the laboring classes, if Erismann's figures are typical. Per- 142 THE PSYCHOLOGY OF ADOLESCENCE haps grip precedes biceps. This would indicate greater power to cling and hang than to move about, as Robinson found in very young infants, where this power may be surprisingly developed. Grip is perhaps the most generalized form of hand power, and the phyletic correlate of its pubescent incre- ment is probably without suggestion of arboreal life, but reminiscent of the later development of hand power in the race. The words grasp, apprehend, comprehend, etc., in their etymologies suggest the close relation between mental and manual ability, and remind us of Huxley's statement that man excels the higher anthropoids no more in varied powers of mind than he does in those of the hand.
The greatest number of taps that can be made in a given brief time interval is an important determination for the de- velopment of accessory muscular control. The time between two successive taps in such a series is from four to six times greater than between successive vibrations that the ear can distinguish before sound-waves fuse into a note or than the successive impulses that cause a tetanic muscular con- traction, and comes nearer that of a simple reaction time. This measurement is very important, and marks one of the factors of motor ability. Not only for children, but even for adults further tests are greatly needed. How rapidly two like simple volitional contractions can follow each other is perhaps the best index we have of will time, and is not much behind the rate of most rapid clear articulation of successive syllables. The maximal adult rate of wagging the forefin- ger does not vary much from that of trotting the leg, whis- pering the sounds t and k, vibrating the head or lower jaw, but none of them have been tested for children.
The best determination of maximal tapping rate at differ- ent ages yet made is that of Bryan.^ This he sought to as- certain for each of the six joints from finger-tip to shoulder, clamping the other joints in each test. He found that rate in- creased almost steadily from the age of six to sixteen in boys, but that for girls there was a falling oflf at fourteen or fifteen, with indication of a subsequent rise. The shoulder was the * On the Development of Voluntary Motor Ability. American Journal of Psy- chology, Nov., 1892.
GROWTH OF MOTOR POWER AND FUNCTION ^43 slowest joint for all ages after seven. At six elbow and wrist were both decidedly ahead of both shoulder and finger, the chief tests of which latter were made with the knuckle-joint. At sixteen the finger had overtaken the fastest joint, while the shoulder had not, suggesting advance of this power down the arms with advancing age. The central or fundamental movements of the shoulder matured earlier than the more accessory wrist. The lowest mean rate was 17.5 taps in five seconds in shoulders of girls at the age of six, and the fastest was at the age of sixteen in the right wrist of boys, viz., 35.9. The rate of the shoulder increases most slowly, of the elbow slightly faster, and that of the wrist and fingers yet more rap- idly. At six the finger-joint in boys is slowest, but at sixteen it has surpassed the elbow. Wrist and finger movements do not grow much ahead of those of the shoulder till the eleventh year, when they develop at a rate relatively much faster.
Puberty thus seems to mark the chief stage of nascency for increased rate of this finger movement, a point of great and obvious importance for many kinds of hand training. There is some indication that this development is not con- tinuous, but that years of rapid growth are interspersed with years of slow growth. This is also suggested in Gilbert's ^ re- sults. He found, in testing the greatest number of taps that could be made in five seconds, that from the ages of six to nine girls excelled, but for the next ten years boys surpassed them, most so from fourteen to fifteen. During the w^hole period of thirteen years, boys increased from 22 to 36.7 taps. The mean variation was greatest for girls at ten and for boys at thir- teen, and there was great difference in fatigue. In his New Haven ^ studies, he found that there was a steady increase of both sexes to twelve; then a year of decline for boys and two for girls; after which there was a steady increase of the greatest number of taps per second to sixteen for girls and seventeen for boys.
In Chicago from thirty to fifty children of each sex and of each age from eight to eighteen tapped as rapidly as pos- sible for thirty seconds, and it was found that for every age ^ Studies in Psychology. Univ. of Iowa, 1897.
* Researches on the Mental and Physical Development of School Children. Studies from the Yale Psy. Lab., vol. ii.
