CRANIUM. - The bony head is made up of two distinct portions: the cerebral cranium, which contains the brain (v. BRAIN), and the facial cranium, which contains various organs of the senses and the organs of mastication. The cranium properly so called, or cerebral cranium, is essentially a bony box protecting the most important part of the central nervous axis. Its study has therefore always been of singular interest from the various points of view — anatomical, physiological, anthropological and clinical. The bones composing it are essentially eight: the frontal, the ethmoid, the sphenoid, the occipital, the two parietals and the two temporals. Between them, however, supernumerary ossicles are often intercalated, more or less numerous and developed, called wormian bones. These bones are classifiable among the flat bones and are made up of two laminae of compact tissue, one external and one internal, enclosing between them a layer of spongy tissue, extremely variable in thickness, called the diploe. The inner table (or lamina) is in contact with the brain and molds itself, like wax, over its irregularities. Thus the grooves of the inner table correspond, as in a negative, to the cerebral convolutions, while the prominences (mammillary eminences) correspond to the cerebral sulci. Similarly, on the inner table are found the negative impressions of the arteries and of the large venous channels that run over the surface of the hemispheres. The study of cerebral morphology through such impressions is, however, subject to many reservations because of the uncertainty of the determinations.
The bones of the facial cranium are distinguishable into two sets, an upper one (upper maxillary portion) and a lower one (lower maxillary portion). The lower portion consists of the mandible alone, whereas the upper portion consists of thirteen distinct bones: the upper maxilla, the malar bone (zygomatic), the lacrimal (suguis), the inferior concha, the nasal and the palatine, all paired, arranged symmetrically with respect to the median plane of the cranium, and the vomer, unpaired. The human cranium is studied descriptively in anatomy, but its interest is still general among anthropologists, who through the most disparate comparisons seek to define precisely the characters of differentiation of species, race, sex, individual ages, etc. Among the classic names of scholars in the history of anthropology who are most closely linked to craniological research, mention should be made of: C. M. Daubenton (1744), G. F. Blumenbach (1775), P. Camper (1791), G. Prichard (1807), and then Geoffroy Saint-Hilaire, F. Cuvier, A. Forville, G. B. Parchappe, G. Serra, P. P. Broca, G. Sergi. But the array of researchers could go on for a long time, because the morphological and metric study of bones has always constituted an essential chapter for anthropology (v. ANTHROPOLOGY). This may seem exaggerated, inasmuch as the human organism does not reveal itself only through its bones, but it is understandable for various reasons: in practice the material for comparison is always more readily constituted by bones, while in the case of fossil men they are the only things preserved. For this very reason anthropologists have found themselves facing complex problems concerning the origin of human forms and the related transformations, which have likewise had to be resolved through skeletal remains. The cranium has naturally been the object of the greatest part of the research, and in reality it is in it that the surest characters of differentiation between the most disparate groups are revealed, together with the traces of the phyletic evolution realized over time. We shall mention the most important observations for racial classification, which also serve as a presupposition for the study of fossils.
Of the individual bones composing the cranium, the morphology is defined in relation to group or individual differentiations, but what is studied in particular is the shape also of the entire cerebral cranium viewed according to determinate norms, and of the facial one in its essential portions.
