nearly ceased growing, and soon after, or, as some think, a few years before puberty, has reached its greatest weight. The skull continues to grow in length and breadth, at least till twenty, an increase that is divided between scalp, bone, and space inside, so that the size of the head is no sure index of the size of the brain. From birth to maturity the weight of the brain increases about three-and-a-half-fold and that of the spinal cord between sevenfold and eightfold. The cord be- comes relatively to the brain twice as heavy between early in- fancy and manhood.
* Am. Text-Book of Physiol., p. 726 et seq. See Spitzka's Brain Weight of q6 Famous Men. Phil. Med. J., May 2, 1903.
io6 THE PSYCHOLOGY OF ADOLESCENCE The following table shows its absolute and relative growth in both sexes up to seventy, according to the data of Vie- rordt, correlated by Miihlmann with the results of other in- vestigators and his own: ^ Age.
Brain Weight.
Absolute.
Relative per cent 2. II In general terms, therefore, we may say the brain in most persons nearly or quite ceases to gain in weight at or before the dawn of adolescence, its declining increment almost sug- gesting insufficiency, and to one writer degeneracy. It has long been supposed that some important modulation in the method or direction of growth occurs at this important epoch, evolution of mass passing over to involution of texture. Changes in structural complexity may continue indefinitely; or, again, certain parts may diminish in mass and others increase in equal ratio; or, yet again, changes in chemical constitution may be characteristic of increasing age.
The processes of cell division in the brain seem to end > Op. cit.
PARTS AND ORGANS DURING ADOLESCENCE 107 three or four months before birth. During the prepubertal period, according to the careful studies of Kaiser, the number of cells that pass from the small undeveloped stage to full maturity is nearly equal to that functioning at birth, so that the number of active cells nearly doubles by the end of the fourteenth year. In the boy of fifteen, the volume of cell bodies is already, on the average, one hundred and twenty- four times their size at birth, and by thirty will have increased to one-hundred-and-fifty-fold. New cells are perhaps not formed at all, but developed out of granules, many of which never do develop, but suggest possibilities in the neural ele- ments yet unrealized. Donaldson ^ concludes that the amount of increase in the mass of individual nerve-cells " ranges be- tween zero and fifty-thousand-fold," and assumes in the three- fourths of the total volume of the central nervous system, which he thinks is nerve tissue proper, that there are 3,000,- 000,000 cells, most of which are probably more or less devel- oped. Thus Kaiser's increase in, e. g., the cervical enlarge- ment of from 104,270 at birth to 211,800 at fifteen is really an increase in the' number of developed cells, no intrinsically new ones being formed. Cells constitute but a very small part of the total brain, so that they can increase many fold and not greatly afifect its total size or weight.
Cajal ^ has suggested that the energy of the division of cells in the embryonic period may be later transformed into the work of developing collaterals and finer processes, upon which he assumes not only the number of associations but the size of the cells depend.
Of prime importance for adolescence are these recerA lines of study, of which the first is upon the tangential fibers which connect different parts of the cortex, with which the names of Vulpius,^ Kaes,* and Flechsig are associated. After the projection system has penetrated the cortex, several systems of fibers among the cortical cells, themselves at right angles to the afferent and efferent fibers, slowly develop, some of them continuing to do so well on into the third or fourth • Growth of the Brain. London, 1895.