144 THE PSYCHOLOGY OF ADOLESCENCE except eleven, girls were slightly inferior in rate to boys. Boys' greatest increment was in the thirteenth year, while the tenth, thirteenth, and fifteenth were also years of rapid gain. The continuity of development seemed to be slightly more constant with girls.
That there is a more rapid increase in this power at or near adolescent years seems clear, but it is also undoubted that this power increases rapidly before puberty, and per- haps it would be found to increase fastest earlier yet. Much depends upon the length of time the tapping is continued, and this effort is intense and always very rapidly fatiguing, especially for the young, in whom it would probably be found that tapping for two, five, ten, and twenty seconds would show a rate greatly reduced as the periods lengthen. From available data it would seem that this pubertal increment is much less than for grasping, but it is easy to conceive that the latter may have had greatly increased value at this age in phyletic development, but this is not so apparent for tap- ping. To infer from this movement ancient modes of pad- dling, scratching, digging, or masturbation, as one writer suggests, etc., may seem to some a hazardous conjecture, but perhaps hardly more so, when the factor of thumb opposi- tion is considered, than to see in grasping a survival and development of a power so highly unfolded in arboreal life. It must not be forgotten, however, that complete as they are both are elemental movements when compared with all the manifold activities into which they enter in all the historic stages of human life.
Accuracy of movement is a very different function and marks the volitional control of the will, and the inhibition of irrelevant spontaneous, reflex, or automatic movements. It involves conscious effort and attention. The same motions performed with precision by the lower reflex centers are often at first performed very imperfectly by the higher conscious and self-directed powers. Exactness is one of the chief products of skill and practise, and is probably more indicative of men- tal development than the preceding kinds of muscular action. Motor education here is most liable to cause the evils of pre- cocity, and fatigue has special dangers.
GROWTH OF MOTOR POWER AND FUNCTION HS Many methods have been devised for testing muscular control and precision of movement in adults, but we have as yet few studies which follow the changes in these respects up the school grades. The ataxiagraph, in which a pen attached to the head writes on a frictionless horizontal plate above the involuntary swayings of the body standing, showed, according to Hancock,^ a marked increase of steadiness from the age of five to seven, but even at the latter age, despite the shortness of the body of children, it was greater both later- ally and vertically than Bullard and Brackett,^ or Hinsdale ^ found with adults. The automatograph, which consists of two glass plates with marbles between them so that the upper plate moves without friction and writes unconscious move- ments when the hand is placed on it, showed a marked gain between five and seven in ability to hold the hand still, but at the latter age these movements were several fold greater than with adults. The tremograph, a thimble attached to a pivoted lever moving freely in all directions, showed that children could not hold the index-linger still for half a min- ute, and that in this kind of control, although there was much gain from five to seven, they were at the latter age still further behind adults. Hancock's conclusions were that by young children " fine and completed movements are made with difficulty. Efforts to keep quiet produced strong symptoms of nervous irritation. Movement is inhibited only in part, and awkward swayings and twitchings result. Children in normal healthy growth show a lack of coordination and con- trol paralleled only by ataxic, choreic, and paralytic patients." Girls were steadier than boys before puberty, the left hand less than the right, and the ring-finger less than the others.
In testing the influence of voluntary inhibition by an ap- paratus in which a hammer threatened to strike the eye, but was arrested just in front of it by a glass, Partridge * counted in 583 boys and 557 girls from the ages of five to fifteen the number of winks necessary before such control was gained as * A Study of Motor Ability. Ped. Sem., vol. iii, 1894. ' Boston Med. and Surg. Journal, vols, i and ii of 1888. ' The Stature of Man. Oregon Med. Journal, vol. xciii.
* Experiments upon the Control of the Reflex Wink. Am. Jour, of Psy., Jan., 11 14^ THE PSYCHOLOGY OF ADOLESCENCE entirely inhibited the reflex wink. He found that this was reach much sooner by boys than girls, but his tables showed in general a marked diminution with the boys from eleven or twelve onward in the number of trials before complete control, indicating the increased dominance of volition over reflex im- pulses at this age.
