is 1016 g). Attempts have also been made to evaluate the relative weight of the brain (considered as a whole) across the full range of mammals, in order to determine whether a general law could be discerned governing the relationship between brain weight and body mass or total weight of the animal. It is well known that absolute brain weight varies enormously among mammals, but a simple review of individual values, ordered according to increasing brain weight, reveals that they are also, as a whole, ordered according to increasing body size. This reciprocal dependence of the two measures clearly has physiological significance, but even the earliest research rightly emphasized that body weight is only one of many factors contributing to the variability of brain weight, and in particular of the brain. Hence the idea of attempting a statistical evaluation of the naturally existing ratio between the two weights.
Using an orthogonal graph, with body weight plotted on the abscissa and brain weight on the ordinate, the values (when plotted on a logarithmic scale) follow an ascending straight line from low to high values for the majority of mammals and even for several birds (A. Sacchetti). The graph also includes the lines implied in the procedures used previously by Cuvier and Lapicque, though these have been disproven by the facts as interspecific relationships. Of particular interest is the position of humans (marked by points 1 and 2), which is markedly elevated in terms of the predominance of brain weight. This position can be quantified using an interspecific coefficient, which provides a relative measure of brain weight and thus enables comparisons between different species. Other relationships between these same measures have been highlighted within comparative contexts not only of species but also of races or other systematic entities, always treating them as populations of individuals. This methodological principle of research falls within the scope of modern demographic studies, i.e., population genetics (v. GEOMETRIA).
Regarding the relationship between psychology and the mass development of the brain, it must be said that this exists as one of many factors in the differing encephalic evolution of animals or humans and also has varying significance depending on the structural and morphological differences of individual brain regions. Nevertheless, there is a certain relationship between brain weight and the development of intelligence, even when limited to comparisons within humans. In idiots, for example, brain weight is unusually low, whereas in geniuses it is
The brain is often far above the average for the population to which it belongs (Cuvier’s brain weighed 1861 g, Byron’s 1805 g, Schiller’s 1580 g). This ratio is, however, by no means constant, because of the many biological factors that interfere with it and about many of which our scientific knowledge is still inadequate.
In the brain two hemispheres (right and left) are distinguished, joined in the median part by a horizontal lamina of white matter called the corpus callosum. Each hemisphere has two ends (an anterior or frontal one and a posterior or occipital one) and three surfaces: an internal one facing the other hemisphere, an external one facing the cranial wall, and an inferior one facing the base of the skull. The surface of the hemispheres, called the cerebral cortex (corteccia), pallium, or mantle, is smooth in many animals, hence their name of lissencephalic (R. Owen). In man, on the contrary—as in higher animals or gyrencephalic animals—it presents numerous elevations and furrows, the former called convolutions or gyri, separated from one another by fissures or sulci.
The presence of these folds indicates a considerable development of the gray matter that constitutes the cortex in relation to the volume of the brain. There are fissures that delimit entire lobes on the individual cerebral surfaces and are the deepest and most interesting for phylogenetic comparisons. Among these are the Sylvian fissure and the Rolandic fissure. The frontal lobe is situated anterior to the Rolandic fissure and thus has very precise boundaries. There are then a parietal lobe, an occipital lobe, and a temporal lobe. On these individual lobes an enormous literature by anatomical and anthropological scholars deals with the various convolutions and their phylogenetic, racial, or physiological significance. Also of interest in this regard is the insular lobe, covered by the lips of the Sylvian fissure.
The general structure of the cortex is characterized by layers in which fibers and nerve cells of various types alternate. The nerve cells, however, constitute the essential element of the gray matter, to an average thickness of about 2.5 mm or more. Six different superimposed layers are generally distinguished according to the classification now accepted (Economo, Brodmann, Cajal, etc.), in which pyramidal, fusiform, or granular cells predominate among others; the first two categories belong mainly to projection pathways, while the last, which connects larger elements and is provided with shorter neurites, exercises mainly associative functions, especially in the fourth layer. To this cellular differentiation corresponds a profound differentiation of nerve fibers equally stratified (Elliot Smith and O. Vogt), as well as a different appearance among the areas of the individual lobes. The areas that are morphologically distinct and to which a different functional localization significance corresponds can be classified into projection areas (motor, with the almost total absence of true granules), sensory areas (with an enormous richness of granules), and association areas (with less development of both types of cells). This has been arrived at after an infinite series of experimental researches, clinical observations, and anatomical-pathological studies. It is true, from the physiological point of view, that today the notion of the whole (in this specific case the brain as the unique psychic center) remains fundamental in biology as in psychology; it is necessary, however, in the light of the facts, to deny that all psychic acts, even elementary ones, involve the brain in its entire complex structure. Functional localizations therefore remain a scientifically established fact.
