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