Precision of movement may be measured as to force, as in holding a weight or pressing a dynamometer steadily so as to maintain a constant muscular tension, or as to direc- tion, as in making regular movements, as in drawing a straight line, or writing in ink of one color over letters in another, etc. The best tests with regard to age have so far been made only in the latter class. This is unfortunate when we consider the great importance of accuracy, the many devices which have been applied to adults, and the ease with which these and other methods might be used for the study of the young.
Kinesthetic sensation, and delicacy of touch and perhaps of eye, are closely connected with precision of movement. Gilbert,^ on averaging results of one hundred children for each age from seven to seventeen, found that lifted weights were dis- tinguished more finely and with a nearly steady increase of accuracy in judgment with increasing age, and that at thir- teen for girls and fourteen for boys this discrimination reached its highest point; that it fell off at fifteen and sixteen; and was slowly increasing at seventeen, but had not regained for either sex its maximal sensitiveness.
Bryan ^ applied two tests which gave significant results. Children placed a metallic stylus in the open arms of a plat- inum V-shaped figure forming an acute angle of 2. 2 J de- grees and sought to bring the point to the apex of the V without contact with the arms, failure to do so being elec- trically recorded. These writing movements were made up, down, and with both hands. In all cases, as the apex was approached and the task became harder, the absolute number of errors increased, but fewer errors in proportion to * Researches in the Mental and Physical Development of School Children. Studies from Yale Psy. Lab:, vol. ii.
' On the Development of Voluntary Motor Ability. American Journal of Psy chology, Nov., 1892.
GROWTH OF MOTOR POWER AND FUNCTION I47 the space were made. The most striking result was the great rate of gain between the ages of six and ei-ght and the rela- tively slight gain in early pubescence. Curiously, too, in these tests the average superiority of the right hand over the left was greatest at six and eight, and rather steadily de- clined up to sixteen.
Bryan's second test for accuracy was a modified clinical test for ataxia, viz., putting a sharp metallic point in a small hole several times without touching the edge. Here the gain between six and eight was less striking, the mean deviations were larger, and the decrease in their absolute size between six and sixteen was much greater than in the V movements.
From both tests it appeared that accuracy improved much faster during the first two or three years after the age of six than later. This was especially true of the right hand. Al- though the right hand is superior, the left improves between six and sixteen by a greater absolute amount. This is true for both sexes, and there was slight difference between the sexes. That nearly half the gain from six to sixteen often takes place in the first two years, and that puberty has an almost relatively retarding effect, suggests again that periods of increase in strength alternate with those of control and per- haps at certain stages have an almost inverse ratio. This is of the greatest significance for motor education, suggest- ing that for a few years the stress should incline to the larger sthenic or coarser strength forms of development, and that precision should have less relative emphasis. Motor activi- ties involving accuracy, which may be accentuated during years that precede puberty, should now yield somewhat to those involving fundamental rather than accessory development. The dangers involved in a violation of this rule will be de- scribed later.
There are many facts that seem to suggest that in ado- lescence the right hand precedes the left, and is not usually quite overtaken, so that this predominance is greater after puberty. If this be so, the relation of the two hands in man is somewhat analogous to the relation between the male and female body in muscular development.
Quetelet found that the left hand was always behind the 148 THE PSYCHOLOGY OF ADOLESCENCE right in strength of grip from one-sixth to one-eighth or less, but that it was most behind it at sixteen and at the two years preceding and following, and that at twenty and twenty-one the difference between them was again increased, this dift'er- ence being greater in boys than in girls. In girls he found the predominance of the right hand was more marked in the early teens than before or after.
In the first Chicago Report on Child-Study Investigation, Christopher found '' that marked differences in the strength of the two hands does not occur in boys till after fourteen, and in girls till after thirteen," and that twelve-year-old pupils in the seventh and eighth grades showed much more marked increment of strength in the right hand than those in the grades below. Thus unidexterity, although developed before school age, increases rapidly during early adolescent years, and as there appears to be some connection between unidex- terity and speech, which is innervated from the same hemi- sphere as is the preferred hand, Smedley raised the inter- esting question whether the pubescent increase in unidex- terity does not parallel the change of voice. He found that with the increment in both sexes, the superiority of the right hand over the left was more and more marked with increase of age and strength. " The curve representing the* strength of the boy's left hand runs nearly parallel to that representing his growth in weight, and in the earlier years of adolescence the curve representing the strength of the right hand gradu- ally approaches the curve of weight." The strength of grip in the right hand increased most rapidly in girls at thirteen and in boys at sixteen, after which the rate of increase rapidly declined. Of twelve-year-old pupils, it was found that those in the higher grades excelled those in the lower, and also that this was much more marked in the right hand than in the left.