The shape of the cerebral cranium viewed from above, vertically to its maximum contour, often serves for a good racial differentiation, albeit with reservations relative to the first attempts of Giuseppe Sergi, who is the ingenious initiator of the method. Around 1895 he distinguished the following forms: 1) ellipsoid; 2) pentagonoid; 3) rhomboid; 4) ovoides; 5) beloid; 6) cuboid; 7) sphenoid; 8) sphenoid; 9) platycephaly. Frassetto subsequently reduced them to 6, but always following the same methodological principle of observation. In any case they give an idea of the real contour of the cranium (in the vertical norm) and morphologically integrate the results obtained from the study of the cephalic index introduced by A. Retzius (1842). This is the principal craniometric index used thus far, as the percentage ratio of the maximum width of the cranium to its length. Others are used which take into account height or other combinations of measurements, but racial differences are always more notable in the aforementioned norm of observation. On the basis of the cephalic index (or cranial, as it is more properly called when determined on skeletal remains) the following classification of values (and therefore of forms) is used: dolichocephalic (long crania) with an index up to 74.9; mesocephalic (crania of medium length) with an index from 75 to 79.9; brachycephalic (short crania) with an index from 80 upward. But this classification too evidently has only an indicative purpose. The limits between the aforementioned forms are arbitrary and in practice, apart from hybrids and mixtures, one never finds a race made up of individuals belonging to a single index category. In nature no discontinuity of the kind has been observed except by probing much more deeply into the analysis of the individual characters determining the overall forms. It should be noted in any case that modern studies, while not renouncing the diagnostic usefulness of cranial indices in general, seek to resolve the problems of classification without resorting to arbitrary subdivisions of the field of variability naturally presented by individual groups. Indeed the study of this variability is being probed ever more deeply, both with graphic representations and with suitable statistical methods of processing the material.
An example. One places on a Cartesian graph orthogonally on the abscissae a scale of possible values of the cranial index, on the ordinates a scale of individual frequencies. By plotting the data of two human groups on said graph, one obtains broken lines which, when by an arithmetical device the group means are arranged on the same vertical and the areas of the graph remain equal in overall extent, represent the real individual variability comparable within the categories or seriations being compared. In the accompanying figure the broken line relating to the Romans shows the existence of a transvariation, that is, of a mixture of two different types within the same group, types which, taken individually, would present a monocuspidal distribution. They correspond to Homo mediterraneus and Homo alpinus (v. RAZZE UMANE). The broken line of the Melanesians also reveals a heterogeneity of the group with the presence of dolichomorphic Melanesians (constituted by various cranial varieties according to G. Sergi) and of brachymorphic Melanesians, who appear as a group secondarily mixed with the principal dolichomorphic one.
On the same metric measurements of the cranium, the study of correlations has a particular value; it has brought to light racial differences of notable systematic significance, while it has clarified different behaviors of different cranial regions. Indeed, the correlation existing between length and width of the cranium has been examined in individual groups of individuals (belonging to the most disparate races) and determined positive values have been found which reveal that, as length increases, width also increases on average from one individual to another. But the same procedure applied to the racial means of several human groups has given a null and perhaps negative correlation. This means that the relationship between the two measurements is much more variable between different races than it is between individuals of the same race. The importance of the result is evident when one considers that instead there are cranial diameters (e.g., of the facial skeleton) that behave in a completely inverse manner: the racial differences in the aforesaid relationships are much smaller compared with the individual ones; in other words, the interracial correlation is greater than the intraracial one. It is a completely opposite behavior among different measurements that has been demonstrated only recently by biometric means, contrary to the pessimistic predictions of various authors (v. the works of A. Sacchetti).
Among the other craniometric indices most used as an indirect measurement of shape, mention should be made of: the vertico-longitudinal index (which serves to evaluate relatively the cranial height according to the categories of chamaecephali, orthocephali, hypsicephali), the frontoparietal index, the total facial index (which serves to evaluate relatively the facial height according to the categories of euryprosopes, mesoprosopes, leptoprosopes), the orbital index (which serves to evaluate relatively the height of the orbit according to the categories of chamaeconchs, mesoconchs, hypsiconchs), the nasal index (which serves to evaluate relatively the width of the nasal aperture according to the categories of leptorrhines, mesorrhines, platyrrhines). But there are some, such as the total facial index, which, while it is generally used for racial differentiation, has today proved absolutely unsuitable for that purpose (with the aforesaid biometric methods) because of its strong individual variability and therefore its very scant diagnostic value in the field of human systematics. In general, moreover, these metric measurements must always be integrated with the particular morphological analysis of the individual skeletal regions, together with the study of their topographic position with respect to the fundamental planes of orientation of the cranium. It is on the latter that the activity of anthropologists has been particularly exercised, e.g., for the judgment of prognathism in fossil men. But the best results have been obtained on physiological and morphomechanical premises.
