any　emotion。　I　am　assured　by　an　eminent　authority　that　young　children　do　not　tremble；　but　go　into　convulsions　under　the　circumstances　which　would　induce　excessive　trembling　in　adults。　　Trembling　is　excited　in　different　individuals　in　very　different　degrees。　and　by　the　most　diversified　causes；by　cold　to　the　surface；　before　fever…fits；　although　the　temperature　of　the　body　is　then　above　the　normal　standard；　in　blood…poisoning；　delirium　tremens；　and　other　diseases；　by　general　failure　of　power　in　old　age；　by　exhaustion　after　excessive　fatigue；　locally　from　severe　injuries；　such　as　burns；　and；　in　an　especial　manner；　by　the　passage　of　a　catheter。　　Of　all　emotions；　fear　notoriously　is　the　most　apt　to　induce　trembling；　but　so　do　occasionally　great　anger　and　joy。　I　remember　once　seeing　a　boy　who　had　just　shot　his　first　snipe　on　the　wing；　and　his　hands
  ＇1＇　See　the　interesting　cases　collected　by　M。　G。　Pouchet　in　the　‘Revue　des　Deux　Mondes；'　January　1；　1872；　p。　　79。　　An　instance　was　also　brought　some　years　ago　before　the　British　Association　at　Belfast。　trembled　to　such　a　degree　from　delight；　that　he　could　not　for　some　time　reload　his　gun；　and　I　have　heard　of　an　exactly　similar　case　with　an　Australian　savage；　to　whom　a　gun　had　been　lent。　Fine　music；　from　the　vague　emotions　thus　excited；　causes　a　shiver　to　run　down　the　backs　of　some　persons。　There　seems　to　be　very　little　in　common　in　the　above　several　physical　causes　and　emotions　to　account　for　trembling；　and　Sir　J。　Paget；　to　whom　I　am　indebted　for　several　of　the　above　statements；　informs　me　that　the　subject　is　a　very　obscure　one。　As　trembling　is　sometimes　caused　by　rage；　long　before　exhaustion　can　have　set　in；　and　as　it　sometimes　accompanies　great　joy；　it　would　appear　that　any　strong　excitement　of　the　nervous　system　interrupts　the　steady　flow　of　nerve…force　to　the　muscles。＇2＇
  The　manner　in　which　the　secretions　of　the　alimentary　canal　and　of　certain　glandsas　the　liver；　kidneys；　or　mammae　are　affected　by　strong　emotions；　is　another　excellent　instance　of　the　direct　action　of　the　sensorium　on　these　organs；　independently　of　the　will　or　of　any　serviceable　associated　habit。　There　is　the　greatest　difference　in　different　persons　in　the　parts　which　are　thus　affected；　and　in　the　degree　of　their　affection。
  The　heart；　which　goes　on　uninterruptedly　beating　night　and　day　in　so　wonderful　a　manner；　is　extremely　sensitive　to　external　stimulants。　The　great　physiologist；　Claude　Bernard；＇3＇　has　shown　bow　the　least　excitement　of　a　sensitive　nerve　reacts　on　the　heart；　even　when　a　nerve　is　touched　so　slightly　that　no　pain　can　possibly　be　felt　by　the　animal　under　experiment。　　Hence　when　the　mind　is　strongly　excited；　we　might　expect　that　it　would　instantly　affect　in　a　direct　manner　the　heart；　and　this　is　universally　acknowledged　and　felt　to　be　the　case。　Claude　Bernard　also　repeatedly　insists；　and　this　deserves　especial　notice；　that　when　the　heart　is　affected　it　reacts　on　the　brain；　and　the　state　of　the　brain　again　reacts　through　the　pneumo…gastric　nerve　on　the　heart；　so　that　under　any　excitement　there　will　be　much　mutual　action　and　reaction　between　these；　the　two　most　important　organs　of　the　body。
  ＇2＇　Muller　remarks　（‘Elements　of　Physiology；'　Eng。　　translat。　　vol。　　ii。　　p。　934）　that　when　the　feelings　are　very　intense；　〃all　the　spinal　nerves　become　affected　to　the　extent　of　imperfect　paralysis；　or　the　excitement　of　trembling　of　the　whole　body。〃
  ＇3＇　‘Lecons　sur　les　Prop。　　des　Tissus　Vivants；'　1866；　pp。　　457…466。
  The　vaso…motor　system；　which　regulates　the　diameter　of　the　small　arteries；　is　directly　acted　on　by　the　sensorium；　as　we　see　when　a　man　blushes　from　shame；　but　in　this　latter　case　the　checked　transmission　of　nerve…force　to　the　vessels　of　the　face　can；　I　think；　be　partly　explained　in　a　curious　manner　through　habit。　We　shall　also　be　able　to　throw　some　light；　though　very　little；　on　the　involuntary　erection　of　the　hair　under　the　emotions　of　terror　and　rage。　　The　secretion　of　tears　depends；　no　doubt；　on　the　connection　of　certain　nerve…cells；　but　here　again　we　can　trace　some　few　of　the　steps　by　which　the　flow　of　nerve…force　through　the　requisite　channels　has　become　habitual　under　certain　emotions。
  A　brief　consideration　of　the　outward　signs　of　some　of　the　stronger　sensations　and　emotions　will　best　serve　to　show　us；　although　vaguely；　in　how　complex　a　manner　the　principle　under　consideration　of　the　direct　action　of　the　excited　nervous　system　of　the　body；　is　combined　with　the　principle　of　habitually　associated；　serviceable　movements。