io8 • THE PSYCHOLOGY OF ADOLESCENCE decade of life at least. These, which Kaes calls the second and third association layers of Meynert, and the lamina of Baillaryer, all of which Vulpius calls the middle layer, be- gin an augmented development in the occipital and in parts of the parietal regions at about seventeen or eighteen years, and continue to increase in definiteness for at least two decades. This is best seen in the convexities of the convolutions, and less in their median and lower surfaces, except for radial fibers. They may represent Hughlings Jackson's highest level. These superradial or tangential fibers seem to grow steadily in caliber and perhaps in number from the age of eighteen on, the short fibers maturing earlier than the long ones. Kaes thinks the brain increases enormously in complexity after six- teen or seventeen, the brain of his man of thirty-eight being nearly twice as rich in fibers here as that of his boy of eight- een, and he holds that medullations and increase of brain power may continue at least till fifty. Kaes and Vulpius are in impressive agreement that the age of the later teens is epochful for the development of the middle layer, which then begins a new and prolonged period of growth. This causes increased thickness of the parietal and central regions, some in the temporal, and is probably followed by growth in the frontal regions last, all this occurring in regions that were far less rich in fibers in early adolescence. The development of these fibers seems to follow more closely than any other brain structure we know the growth of intelligence. They decline in numbers with old age; their growth is checked by poor nutrition, and they reach their maximum in the centers for sight and hearing by the end of the second year, while in the speech area they increase rapidly till the eleventh and slightly till the thirty-third year. Of the three layers of these fibers, the middle, although beginning in the ninth month and having the least fibers, makes no marked increase till the beginning of adolescence, and grows fastest during the years of early adult manhood and womanhood, continuing till thir- ty-eight at least. Kaes thinks there is an arrest in their rate of growth at about forty. The outer layer of tangential fibers is normally far smaller than the inner, save in idiots, where it is very large, as it also is in reptiles before the pro- jection system is much developed, and of which it may have PARTS AND ORGANS DURING ADOLESCENCE lOQ been a functional precursor. Kaes holds that these idiots revert to an ancient type, fibers closest connected with smell (which is the oldest cortical structure) being best developed. Perhaps the rule is that the finer the structure the later on in life it continues to develop.^ The second recent line of brain study bearing upon ado- lescence has been opened by Flechsig, which may be con- cisely summarized as follows: A considerable part of early growth of the brain before and after birth is due to the increase of medullation, myelinization, or the sheathing of the fibers, which is generally held to be the surest concomitant of the development of their function. The various bundles of fibers acquire this sheath not all together, but strand by strand. First come the fibers connecting adjacent centers in the spinal cord, which mediate reflexes of the same level or segment, which medullate the last part of the fifth fetal month. Next come the fibers connecting different vertical levels of the cord; then the sensory column of Gall and Bur- dach; then the pyramidal or volitional bundles. Nearly all the cord, save this column of fibers, is meduUated by the time of birth, but the brain is very undeveloped; so that the new-born child is a spinal creature. Passing up, he finds three sheafs of sensory fibers connecting with the brain which mature at different times, and carrying tactile and kinesthetic sensations to the central convolutions, the latest of which connects with the speech centers. Of the senses judged thus by medullation, smell comes first and hearing develops last. Not only this method of reasoning, but the results of these researches are now more or less widely accepted.
The novel and even revolutionary part of the above pub- lication of Flechsig, however, is his claim that, whereas it has generally been held that all parts of the cortex were con- nected with the lower centers of sense, two-thirds of it has no such connection. Only one-third of the gray cerebral mantle " has any direct connection with the processes which bring * See F. Burk. From Fundamental to Accessory in the Development of the Nervous System and of Movements. Pedagogical Seminary, October, 1898, p. 8 et seq.
no THE PSYCHOLOGY OF ADOLESCENCE sense-impression to consciousness and excite the muscles and mechanism of movement." The rest of it, more or less de- tached from sense and motion, has the functions of reason, science, judgment, moral and esthetic feeling, etc. This two- thirds is reduced to one-half in the higher apes and in idiots, and vanishes in the rodents. It is almost inevitable to ask whether this higher two-thirds does not come into predomi- nant function during adolescence, or, at least, if there is then an increased prepotence of it.