The brightest Chicago pupils were more decidedly uni- dextrous than the average pupils; the latter more so than the dull, and they yet more so than pupils of the Reform School. This was found to be the case both on the basis of strength tests and on those of the greatest number of taps, the left hand more nearly approaching the right in both these respects the lower in the above order the pupils stood.
GROWTH OF MOTOR POWER AND FUNCTION H9 With accuracy alone the case is different. Bryan found that with his tests the right hand improves faster the first two or three years after the age of six than later. This, too, in- dicates that precision or control follows a different law than growth in strength or speed. Education of one side causes in- direct education of the other, as Scripture and others have shown. For thirteen days, e. g., Miss Brown at Yale squeezed a mercury dynamometer bulb, and thereby increased the strength of her right hand from 28 to 48 points, while the left hand, without exercise, showed a gain of 12 points. Miss Smith daily inserted a needle-point into a small hole, so that error was electrically recorded, 200 times a day for ten days, and reduced her error from 39 to 12 per cent. Meanwhile the untrained left hand fell from 50 to 24 per cent.
Here, as in the vast body of clinical, anatomical, physiolog- ical, and popular literature on the subject, the problem of bi- lateral asymmetry is suggestive yet tantalizing. Attention has more influence on the favored side. Up to a certain point excess of development of one side tends to bring up the other, and beyond this variable and indeterminable point, to increase at the expense of the weaker side. This disparity has great practical advantages in skill, tools, etc., and can not be called a deformity because nature gives nowhere any inti- mation that perfect parity of form or function was intended. This form of specialization must have some biological ad- vantages analogous to those of nascent periods for different organs and parts, as if the difficulties in growth were econom- ically overcome when the energies were focused, and then when a point of higher development was reached in one direc- tion, this was a stimulus to proportionate growth in related parts. The vicarious performance of the functions of one half of the body by those of the other suggests analogies with cases where fundamental muscles do the work of those that are accessory. Probably in legs, and perhaps in other parts and functions, adolescence involves a marked increase of disparity in the dual halves of the body.^ Of the power of adolescents to resist fatigue we still know very little that can be called scientific. Periods of rapid * See Bilateral Asymmetry of Function. Hall and Hartwell. Mind, xxxiii.
150 THE PSYCHOLOGY OF ADOLESCENCE growth are commonly thought to be marked by increased fatiguability, but fatigue has very different forms. Some individuals have great power to suddenly summon their resources for a very concentrative and intense effort, which leaves them soon exhausted, while others, lacking this sum- mative power, resist fatigue by endurance under long-sus- tained effort. The power to summate and power to plod rarely go together, but seem to be usually distinct diatheses. There is much to suggest that early adolescence develops in the direc- tion of spurty rather than in that of sustained effort, or that the latter comes later. The known changes in circulation, the con- jectured modification of the nervous centers, phyletic anal- ogy with the longer than diurnal rhythms of work and rest elsewhere discussed, and common observation as well as the general concept of plasticity to be shaped by culminative stresses that break out new ways across old ones — all these suggest this temporal primacy of erethic over plodding incre- ment.
The ergogram pictures a very specific form of fatigue and shows a very wide range of individual differences. In Chicago the ergograph was loaded with seven per cent the weight of the individual and lifted with the right middle finger forty-five times in ninety seconds. From six to twelve years of age both boys and girls increase quite steadily, girls' absolute endurance and also their rate of increase being less than boys'. From twelve and a half to eighteen and a half boys increase much more rapidly than before, gaining fastest from fourteen to seventeen. In Chicago girls, the ergograph increment began at ten and was greatest from tw^elve to fourteen, with subse- quent marked decline, the gain from fourteen to eighteen being hardly equal to half that of from twelve to fourteen. In endur- ance, as measured by ergograms as well as in the strength of grip, '' boys surpass girls at all ages, and this differentiation becomes very marked after the age of fourteen, after which girls increase in strength and endurance but very slightly, while after fourteen boys acquire almost exactly half of their total power in these two directions," these curves being almost identical with those of vital capacity.^ Muscular differentia- tion of the sexes practically begins at thirteen.
