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Our third principle is the direct action of the excited nervous system on the body； independently of the will； and independently； in large part； of habit。 Experience shows that nerve…force is generated and set free whenever the cerebro…spinal system is excited。 The direction which this nerve…force follows is necessarily determined by the lines of connection between the nerve…cells； with each other and with various parts of the body。 But the direction is likewise much influenced by habit； inasmuch as nerve…force passes readily along accustomed channels。
The frantic and senseless actions of an enraged man may be attributed in part to the undirected flow of nerve…force； and in part to the effects of habit； for these actions often vaguely represent the act of striking。 They thus pass into gestures included under our first principle； as when an indignant man unconsciously throws himself into a fitting attitude for attacking his opponent； though without any intention of making an actual attack。 We see also the influence of habit in all the emotions and sensations which are called exciting； for they have assumed this character from having habitually led to energetic action； and action affects； in an indirect manner； the respiratory and circulatory system； and the latter reacts on the brain。 Whenever these emotions or sensations are even slightly felt by us； though they may not at the time lead to any exertion； our whole system is nevertheless disturbed through the force of habit and association。 Other emotions and sensations are called depressing； because they have not habitually led to energetic action； excepting just at first； as in the case of extreme pain； fear； and grief； and they have ultimately caused complete exhaustion； they are consequently expressed chiefly by negative signs and by prostration。 Again； there are other emotions； such as that of affection； which do not commonly lead to action of any kind； and consequently are not exhibited by any strongly marked outward signs。 Affection indeed； in as far as it is a pleasurable sensation； excites the ordinary signs of pleasure。
On the other hand； many of the effects due to the excitement of the nervous system seem to be quite independent of the flow of nerve…force along the channels which have been rendered habitual by former exertions of the will。 Such effects； which often reveal the state of mind of the person thus affected； cannot at present be explained； for instance； the change of colour in the hair from extreme terror or grief； the cold sweat and the trembling of the muscles from fear； the modified secretions of the intestinal canal；and the failure of certain glands to act。
Notwithstanding that much remains unintelligible in our present subject； so many expressive movements and actions can be explained to a certain extent through the above three principles； that we may hope hereafter to see all explained by these or by closely analogous principles。
Actions of all kinds； if regularly accompanying any state of the mind； are at once recognized as expressive。 These may consist of movements of any part of the body； as the wagging of a dog's tail； the shrugging of a man's shoulders； the erection of the hair； the exudation of perspiration； the state of the capillary circulation； laboured breathing； and the use of the vocal or other sound…producing instruments。 Even insects express anger； terror； jealousy； and love by their stridulation。 With man the respiratory organs are of especial importance in expression； not only in a direct； but in a still higher degree in an indirect manner。
Few points are more interesting in our present subject than the extraordinarily complex chain of events which lead to certain expressive movements。 Take； for instance； the oblique eyebrows of a man suffering from grief or anxiety。 When infants scream loudly from hunger or pain； the circulation is affected； and the eyes tend to become gorged with blood： consequently the muscles surrounding the eyes are strongly contracted as a protection： this action； in the course of many generations； has become firmly fixed and inherited： but when； with advancing years and culture； the habit of screaming is partially repressed； the muscles round the eyes still tend to contract； whenever even slight distress is felt： of these muscles； the pyramidals of the nose are less under the control of the will than are the others and their contraction can be checked only by that of the central fasciae of the frontal muscle： these latter fasciae draw up the inner ends of the eyebrows； and wrinkle the forehead in a peculiar manner； which we instantly recognize as the expression of grief or anxiety。 Slight movements； such as these just described； or the scarcely perceptible drawing down of the corners of the mouth； are the last remnants or rudiments of strongly marked and intelligible movements。 They are as full of significance to us in regard to expression； as are ordinary rudiments to the naturalist in the classification and genealogy of organic beings。
That the chief expressive actions； exhibited by man and by the lower animals； are now innate or inherited；that is； have not been learnt by the individual；is admitted by every one。 So little has learning or imitation to do with several of them that they are from the earliest days and throughout life quite beyond our control； for instance； the relaxation of the arteries of the skin in blushing； and the increased action of the heart in anger。 We may see children； only two or three years old； and even those born blind； blushing from shame； and the naked scalp of a very young infant reddens from passion。 Infants scream from pain directly after birth； and all their features then assume the same form as during subsequent years。 These facts alone suffice to show that many of our most important expressions have not been learnt； but it is remarkable that some； which are certainly innate； require practice in the individual； before they are performed in a full and perfect manner； for instance； weeping and laughing。 The inheritance of most of our expressive actions explains the fact that those born blind display them； as I hear from the Rev。 R。 H。 Blair； equally well with those gifted with eyesight。 We can thus also understand the fact that the young and the old of widely different races； both with man and animals； express the same state of mind by the same movements。
We are so familiar with the fact of young and old animals displaying their feelings in the same manner； that we hardly perceive how remarkable it is that a young puppy should wag its tail when pleased； depress its ears and uncover its canine teeth when pretending to be savage； just like an old dog； or that a kitten should arch its little back and erect its hair when frightened and angry； like an old cat。 When； however； we turn to less common gestures in ourselves； which we are accustomed to look at as artificial or conventional； such as shrugging the shoulders； as a sign of impotence； or the raising the arms with open hands and extended fingers； as a sign of wonder； we feel perhaps too much surprise at finding that they are innate。 That these and some other gestures are inherited； we may infer from their being performed by very young children； by those born blind； and by the most widely distinct races of man。 We should also bear in mind that new and highly peculiar tricks； in association with certain states of the mind； are known to have arisen in certain individuals； and to have been afterwards transmitted to their offspring； in some cases； for more than one generation。
Certain other gestures； which seem to us so natural that we might easily imagine that they were innate； apparently have been learnt like the words of a language。 This seems to be the case with the joining of the uplifted hands； and the turning up of the eyes； in prayer。 So it is with kissing as a mark of affection； but this is innate； in so far as it depends on the pleasure derived from contact with a beloved person。 The evidence with respect to the inheritance of nodding and shaking the head； as signs of affirmation and negation； is doubtful； for they are not universal； yet seem too general to have been independently acquired by all the individuals of so many races。
We will now consider how far the will and consciousness have come into play in the development of the various movements of expression。 As far as we can judge； only a few expressive movements； such as those just referred to； are learnt by each individual； that is； were consciously and voluntarily performed during the early years of life for some definite object； or in imitation of others； and then became habitual。 The far greater number of the movements of expression； and all the more important ones； are； as we have seen； innate or inherited； and such cannot be said to depend on the will of the individual。 Nevertheless； all those included under our first principle were at first voluntarily performed for a definite object；namely； to escape some danger； to relieve some distress； or to gratify some desire。 For instance； there can hardly be a doubt that the animals which fight with their teeth； have acquired the habit of drawing back their ears closely to their heads； when fe