Again, Hughlings Jackson, as is well known, has long held that the central nervous system was made up of three superposed levels closely correlated, but maintaining so much independence that each might be more or less separately involved in an epileptic attack. His lowest level consists of the gray cells of the cord, medulla, and pons. If this only is involved in disease, the limb may be convulsed, or there may be respiratory spasms, etc., while the higher powers are not much affected. The next higher or mid-level receives sense- impressions indirectly from and its motor discharges are indi- rect through the lowest level, and comprises the basal gan- glia, the centers for special sense, and the central convolu- tions of the cortex. The epilepsies of this level are more complex and widely irradiated, and instead of involving syn- chronous convulsions of remote parts, may progress regu- larly from one to another as they are structurally connected, and loss of consciousness, if it comes, is later, but more un- certain and incidental. So accurate was this view, and so exact can diagnosis by its aid now be, that many an opera- tion in the lateral convolution is correctly indicated and suc- cessfully made. Jackson's highest level is connected with the second much as that is with the first, and may be said to approximately coincide with Flechsig's higher two-thirds of the brain front and back, which has no direct pyramidal con- nections. When epilepsy occurs here consciousness is lost at first in such an attack. In this scheme the simplest structures and functions are the oldest. The lowest stratum represents reflex movements of limbs, of eyes, tongue, digestion, etc., and the highest uses these same organs and motions for voli- tion, gesture, facial expression, words, and emotion. The lowest is presentation, the middle layer is representation, to PARTS AND ORGANS DURING ADOLESCENCE m use the terms familiar in Scotch philosophy, and the top level is for re-representation.
Each of these structures has its nascent period when the energies of growth tend to focus not exclusively but only with more or less predominance upon it, and I think we may say that, at last, we seem to have within our reach some con- ception of what brain changes are involved in adolescence. It appears to be the period of acceleration for the mid-stratum of tangential fibers of Vulpius and Kaes, for Flechsig's two- thirds of the brain that lacks direct connection with the pro- jection system, and for Jackson's highest level. Certain types of insanity are rare before puberty, because the child can not reason according to adult standards until fourteen, the age at which Aristotle would begin the education of reason. Be- fore this comes the age of the spinal reflex and automatic nascency of the late prenatal life and the early months of infancy. Then comes the stage of controlled muscular actions — walking, plays — when drill, habituation, memory, and instinct culminate, which is associated with the mid-level regions of the brain. Lastly comes the age of rational thought, higher logical correlation, personal opinion and conviction, higher esthetic enjoyments, deliberate choice, and willed action. One common and now most interesting form of arrest here is to be discussed at length in Chapter IV. Each lower level, however, must have its full develop- ment, for it is a necessary condition for the unfoldment of the higher. Logical methods, on the other hand, if too early, tend to stultify and violate the law that fundamental must always precede accessory structures and functions.
Again, on this view, thought is more independent of mus- cular activity or motor innervation than we have considered it to be, and the sphere of gymnastic and motor education is per- haps more restricted. The field of sense-training and object- lesson methods are also limited. We can train the sense centers and cultivate eye and ear mindedness in the regions contigu- ous to the Rolandic fissure by manual or athletic culture, but all this only supplies the data and lays the foundation for the education of the highest level. The dawn of reason, marked by the appetency for crude logical processes, the shadowy grasping of new and great conceptions, and the THE PSYCHOLOGY OF ADOLESCENCE silent reverie and dreams of dawning adolescence which we shall trace later when the senses and muscles are inert, may be the first psychic function of new neural parts. At first, their activity is tentative, intermittent, incidental, dim, and unformulated; but they are designed later to dominate more and more of Hfe, and to subordinate, and in abnormal devel- opments even to precede and arrest the lower levels.^ F. Growth of Skin and Internal Tract. — Of several care- ful measurements of the dermal surface, that of Meeh,^ who laid on strips of paper to follow the folds and changes of level, is perhaps the best. Even this method was by no means exact. His table from the ages of nine to twenty-six is as follows: Age.
Height in cm.
Weight in g.
Surface in cm.
No. of cm. surface to I kg. weight.
Yrs. Mos.
