Brain

BRAIN. - The encephalon (from Gr. ἐγκέφαλος) constitutes the most important center of the entire nervous system and is located in the cavity of the skull, while the spinal cord occupies the vertebral canal. In it there are primary centers that have direct relations with the periphery (for example, for the transmission of sensory impulses) together with centers of correlation, coordination and association that are particularly developed in man.

As a whole, the shape of the c. is comparable to an ovoid oriented in an anteroposterior direction, but it naturally varies with the variation of the shapes of the cranial cavity that encloses it. In dolichocephalics (v. CRANIO), the c. is relatively narrower and longer; in brachycephalics it is relatively broader and shorter.

The volume of the c. is particularly large in man in comparison with all mammals. It may be said that this is one of the principal characteristics of the physical organization of man. The same may be repeated for weight, even while bearing in mind in this regard the strong racial and individual variations. According to absolute values, the weight of the c. in women is lower than in men (in women, according to Broca, there is a value of 995 g., in men a value of 1157 g., but other authors have obtained even higher values; from the data collected by Chiarugi for a group of 663 Italian males there is 1153 g., for a group of 428 Italian females there is 1016 g.). A relative evaluation of the cerebral or encephalic weight (considered as a whole) has furthermore also been attempted for the complete series of mammals, with the aim of seeing whether a law of general behavior of the aforesaid weight could be identified with respect to the body mass of the animal or even to its total weight. The enormous variability of the absolute weight of the encephalon in the series of mammals was in fact known, but a simple glance at the individual values, arranged according to the increasing weight of the encephalon, had made it noticed how they were also found arranged, on the whole, according to the increasing size of the body mass. Such a mutual dependence of the two weight measurements evidently also had a physiological meaning, but from the most ancient researches it was rightly pointed out that body weight is only one of the many factors of weight variability of the encephalon and in particular of the c. Hence the idea of attempting a statistical evaluation of the relationship naturally existing between the two weights.

Using an orthogonal graph, in which the abscissas represent the body weight and the ordinates the encephalic weight, one obtains a course of values (if one works on logarithms) interpolable with an ascending straight line, from low to high values, for the mass of mammals and also for various birds (A. Sacchetti). In the attached graph there also appear the straight lines implicit in the procedures used previously by Cuvier and Lapicque, but which have been disproved by the facts as an interspecific relationship. Interesting is the position of man (with points 1 and 2), very high in the sense of a prevalence of encephalic weight. This position could be evaluated with an interspecific coefficient whose measure gives a relative idea precisely of the encephalic weight, with the consequent possibility of comparisons between the different species. Other relationships have been brought to light between the two same weight measurements in the comparative sphere not only of species, but of races or other systematic entities, always considering them as populations of individuals. This methodological principle of research falls within the direction of modern demogenetic studies, that is, of population genetics (v. BIOMETRIA).

As regards the relationships between psychology and the massive development of the c., it must be said that these subsist as one of the many factors of the different encephalic evolution of the animal or of man and furthermore have a different bearing as a function of the structural and morphological differences of the individual cerebral portions. In any case a certain relationship exists between encephalic weight and the development of intelligence even in the comparison limited to man. In idiots, for example, the weight of the c. is unusually low; in men of genius, on the other hand,

THE REGIONS AND CYTOARCHITECTONIC AREAS OF THE LATERAL CORTEX IN MAN (A) AND IN CERCOPITHECUS (B)
(ACCORDING TO BRODMANN)

Frontal region

Pyramidal area (proper motor cortex)

Agranular area

Preoccipital and occipital area (Campbell's psychical visual area)

Supratemporal and insular regions

Inferior frontal area (human)

Postcentral region

Parietal region

Striate area (Campbell's sensory visual area)

Temporal region

BRAIN

(by courtesy of Prof. A. Sacchetti)

it is very often above the average of the population to which it belongs (Cuvier's encephalon weighed 1861 g., Byron's 1805, Schiller's 1580). This relationship is not, however, absolutely constant, owing to the many biological factors that interfere with it and about many of which our scientific knowledge is still inadequate.

In the c. two hemispheres in particular are distinguished (right and left) joined to each other in the median part by a horizontal lamina of white matter called the corpus callosum. Each hemisphere presents two extremities (one frontal or anterior and one occipital or posterior) and three faces, one internal toward the other hemisphere, one external toward the cranial wall, one inferior toward the base of the skull. The surface of the hemispheres, called the cerebral cortex (cortex), or pallium, or mantle, in many animals is smooth: hence their name of lissencephalics (R. Owen). In man, on the contrary, as in the higher animals or gyrencephalics, it presents numerous protrusions and furrows, the former called convolutions or folds separated from each other 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 c. There are fissures that bound entire lobes on the individual cerebral faces and are the deepest and most interesting for phyletic comparisons. Among them are remembered the fissure of Sylvius and that of Rolando. The frontal lobe is precisely situated anteriorly to the fissure of Rolando and thus has very precise limits. There are then a parietal lobe, an occipital one and a temporal one. On these individual lobes an enormous literature, by anatomical scholars and anthropologists, deals with the various convolutions and their phyletic, racial or physiological significance. Interesting in this regard is also the lobe of the insula covered by the lips of the fissure of Sylvius.