Among the projection centers, for example, the tactile sphere around the Rolandic fissure, the olfactory sphere on the internal surface, the visual sphere on the occipital lobe, and the auditory sphere on the first temporal convolution should be noted. Of physiological importance in the frontal lobe (which is especially developed in man) is the inferior convolution of the lateral surface (Broca’s convolution), which possesses the movements necessary for articulated speech. Similar related centers are also found in the superior temporal convolution and in the inferior temporal convolution. Among the fibers contained in the white matter of the hemispheres, particular attention is given to the elements differentiated according to their origin, their termination, and their anatomical significance. According to Meynert, there are three types of fibers: association fibers, intended to connect two points of the cortex in the same hemisphere; commissural fibers, which put the two hemispheres in relation with each other; and projection fibers, which finally connect the gray matter of the cortex both with the central nuclei of the base and with the formations of the neuraxis beneath them. Thus the so-called pathways are identified, which synthetically can
Cerebrum = Structure of the cerebral cortex (according to Brodmann and Vogt, from Economo); on the left, the six laminated layers (cytoarchitectonics); on the right, the six corresponding planes of fibers (myeloarchitectonics).
They are distinguished into an ascending or sensory pathway and a descending or motor pathway. The functional significance of these two expressions is implicit and evident.
The racial differentiations of the human cerebrum, besides concerning in particular its weight and volume, are also of a macroscopic or microscopic morphological nature (cytoarchitectonic and myeloarchitectonic). Its general shape varies, for example, with the different cranial forms of the races. According to Chiarugi, in brachycephalic human groups the sulci running in an anteroposterior direction would be deeper than in dolichocephalic groups. The indices and measurements of individual encephalic portions would also differ. The Sylvian fissure would be differently inclined, as seems to be the case in fossil men of the Paleolithic. Other notable differences concern the lobe of the insula, according to specific studies by S. Sergi, in particular comparison with Hylobates syndactylus.
Our particular knowledge of the remains of the most ancient men is based on the cubic capacity of the cranial cavity and on the endocranial cast. The morphology of the folds that have left impressions is indirectly inferred from the latter. Naturally, these observations are imprecise and incomplete, and their conclusions must be accepted with caution (for cranial volume V. PALEANTHROPOLOGY).
As for sulcation, what strikes one about the Neanderthals is their generic simplicity and the coarse appearance of their convolutions. Particular attention should be paid to the foot of the third frontal convolution, which is certainly present in Neanderthals, as S. Sergi has demonstrated for the Saccopastore skull. In the cast of the Piltdown skull (Lower Paleolithic), Keith finds a simple and primitive arrangement of parts, but not so simple and primitive as to place the Piltdown brain in a class distinct from that of the modern human brain.
S. Sergi concludes: it may be considered that the brain of the Neanderthals, while possessing, in volume and general morphology of the neopallium, the characteristics of living hominids, diverges from these by a set of primitive, intermediate characters between those of man and anthropoid apes. Some differentiating characters seem, moreover, to represent a less advanced evolution "to which there must have corresponded a more limited psychic activity." This assertion, however, is very uncertain, all the more so since conclusive data are lacking on the encephalic morphology of even more ancient men such as Pithecanthropus.
Turning to individual variability in living beings, among the various portions of the cerebrum the following can be said: eight zones have been identified (A. Sacchetti) with more stable sulcation and, without exception, are the oldest; these are often relatively reduced in man compared to the growth of surrounding areas that are more unstable from individual to individual, or to the emergence of new areas and thus of new sulcation and gyration. This is the case for the olfactory area, the superior temporal, the striate, and the limbic areas. All neoformations are unstable, more variable; they are, in the end, those that still tend toward a different individual arrangement, those that represent in man a further perfection not only morphological but also physiological and psychological (these are the associative areas), those that give man his so-called personality not only through direct cerebral formations but, above all, through the interest they arouse, through the psychological conditions of the higher associative sphere (it would suffice to cite some frontal areas and the inferior parietal).
All the data confirm: a) the greater variability of the areas and parts that are more complex from a functional point of view; b) the greater variability of the cellular layers that preside over the characteristic functions of the areas; c) the greater variability of the zones that appear late in individual development and in the course of possible phylogeny, which are typically human due to their particular development in man.
Where homologies with anthropoid apes and monkeys are easier, where we witness the relative reduction of parts in man (for example, following the planimetric coefficients of Tilney), where there is a relative lesser complexity and functional typicality, and where formative processes occur early in ontogenetic development, there is generally less variability.
Historical background. — In the medical writings collected under the name of Hippocrates (468–377 B.C.), which form the basis of ancient medicine, two different concepts are found regarding the function of the cerebrum. Contrary to the Homeric belief, which considered the diaphragm the seat of spiritual functions, Hippocrates states: "it is through the cerebrum that we become mad, that delirium seizes us, that fear and pain assail us..."; "We know that pleasure and joy on the one hand, and pain and sorrow on the other, are referred to the cerebrum." On the other hand, the writings on the cardinal humors consider the cerebrum the place where phlegm, the cold and moist humor (the humor of winter and old age), is prepared. The phlegm, descending from the head as "catarrh" or "rheumatism," is the cause of pneumonias, pleurisy, tuberculosis, ascites, and diarrhea; while bile is responsible for the heat of fever, phlegm causes trembling. Apoplexy and paraplegia are due to phlegm, which clogs the vessels.
