Fourth Period.—But in the case of cultured persons, those who form the class of brain-workers, the brain continues to grow, although extremely slowly, up to the age of thirty-five or even forty, thanks to the sutures which still remain completely intact and which still make an expansion of the bony envelope possible.
After this comes the beginning of the
Fifth Period.—The period of involution, during which the synostosis (closing) of all the cranial sutures will successively occur, until in advanced old age the cranium becomes composed of a single bone, just as in the embryo it was formed of a single membrane.
The synostoses which occurred in the early periods had an evolutive significance and were associated with the growth of the body and the intelligence. These later synostoses, on the contrary, have an involutive significance and are associated with the physiological decay of the organism and at the same time with that of the psychic activities.
The first point at which synostosis takes place is in the region of the obelion, that is, near the middle of the suture which, unites the two parietal bones; shortly afterward, the fronto-parietal sutures begin to unite along the pterion. At the age of forty-five, the obeliac synostosis has progressed as far as the lambda, and that of the fronto-parietal suture to the bregma; and at fifty the ossification is very nearly accomplished, at least on the right-hand side (according to Broca's series of crania). At seventy the squama of the temporal bone unites with the parietal, and at eighty the entire cranium has become a single bone.
These processes are subject to no small number of individual variations; there have been cases of persons who, although very old, still preserved many of their cranial sutures intact and their psychic activities remained correspondingly alert (men of genius). Conversely, the closing of the sutures sometimes begins as early as the thirty-fifth year. A diagnosis of age, as determined by the skeleton, is consequently only approximate.
During the periods of growth the cranium may exhibit transitory anomalies; it is very common to encounter in the heads of children of the lower social classes, who are consequently subject to denutrition, malformations which represent various degrees and forms of plagiocephaly, and which subsequently disappear completely, as the development of the cranium advances. Anomalies of form must therefore be judged differently in the case of the child than in that of the adult.
It may even happen that the five primitive nodules persist for a long time and even remain as a definitive form of the adult cranium constituting, according to Sergi, a distinct variety, the pentagonal cranium. But this is quite rare. From the frequency with which this form is to be observed in schools attended by children of the poorer classes, it is better to regard it as due to a delay in morphological evolution, which will probably disappear later on.
Normal Forms of the Cranium
We are indebted to Sergi for an exact knowledge of the normal forms of the cranium. Such forms are racial characteristics and are invariable, as Sergi has succeeded in proving by a comparison of the most ancient forms of the cranium with recent forms. Accordingly this authority takes the cranial formation as the basis for his classification of races. We have no direct interest, so far as concerns the special scope of our own science, in the value of this theory of classification—a theory, by the way, already divined, although very imperfectly and under a different form, by French and German anthropologists. Sergi's studies of cranial forms interest us solely as a diagnostic test of normality as compared with abnormality. For it is due to these researches that certain forms that used to be considered pathological, have come to be recognised as normal.