The general structure of the cortex is characterized by layers where fibers and nerve cells of various types alternate. The nerve cells, however, constitute the essential element of the gray matter for an average thickness of about 2.5 mm. or even more. They are generally differentiated into six different superimposed layers according to the classification today accepted (Economo, Brodmann, Cajal, etc.), where among others pyramidal, or fusiform, or granular cells prevail; the first two categories forming part predominantly of projection pathways; the last, which connects elements of greater size and which is provided with shorter neurites, exercises predominantly associative functions, especially in the ♂ 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 distinguished morphologically and to which a different meaning of functional localization corresponds can be classified into projection areas to the movements necessary for articulate speech. Similar correlated centers are, however, also found in the superior temporal gyrus and in the inferior temporal gyrus. Among the fibers contained in the white matter of the hemispheres, the elements of differentiation are studied particularly according to their origin, their termination and their anatomical significance. There are, according to Meynert, fibers of three types: association fibers, intended to unite two points of the cortex in the same hemisphere; commissural fibers, which bring the two hemispheres into 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 lying beneath them. Thus the so-called pathways are identified, which can synthetically tion (motor areas with the almost total absence of true granules), sensory areas (with an enormous wealth of granules), association areas (with lesser development of both types of cells). This was arrived at after an infinite series of experimental researches, clinical and anatomo-pathological observations. It is indeed true, from the physiological point of view, that today the notion of the whole (in the specific case the brain as a single psychic center) remains as fundamental in biology as in psychology; it is necessary, however, to deny, in the light of the facts, that all psychic acts, even elementary ones, involve the brain in all its complex structure. Functional localizations therefore remain a scientifically established fact.

Among the projection centers, for example, one should note the tactile sphere, around the fissure of Rolando; the olfactory sphere, on the internal surface; the visual sphere, on the occipital lobe; and the acoustic sphere, on the first temporal convolution. Physiologically important in the frontal lobe (which is above all developed in man) is the inferior convolution of the lateral surface (convolution of Broca), which presides over be distinguished into an ascending or sensory pathway and a descending or motor pathway. The functional meaning of the two expressions is implicit and evident.

The racial differentiations of the human c. [cervello], besides concerning in particular 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 shapes of the races. According to Chiarugi, in brachycephalic human groups the sulci running in an anteroposterior direction would be deeper than in dolichocephalics. The indices and measurements of individual encephalic portions would also be different. The fissure of Sylvius would be differently inclined, as seems to be the case for the fossil men of the Paleolithic. Other notable differences concern the lobe of the insula, according to the specific studies of S. Sergi, in particular comparison with Hylobates syndactylus.

Particular knowledge about the remains of the most ancient men is based on the cubic capacity of the cranial cavity and on the cast of the endocranium. On the latter are indirectly based the findings concerning the morphology of the folds that have left impressions. These are naturally imprecise and incomplete observations, whose conclusions must be accepted with reserve (for endocranial volume v. PALEANTROPOLOGIA). As regards the sulcation, what is striking in the Neanderthals is their generic simplicity and the coarse appearance of the convolutions. Particular attention must be given to the foot of the third frontal convolution, which is certainly present in the Neanderthals, as S. Sergi demonstrated for the skull of Saccopastore. In the cast of the skull of Piltdown (Lower Paleolithic) Keith finds a simple and primitive arrangement of the parts, but not so simple and so primitive as to place the c. of Piltdown in a class distinct from that of the modern human c.

One concludes with S. Sergi: it may be held that the encephalon of the Neanderthals, while in volume and general morphology of the neopallium it possesses the characteristics of living hominids, departs from these through a set of primitive characters, intermediate between those of man and of the anthropomorphs. Some characters of differentiation seem, indeed, to represent a less advanced evolution "to which a scantier psychic activity must have corresponded." This assertion is, however, very uncertain, all the more so since conclusive data on the encephalic morphology of even more ancient men such as Pithecanthropus are lacking.

Coming down to the differences of individual variability, for the living, among the individual portions of the c., the following may be said: eight zones have been identified (A. Sacchetti) with a more stable sulcation, and without exception they are the most ancient; those which often in man have undergone a relative reduction with respect to the growth of surrounding areas that are more unstable from individual to individual, or to the arising of new areas and hence of a new sulcation and gyration. Thus it is for the olfactory area, the superior temporal, the striate, the limbic. All the new formations prove unstable, more variable; they are fundamentally 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 (they are the association areas), those that give man so-called personality not only through the direct cerebral formations but, above all, through the interest they arouse, through the psychological conditions of the higher associative sphere (it would suffice to mention certain frontal areas and the inferior parietal).

All the data confirm: a) the greater variability of the areas and of the parts that are more complex from the 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 phylogenesis, which are typically human because of their particular development in man.