The followers of Plato in medicine, the so-called dogmatici, consider the marrow as the link between body and soul. They see in the cerebrum, perfect in its spherical form, not only the place where semen is prepared but also the seat of reason and the soul, while the mortal parts of the soul, such as feelings and desires, are placed in the chest and abdomen.
The school of Alexandria, represented by Herophilus (300 B.C.), saw in the brain, and particularly in its ventricles, the seat of the soul. To Herophilus is owed a very exact anatomical description of the brain; parts such as the calamus scriptorius and torcular still bear his name.
Galen (A.D. 131–200), physician to the emperor Hadrian, recognized the correlations between psychic and physical factors and professed the doctrine that psychic functions are the expression of the harmony of the humors. He placed in the brain the faculty of drawing conclusions (ψυχὴ λογιστικὴ); while emotional life resides in the heart, desires in the liver. Galen, who also devoted himself to the anatomy of the brain, introduced the ideas of the pneumatic school into his system. According to these, the psychic pneuma is formed from the content of the carotids in the choroid plexuses of the ventricles. Though repeating Hippocratic ideas on the function of mucus, Galen viewed the brain as an insensitive organ, having a movement coordinated with respiration, whose purpose was to convey the pneuma from the ventricles into the nerves. Galenic ideas dominated medical thought in this field for a millennium and a half.
The Byzantine physician Posidonius, in the second half of the 4th century, was the first to deny the demonic origin of mental illness and advanced the hypothesis of the localization of certain cerebral functions. He situated imagination in the anterior lobe, reason in the central ventricles, and memory in the occipital parts; from this he derived the conviction that neuroses and psychoses were due to disturbances in these regions.
Arabic medicine, represented by Avicenna (980–1037), reflected the thought of the Greek world and regarded psychic alterations as discrasias of the brain. The intensity and form of psychoses depend on their localization and intensity. Black and yellow bile as well as putrid phlegm were considered the cause of melancholy, mania, and dementia. As for localization, disturbances of the anterior lobe cause hallucinations, those of the central parts loss of intellect, while occipital lesions cause loss of memory.
The “Salernitan School,” the principal representative of medieval medical thought, classified organs into animated members, of which the brain is dominant, spiritual members, dominated by the heart, and nutritive members by the liver. Speculative medieval medicine contributed little to the progress of scientific knowledge, and it was only the great anatomists of the Renaissance who were able to give a description of the brain superior to that provided by Galen. The Bolognese Costanzo Varolio (1543–75) described, in 1572, with particular accuracy, the base of the brain (pons Varolii). The Leiden professor, Franciscus Sylvius (de la Boë) (1614–72), discovered the true structure of the ventricles (aqueductus Sylvii), and the Swiss Johann Wepfer contested the formation of animal spirits within them. He demonstrated in 1658 that Hippocrates was wrong and that phlegm could not descend from the brain to enter the nasal cavity. The Englishman Thomas Willis (1622–75) gave an exact description of the cerebral vessels (circulus arteriosus Willisii) and of the corpus striatum in his *Cerebri anatome* (1664).
In the 18th century, contributions to the knowledge of the brain were made by the Frenchman Vicq d’Azyr (1748–94; fasciculus Vicq d’Azyr) and the German Samuel Thomas Soemmering (1755–1830) with his work *De basi encephali* (substantia nigra Soemmering).
A lively discussion on the functions of the brain, which also interested the general public, was sparked by the Frenchman Giuseppe Gall. He earned undisputed merit by determining the course of the nerve bundles entering the brain and describing the decussatio pyramidum. In his lectures, begun in 1786, he advanced the theory that 27 mental faculties are distinctly localized on the surface of the brain, such as parental love, carnivorous instinct, friendship, cunning, wisdom, metaphysical spirit, poetic talent, etc. This theory, under the name of phrenology, was contested by official science, and it is to Marie-Jean-Pierre Flourens (1794–1867) that credit is due for laying the foundations of more accurate concepts. By discovering the respiratory center (point vital), he opened the way for research into the vegetative centers in the central nervous system. Among the scientists of the 19th century, Luigi Rolando with his *Saggio sopra la vera struttura del cervello dell’uomo e degli animali* (Sassari 1809), Jules-Bernard Luys and Louis-Pierre Gratiolet with their comparative anatomy of the brain of man and apes, paved the way for deeper knowledge—a path that, through histological and physiological discoveries, led to the present-day understanding marked by the names of Golgi, Martinotti, Baillarger, Gennari, Bechterew, Monakow, Meynert, Edinger, Flechsig, Burdach, Gowers, Deiters, Purkinje, and others.