* Copious and bewildering as is the recent literature upon the embryology of the brain, its structure and growth in lower animals, localization, and especially the anatomy of the adult brain, one examines not only most of the recent comprehen- sive text-books, Ramon y Cajal, van Gehtichten, Kolliker, Barker, Retzius, Edinger, etc., but the new neurological Jahresbericht and the files of the Centralblatt, almost in vain as yet for studies upon brain growth during youth which would seem to be more promising of practical results than work in any other field, and which must be ere long the center of the highest scientific interest when evolu- tionary lines of research attain the importance that belongs to them. Only Vulpius, Kaes, Flechsig, and Jackson shed distinct light on it. See Kaes, Archiv f Psychia- tric u. Nervenkrankheiten, Bd. xxv, p. 695. Neurol. Centralblatt, Bd. x, p. 456, and Bd. x, p. 119; Bd. xiii, p. 410. Miinchen Wochenschrift, Bd. xliii, p. 100. Wiener med. Wochensch., 1895, pp. 1734-1770. Vulpius, Arch. f. Psych., 1891, p. 775. Edenger, Vorlesungen, 6th ed., 1900, p. 424. Flechsig. Gehirn u. Seele, 1896, p. 112. Etudes sur le Cerveau, 1898, p. 221. Neurolog. Centralblatt, 1894, p. 606; 1898, p. 977; 1899, p. 1000; and 1895, pp. 1118, 1177. Localization der Geistigen Vorgange, Leipzig, 1896. Arch. Ital. de Biol., 1901, p. 30. Hughlings Jackson. Brit. Med. J., 1898, vol. i, p. 65, Apathy's fibrillar theory that since 1897 invades the neuron doctrine; Dercum's ameboid make and break theory, 1896; Loeb's ion theory of neuron and muscle action, 1900, and many late studies in neuropathology, e. g., by Ceni, Babinski, Dejerine, and many others, have added a great and new charm to all this field.
' Zeits. f. Biologie, vol. xv.
PARTS AND ORGANS DURING ADOLESCENCE XI3 There is a rapid decline in the relation of surface to weight between birth and maturity estimated at from about eight hun- dred and twelve to three hundred and one centimeters square per kilogram. The epidermis is quite unlike the corium, in that it continues to grow until at least sixty. It is more exempt from atrophic changes than the cutis.
As the skin covers the body from without, the mucous surface lines it within. Owing to its very many folds and the contraction and expansion of the intestines, its area can be determined with even less accuracy than that of the skin.
Early determinations gave the conclusion that the growth of the intestines was relatively less than that of the mass of the body, but Mtihlmann's careful weighings indicate that the converse is true, save perhaps for a very brief period in infancy. The mucous surface, however, probably continues to grow through life. This is especially true of the epithelial layer.
Men Weight in grs.
Length.
Intestinal Weight.
Intestinal Length.
Intestinal as Age.
Abs. g.
Rel. per cent.
Abs. cm.
Rel. per cent.
related to bodily weight.
14 16 17 19 24 From six to twenty years the small intestine increases to about threefold and the large intestine to about fourfold its ca- pacity at birth. The length of the latter grows from about one- sixth that of the former in infancy to one-fourth in maturity. The total length of the digestive canal, which is about five times that of the body in the adult, increases relatively less than stature, for at birth it is about six times the length of the body. In the adult its surface is almost equal to that of the body, while in the carnivorous dog it is half, and in the herbivorous horse nearly twice its body surface. We have little exact data, but it would appear from those at hand that at adolescence the absorptive surfaces of the canal increase less than the body surface, and if so, this suggests increased animal food 9 THE PSYCHOLOGY OF ADOLESCENCE in the past, at the phyletic correlate of this age. At the age of eleven or twelve the waist of boys is increasing more than their height, and in the average boy continues to grow till at least twenty-three. The area of chewing surface is slowly increased by the addition of the later molars, and as we saw there is increase in both length and strength of jaw.