Where homologies with the anthropomorphs and the monkeys are easier, where we witness the relative reduction of the parts in man (for example, following the planimetric coefficients of Tilney), where there is a relatively lesser complexity and functional typicality, and where the formative processes occur early in ontogenetic development, there is generally a lesser variability.

HISTORICAL PRECEDENTS

In the medical writings that, collected under the name of Hippocrates (468-377 B.C.), form the basis of ancient medicine, two different concepts are found regarding the function of the brain. In contradiction to the Homeric belief, which considered the diaphragm the seat of spiritual functions, according to Hippocrates "it is through the brain 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 pains and displeasures on the other refer to the brain." On the other hand, the writings on the cardinal humors consider the brain the place where mucus is prepared, the cold and moist humor (the humor of winter and old age). Mucus, descending from the head as "catarrh" or "rheumatism," is the cause of pneumonias, pleurisies, phthisis, ascites and diarrheas; while bile is responsible for the heat of fever, mucus causes trembling. Apoplexy and paraplegia are due to mucus, which obstructs the vessels.

The followers of Plato in medicine, the so-called dogmatists, consider the marrow as the bond between body and soul. They see in the brain, perfect in its spherical form, not only the place where the seed is prepared but also the seat of reason and of 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.), sees in the brain, and particularly in its ventricles, the seat of the soul. To Herophilus we owe a quite exact anatomical description of the brain; and parts such as the calamus scriptorius and torcular still bear his name.

Galen (131-200 A.D.), physician to the emperor Hadrian, recognizes the correlations between psychic and physical factors and professes the doctrine that psychic functions are the expression of the concert of the humors. He places in the brain the faculty of drawing conclusions (ἐνυχή λογιστική); while the 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. The pneuma psychicón is according to these formed from the content of the carotids in the choroid plexuses of the ventricles. Although repeating the Hippocratic ideas on the function of mucus, Galen sees in the brain an insensitive organ, having a movement coordinated with respiration, whose purpose is to carry the pneuma from the ventricles into the nerves. Galenic ideas dominated medical thought in this field too for a millennium and a half.

The Byzantine physician Poseidonios, in the second half of the 4th century, was the first who, denying the demonic origin of mental illnesses, advanced the hypothesis of the localization of certain cerebral functions. He situated fantasy in the anterior lobe, reason in the central ventricles, memory in the occipital parts; from this derived the conviction that neuroses and psychoses were due to disturbances of these parts.

Arab medicine, represented by Avicenna (980-1037), mirrors the thoughts of the Greek world and considers psychic alterations as dyscrasias of the brain. On their localization and intensity depend the intensity and form of the psychoses. Black and yellow bile as well as putrid mucus are considered the cause of melancholy, mania and dementia. As for localization, disturbances of the anterior lobe cause hallucinations, those of the central parts the loss of intellect, while occipital lesions cause the loss of memory.

The "School of Salerno," the principal representative of medieval medical thought, classifies the organs into membra animata, among which the brain dominates, membra spiritualia, dominated by the heart, and membra nutritiva by the liver. The speculative medicine of the Middle Ages contributed very little to the progress of scientific knowledge, and only the great anatomists of the Renaissance succeeded in giving 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 professor at Leiden, Franciscus Sylvius (de la Boë) (1614-72), discovered the true structure of the ventricles (aquaeductus Sylvii) and the Swiss Johann Wepfer contested the formation of animal spirits in them. He, in 1658, finally demonstrated that Hippocrates was wrong and that mucus 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 eighteenth century, contributions to the knowledge of the brain were made by the Frenchman Vicq d'Azyr (1748-94; fasciculus Vicq d'Azyr), and by the German Samuel Thomas Sömmering (1755-1830) with his book De basi encephali (substantia nigra Sömmering).

A lively discussion on the functions of the brain, which also interested the general public, was sustained by Franz Joseph Gall. He acquired indisputable merits by determining the course of the nerve bundles that enter the brain and describing the decussatio pyramidum. In his lectures, begun in 1786, he advanced the theory that 27 mental faculties are localized in a recognizable manner on the surface of the brain, such as love for one's children, the carnivorous instinct, friendship, cunning, wisdom, the 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 we owe the merit of having laid the foundations of more exact concepts. He, by discovering the respiratory center (point vital), opened the way for the search for vegetative centers in the central nervous system. Among the scientists of the nineteenth century, Luigi Rolando with his Saggio sopra la vera struttura del c. dell'uomo e degli animali

(Sassari 1809), Jules-Bernard Luys and Louis-Pierre Gratiolet with the comparative anatomy of the brain of man and monkeys, opened the way to a deeper knowledge, the way that, through histological and physiological discoveries, led to the present picture, marked by the names of Golgi, Martinotti, Baillarger, Gennin, Bechterew, Monakov, Meynert, Edinger, Flechsig, Burdach, Gowers, Deiters, Purkinje and others.

Alfredo Sacchetti