' Child Study Report, No. 2. Chicago, p. 72.
GROWTH OF MOTOR POWER AND FUNCTION 151 Fatigue was tested by tapping forty-five seconds and comparing the number of taps for the last five seconds with those for the first five. There was a loss of power to resist fatigue at eight years, then a steady increase to twelve for girls and thirteen for boys, with a year of subsequent loss followed by another of gain, both reaching their maximum at fifteen and showing some diminution thereafter. It was found also that *' muscle sense, force of suggestion, voluntary motor ability, fatigue, lung capacity, reaction, reaction with dis- crimination and choice, time memory — all give clear indica- tions of a marked change in the development of this period [puberty]." 1 One of Christopher's most interesting charts shows that the extremes of strength between the weakest and strongest pupils are almost twice as great in the two upper grades as in those below. This greatly increased range of individual variation in early puberty was no less marked in vital capacity and in resistance of fatigue as tested by the ergograph. In vital capacity, still more in strength, and most of all in en- durance as tested by the ergograph, the difference between the extreme individuals increases very rapidly up the grades, sometimes becoming from three or four to six or eight times in the seventh and eighth grades what it was in the first, and being in these respects much greater than the similar but marked differentiation between extremes in weight and height.
Partridge,^ in his study of second breath, had one hundred and sixty-eight cases of overplay, abandon to mental or physical effort, the most striking of which were between the ages of fifteen and twenty, although his data demonstrated nothing concerning age except that the phenomenon is very common at this stage. The descriptions of it in his returns by girls, and especially by boys, in whom it seems more common, is far more graphic and detailed at this age than before or after. When one runs, skates, dances, works, the breath is short and the next effort seems almost impossible. If one still persists, a point is soon reached when all fatigue has passed away and there is as great freshness as at first. The same is true of study late at night beyond the ^ Studies from the Yale Psy. Lab., vol. ii, p. 40.
* Second Breath. Ped. Sem., April, 1897, vol. iv, pp. 372-381.
15^ THE PSYCHOLOGY OF ADOLESCENCE usual hour of sleep. At a certain point sleepiness ceases and one can easily work with perhaps increased clearness and facility far into the night, although later there may be marked signs of reactionary exhaustion. The same is found in the so-called warming-up curve in lifting weights with the finger. The height of the lift declines with every suc- cessive pull by a rather regular curve until the weight does not leave its support at all, but if the effort is continued at each rhythmic interval it is soon lifted, it may be, as high as before, with perhaps several successive periods of recovery and loss. Erethic states in general, very likely physiologic- ally akin to those of inspirationists and ecstatics, are probably connected with the erethic or erectile function of circulation, and quite likely this is an important one in the education of adolescents, which is, however, not yet adequately known.
One of the most fruitful of the methods of experimental psychology has been that of time reactions. The discovery of Helmholtz fifty years ago that the time of a stimulus-wave along a nerve-fiber not only marked an epoch in demonstra- ting that psychic processes were not timeless nor even with electrical velocities, but relatively very slow, opened one of the most voluminous chapters in psycho-physics. By means of this method-instrument we not only determine the com- pleteness of reflex organization, but measure with precision the time required to discriminate forms and colors, to make associations, to recognize, classify, make a choice, etc. In general this time is inversely as complexity and practise, and these studies reveal the very composition of the habit- plexus, the ruts most worn by repetition in thought. That so far most of these studies have been confined to adults is unfortunate, for the reconstructions and mobilizations of this age, with its new interests, suggest that the chief triumphs of this method are to be achieved in applying it to mapping out the psycho-physic organization of the stages of childhood and youth, and that it has a great examination function in the future. All of value yet done here is briefly told.