In the same order of research, note should be taken of those tending to the measurement of cranial curves and angles (particularly at the base of the cranium, on the sphenoid, and at the face), as well as of the capacity of the cranial vault, on which observations of great interest have been made for the corresponding inferences about the mass of the brain, especially in the case of fossil men (v. CERVELLO; PALEANTROPOLOGIA). The most important facial angle is the one that permits the measurement of prognathism and that has given good results for racial differentiation. Note, however, that today prognathism is also determined analytically through the spatial morphomechanical study of the individual bone elements. Particular attention should be brought to this last point. All of modern craniology tends, in fact, not only to establish the systematic value in the differentiation of characters, but also to explain physiologically and mechanically the different architectural and structural adaptations of the skeleton. The function of mastication is evidently connected with the alveolar arch and with the arrangement of the individual bone elements, just as the architecture of the cranial vault depends on the spatial position of the temporal bone, which contains the organs of orientation. Indeed,
This dynamic correlation in craniogenesis was demonstrated by the studies of G. Sergi both on present-day and on prehistoric humans.
Such analysis of the position and shape of the individual bones in relation to the cranial solid as a whole and to its physiological planes of orientation is pursued with specific techniques and specially constructed instruments (v. ANTROPOLOGIA). In the accompanying figures there is an example of the study of the maxilla in isolation with respect to the fundamental planes, and the corresponding geometric diagrams, superimposed on one another, clearly show the differences sought, with a possibility of more rational interpretation. It should be noted in this regard that the reference points of each bone are those architecturally most important in craniogenesis.
In the accompanying figures one can see notable examples of racial differences in various other characteristics of the human cranium.
There are then morphological and structural differences depending on sex which make it possible to distinguish the male cranium from the female one even at the same age. The female cranium is said to preserve in general a morphological complex of greater infantilism which makes it more delicate and slender than the male one. It is in fact also smaller, lighter, the bony ridges for muscular insertions are less prominent, the mastoid processes are smaller, the forehead is more arched, of the bambée form that recalls that of children, the frontal and parietal bosses are more developed, the upper margins of the orbits are sharper, almost cutting, the overall volume of the cranial box is smaller, etc. Naturally all these typical characters serve in the majority of cases, but the anthropologist may find himself faced with exceptions for which the diagnosis remains very uncertain if not impossible. There are in fact cases in which female crania present characteristics of robustness entirely similar to male ones, so much so that they cannot be distinguished, while there are cases of male crania with delicate characters that are entirely confounded with the corresponding characters of the other sex. The norm, however, is the one indicated.
The development of the cranium through the ages of life has likewise been studied so that the examination of various characters permits a good diagnosis of age on the part of the anthropologist. Particularly notable are the transformations of the cranial vault in the child due to the processes of ossification which one observes after birth, within the so-called membranous cranium, which is constituted by the fibrous tissue of the vault, while the cranial base is very soon, during the embryonic period, constituted by cartilage (Kölliker's basilar plate). In the membranous cranium of the child (fontanellar period) numerous points of ossification are observed which gradually go to constitute the bones of the vault. It follows that the angles of the cranial pieces, farther from the aforesaid original centers of ossification, are the last to be filled by bony substance. Therefore the corresponding regions received the name of fontanelles. They interest the obstetrician for his prenatal diagnoses, as well as the anthropologist who observes the different developmental behaviors in the most disparate races. With the disappearance of the fontanelles through complete ossification of the vault, the margins of the individual bones assume a dentate shape which permits a kind of reciprocal interpenetration, in the manner of inlay, of the contiguous elements. The complication of these dentations at the so-called sutures of the cranium is different from region to region, but generally increases through the ages according to different timings, however, in different racial groups. All these elements are precious for the diagnoses of age of subjects. The specialized anthropologist knows in any case how to distinguish the not rare cases of anomalous obliteration of the sutures as the final stage of synostosis. The membrane remaining at the sutures is in fact gradually invaded by bony tissue (as previously the fontanelles), so that the different pieces of the cranium, albeit at different moments, come to fuse completely with one another. One must naturally distinguish also in this a normal physiological process which leads to the picture of the classic cranium of the old person, constituted by a single bony piece, from a pathological process which can even lead to strong anomalies of cranial forms (microcephaly, scaphocephaly, plagiocephaly, etc.). It has also been discussed, in this regard, whether a different progression of the obliteration of the cranial sutures can explain the different cephalic forms according, for example, to the schemes of G. Sergi. But perhaps it is a matter of only one of the dynamic factors of the craniogenesis of forms, a factor which can in any case enlighten us when we wish to interpret its nature and proximate biological causes, at least for individual ethnic groups.
