The length of the cord at birth equals 14 em., and only at the be ginning of the fifth year is an increase of about 10 em. appreciable; it very slowly reaches its definite length of 43-45 ent. Whereas at birth, it corresponds to about 29.5 per cent. of the total body length, it steadily looses ground after the end of the first year, ultimately rep resenting only- 25 to 26 per cent. of the same. The configuration of the spinal cord also undergoes considerable morfification, especially at the level of the enlargements, where, in the newborn, and not infrequently for some time afterward, the inequality of the two diameters on cross section is scarcely appreciable.
Let us now briefly outline the histologic peculiarities of the child's nervous system. Developmental processes characterized by an in crease in size and number of the various constituent elements take place throughout the central organs, and gradually', the complex struc tures formed by the agglomeration of membranes, blood vessels, nerve fibres, etc., appear. The luxuriant growth of the cellular elements (cell and nuclear division) especially at the very beginning of life, the streaming out of axis cylinders and blood vessels, the splitting off of glia fibres, realize, all taken together, a histologic aspect, which has been designated not wholly- without l'eaS011, as the nearest approach to true inflammatory reaction. As regards nervous tissue proper, it is to be noted that in the newborn, the ganglionic cells do not present everywhere the specific characters of the adult type of cell; this being especially true of those of the cerebral hemispheres. In part, they remain dense of texture and present, in both nucleus and cell-body, structural peculiarities (Arborio) or an embryonal character; the characteristic configuration more particularly of the pyramidal cells is still wanting. Nuclear division is to be seen in cells undergoing devel opmental changes; being especially common at the very outset and becoming ever rarer in subsequent phases of development. Pigment is totally wanting at birth, and only at a later period does it very grad ually appear. The dark brown pigment of the locus creruleus appears at about the end of the first year; that of the nucleus of the vagus and substantia nigra, in progressively increasing quantity after the third or fourth year; the light yellow pigment of the posterior spinal ganglia is rarely found before the sixth, while that of the spinal cord only appears after the seventh or eighth year. In childhood the cells of the cerebral hemispheres are devoid of pigment.
It is in connection with the great mass of nerve fibres of the cen tral organs, however, that the most marked changes are observed, Sprouting of nerve fibres (differentiation of new fibrils) takes place throughout the central nervous system from the very beginning of life. The extra-uterine increase in volume of its various segments is due, above all, to the fact that great numbers of nerve fibres become in vested with myelin sheaths only after birth; that being especially true of those of the cerebral hentispheres. Thus, while the spinal cord pos sesses, at birth, practically the full measure of its myelin constituent, on the contrary, the brain stern and cerebellum, with the greater part of their substance, and extensive areas of the cerebrum are totally devoid of it; a fact which explains the grayish hue observed over large portions of a freshly cut surface of this organ as was previously stated.
In children born at the eighth or ninth month, practically the only tracts of fibres which are provided with myelin sheaths, are those sub servient to general sensibility, tactile and musele-sense; at a later period, the pyramidal tracts may also bc partly invested. In chil dren born at full term, in addition to the above-named tracts, the fibres belonging to the olfactory and visual systems, as well as to certain segments of the corona radiata, also possess their myelin sheaths in greater or lesser numbers; whereas considerable portions of the temporal, occipital and frontal lobes, as well as of the commissural system are still practically wholly unprovided. The researches of Flechsig, Siemerling, v. Alonakow, Probst and others, have shown that the various tracts of fibres are not affected simultaneously by the pro cess of myclinization, the individual fasciculi receiving tbeir sheaths at different periods. We know however, that the constituent fibres of tracts belonging to the same system, or to systems associated in physio logic activity, do become invested with myelin at the same time. Only after the ninth month of extra-uterine life is the projection system of fibres fully- provided with myelin. At birth, the cerebral cortex con tains but very few tangential fibres widely scattered over its entire surface; it is only in the course of many years that these intracortical tangential fibres, as well as the numerous and larger subcortical asso ciation tracts, acquire their full development. Whether these various fasciculi, once formed, retain their original structural proportions, or whether they are influenced by the varying degree of functional ac tivity, so that new axis cylinders and myelin sheaths develop in num bers adequate to the exigencies of acquired physiology-, as Edinger has suggested, the problem is hardly solvable, owing to the appreciable variations encountered in the histologic configuration of these same tracts.
The very considerable alterations which occur in the central ner vous sy-stcm in the course of development, affect not only- its ganglionic cells and their axis cylinder processes, but its supporting framework or neuroglia tissue as well; being especially active in the very first phase of extra-uterine life and most marked on the surface of the brain and in the ependymal structures. Increase in number and differentia tion of cellular elements, structural reorganization of fibrillar networks, take place, affecting especially the central gray matter which surrounds the SyHan aqueduct and the central canal of the spinal cord. Both these structures are relatively' wide and gaping in infancy and early childhood; a condition which is particularly favorable to the free cir culation of bacteria and toxins over considerable portions of the im portant columns of gray matter which constitute their limiting walls.