Growth of Glands and Other Organs. — We shall never understand many of the deepest problems involving the rela- tion of the mind to the body until we can write a new chap- ter of psycho-physiology on glandular psychology. Secre- tions and excretions, both internal and external, including the functions of what we may still call the sexual glands, although unlike others, they secrete only living cells, condi- tion many psychic states in a way hardly less basic than they do all other physiological processes, and for fundamental feel- ings and instincts they are probably quite as important as the brain itself.
Kidneys. — Both the volume and the weight of the kidneys increase till well into the third decennium of life, but their growth relatively to that of the body weight begins to fall behind in the second year of life. Vierordt has compiled from various measurements of men the following table, which is more comprehensive than Benecke's '} Age.
Abs.
Rel.
During most of the life of the individual the kidney cells simply replace their slight loss without growing, and after the third decennium usually begin to atrophy.
^ Constitution und constitutionelles Kranksein des Menschen. Marburg, 1885.
PARTS AND ORGANS DURING ADOLESCENCE "S Urea is known as the chief nitrogenous waste and prod- uct of the proteid element of food; it is chiefly prepared for ehmination in the Hver, the largest gland, and is provided with the richest blood supply. It is discharged thence into the blood and taken up again by the kidneys for final elimina- tion. One of the most interesting and important chapters in modern physiology and another in practical medicine are devoted to the study of its history, and a well-marked group of diseases are traced to its accumulation. Despite several careful studies, we are still uncertain concerning its relations to adolescence. Uhle found a decrease of urea from eight to eleven years of age, and still more from thirteen to sixteen, at which latter age the amount secreted reached about the normal adult standard, which was about half that before six. Marro found a marked increase in urea and also in urine at about twelve in girls, and again at fourteen, with a later decline. Uhle and Mosler have shown that there is a marked diminu- tion in phosphoric acid secreted in the urine at puberty, which they think combines with bases like lime to form bone, which grows so rapidly at this period. Marro ^ found a distinct augmentation of chlorin at fourteen and fifteen, and also of H2SO4 and of P2O5. As under the influence of the whip- stroke, which adolescence applies to growth, boys exceed girls in the amount of carbonic acid exhaled, so also they do in azotic urinal deposits. Physicians report occasional cases of albumen in the urine of adolescents to an extent that sug- gests Bright's disease, but these regulate themselves to nor- mality later; even sugar may be excessive. On the whole, while many more determinations for different ages are needed before kidney secretions can be correlated with growth, it is highly probable that these, like so many other functions, have a wider range of average and also of individual variation within the limits of normality than at the age just preceding the growth increment or at any subsequent period save that of senescent involution, which is in so many ways an inverted picture of adolescence.
The increase in the amount of urine at different ages is shown in the following table: ii6 THE PSYCHOLOGY OF ADOLESCENCE Age.
8 9 lO II 12 13 Amount in 24 hours.
To I k. weight.
Micttiritional obscenities, which our returns show to be so common before adolescence, culminate at ten or twelve, and seem to retreat into the background as sex phenomena appear. The former are of two classes, fouling persons or things secretly from adults but often openly with each other, and less often ceremonial acts connected with the act or the product that almost suggests the scatological rites of savages, unfit for description here, but of great interest and impor- tance for the psychic side of the recapitulation theory, and although rare, sometimes elaborately developed.^ No less significant are the phenomena of what we may call urino- phobia, sometimes developed at this age of fears, which remind us of the many centuries when urinomancy and urinos- copy vied with astrology as the chief factors in a physician's training. Many adolescents whose attention is called to the subiect by the scare advertisements perhaps naturally scan their urine daily for years, sufifer acute and generally secret fears as they find it now cloudy, turbid, with red or white deposits or with iridescent film, test its odors and imagine incipient, fatal, and offensive diseases; seek conso- lation by furtive comparisons with that of other persons; modify their diet, exercise, and general regimen; associate this with the yet more serious sex fears later described, and sometimes muster up courage to consult friends or throw them- selves into the hands of quacks. Of one hundred and nineteen students, all but five confess to a period of mild or intense fear which a little incidental instruction would have shown to be groundless.^