In conclusion the human cranium differentiates itself in ages, in sexes, in races, according to modalities that are still the object of study with the modern methods of statistical and anthropological analysis. Very often the ultimate racial differences have not been explained in their nature and their temporal evolution is not known. The so-called connecting links between the different forms are lacking. The problems, however, are being deepened and study tends ever more to clarify the modalities of behavior of individual characters through the generations, as well as in the cases of miscegenation or hybridism and those of the most disparate environmental influences. The problem of racial mixtures is fundamental and often explains the genesis of new forms. Particular mixtures of different types serve at times as indirect experience for seeing the architectural adaptation of determinate bony structures (the example of the figure where two diagrams of the maxilla in Arbore individuals are superimposed served precisely, in the context of study, for the explanation of the facial metamorphism of those populations that live on Lake Stefania at the boundary of the habitation of the Negro world with the Hamitic one, in which conversely European facial forms dominate). Finally the problems of the pliability of characters as a function of the environment have also been the object of study in the case of the cranium, especially as regards its general form according to the view of the cephalic index. The researches of F. Boas on immigrants in America are known, as are the data of Snigrew and Talko-Hryncewicz for Lithuania, of Fishberg for New York, of Elkind for Warsaw, of Zakrewsky, Majer and Kopernicki for Galicia, of Hirsch for Massachusetts, of Guthe for Boston, as well as the investigations of Frets, Bryn, Schreiner, Geipel, Abel, Pessler, the recent ones of Dornfeldt and those of Sacchetti. The conclusions that seem most probable, according to the latter, suggest a norm of reaction of individual characters (such as the shape of the head), in the strictly genetic sense of the expression and in the case of transformations ascertained following a change of environment (through emigration of entire groups, etc.). In particular the cephalic index in comparison with all the other anthropometric characters of the bony head has shown a great and significant interracial variability; this result might seem in opposition to the aforesaid knowledge on the possible variations of the character following environmental solicitations. But a careful analysis of the phenomena shows that even those small variations of the shape of the head, which so preoccupied anthropologists at the beginning of the century (so that they divided into two contrasting camps in admitting or excluding environmental influences), must not be considered in contrast with the very aptitude of the character to the differentiation of groups, and even less with their real different morphological and demogenetic constitution, if one considers the strictly genetic and physiological aspect connected with it. It is a principle of reactivity linked to the individual genetic patrimony and which manifests itself through the phenotype. Thus alone is it explained how, faced with the same social and geographic environment (for example, in America, U.S.A.), racially different populations (brachycephalic Polish Jews and dolichocephalic Mediterranean Sicilians) reacted in a manner opposite to one another, irrespective of the individual forms of mixture with the local populations (Boas). Similarly it has happened for certain groups of emigrants in Italy (Sacchetti). Reactivity to the environment and heredity do not therefore turn out to be two contrasting concepts, at least for certain characteristics of the c. in anthropology. But similar examples are already cited in various fields of application of genetics, particularly of demogenetics. Thus to the static and exclusively morphological conception of the study of the human c. there has today been added a new method of investigation in the field of demogenetic determinism, always in relation to the reciprocal mechanical adaptation of forms, whereby one approaches ever more closely and better the explanation of the real complex phenotypic manifestation of human groups, as they are, in their natural environment and in their potential reactivity to external factors.
Bibliography33 works
Alfredo Sacchetti