  When　animals　suffer　from　an　agony　of　pain；　they　generally　writhe　about　with　frightful　contortions；　and　those　which　habitually　use　their　voices　utter　piercing　cries　or　groans。　Almost　every　muscle　of　the　body　is　brought　into　strong　action。　With　man　the　mouth　may　be　closely　compressed；　or　more　commonly　the　lips　are　retracted；　with　the　teeth　clenched　or　ground　together。　There　is　said　to　be　〃gnashing　of　teeth〃　in　hell；　and　I　have　plainly　heard　the　grinding　of　the　molar　teeth　of　a　cow　which　was　suffering　acutely　from　inflammation　of　the　bowels。　The　female　hippopotamus　in　the　Zoological　Gardens；　when　she　produced　her　young；　suffered　greatly；　she　incessantly　walked　about；　or　rolled　on　her　sides；　opening　and　closing　her　jaws；　and　clattering　her　teeth　together。＇4＇　With　man　the　eyes　stare　wildly　as　in　horrified　astonishment；　or　the　brows　are　heavily　contracted。　Perspiration　bathes　the　body；　and　drops　trickle　down　the　face。　The　circulation　and　respiration　are　much　affected。　Hence　the　nostrils　are　generally　dilated　and　often　quiver；　or　the　breath　may　be　held　until　the　blood　stagnates　in　the　purple　face。　If　the　agony　be　severe　and　prolonged；　these　signs　all　change；　utter　prostration　follows；　with　fainting　or　convulsions。
  A　sensitive　nerve　when　irritated　transmits　some　influence　to　the　nerve…cell；　whence　it　proceeds；　and　this　transmits　its　influence；　first　to　the　corresponding　nerve…cell　on　the　opposite　side　of　the　body；　and　then　upwards　and　downwards　along　the　cerebro…spinal　column　to　other　nerve…cells；　to　a　greater　or　less　extent；　according　to　the　strength　of　the　excitement；　so　that；　ultimately；　the　whole　nervous　system　maybe　affected。＇5＇　This　involuntary　transmission　of　nerve…force　may　or　may　not　be　accompanied　by　consciousness。　Why　the　irritation　of　a　nerve…cell　should　generate　or　liberate　nerve…force　is　not　known；　but　that　this　is　the　case　seems　to　be　the　conclusion　arrived　at　by　all　the　greatest　physiologists；　such　as　Muller；　Virchow；　Bernard；　&c。＇6＇　As　Mr。　Herbert　Spencer　remarks；　it　may　be　received　as　an　〃unquestionable　truth　that；　at　any　moment；　the　existing　quantity　of　liberated　nerve…force；　which　in　an　inscrutable　way　produces　in　us　the　state　we　call　feeling；　MUST　expend　itself　in　some　directionMUST　generate　an　equivalent　manifestation　of　force　somewhere；〃　so　that；　when　the　cerebro…spinal　system　is　highly　excited　and　nerve…force　is　liberated　in　excess；　it　may　be　expended　in　intense　sensations；　active　thought；　violent　movements；　or　increased　activity　of　the　glands。＇7＇　Mr。　Spencer　further　maintains　that　an　〃overflow　of　nerve…force；　undirected　by　any　motive；　will　manifestly　take　the　most　habitual　routes；　and；　if　these　do　not　suffice；　will　next　overflow　into　the　less　habitual　ones。〃　Consequently　the　facial　and　respiratory　muscles；　which　are　the　most　used；　will　be　apt　to　be　first　brought　into　action；　then　those　of　the　upper　extremities；　next　those　of　the　lower；　and　finally　those　of　the　whole　body。＇8＇
  ＇4＇　Mr。　Bartlett；　〃Notes　on　the　Birth　of　a　Hippopotamus；〃　Proc。　　Zoolog。　　Soc。　　1871；　p。　　255。
  ＇5＇　See；　on　this　subject；　Claude　Bernard；　‘Tissus　Vivants；'　1866；　pp。　316；　337；　358。　　Virchow　expresses　himself　to　almost　exactly　the　same　effect　in　his　essay　〃Ueber　das　Ruckenmark〃　（Sammlung　wissenschaft。　Vortrage；　1871；　s。　　28）。
  An　emotion　may　be　very　strong；　but　it　will　have　little　tendency　to　induce　movements　of　any　kind；　if　it　has　not　commonly　led　to　voluntary　action　for　its　relief　or　gratification；　and　when　movements　are　excited；　their　nature　is；　to　a　large　extent；　determined　by　those　which　have　often　and　voluntarily　been　performed　for　some　definite　end　under　the　same　emotion。　　Great　pain　urges　all　animals；　and　has　urged　them　during　endless　generations；　to　make　the　most　violent　and　diversified　efforts　to　escape　from　the　cause　of　suffering。　Even　when　a　limb　or　other　separate　part　of　the　body　is　hurt；　we　often　see　a　tendency　to　shake　it；　as　if　to　shake　off　the　cause；　though　this　may　obviously　be　impossible。　　Thus　a　habit　of　exerting　with　the　utmost　force　all　the　muscles　will　have　been　established；　whenever　great　suffering　is　experienced。　　As　the　muscles　of　the　chest　and　vocal　organs　are　habitually　used；　these　will　be　particularly　liable　to　be　acted　on；　and　loud；　harsh　screams　or　cries　will　be　uttered。　But　the　advantage　derived　from　outcries　has　here　probably　come　into　play　in　an　important　manner；　for　the　young　of　most　animals；　when　in　distress　or　danger；　call　loudly　to　their　parents　for　aid；　as　do　the　members　of　the　same　community　for　mutual　aid。
  ＇6＇　Muller　（‘Elements　of　Physiology；'　Eng。　　translat。　　vol。　　ii。　　p。　　932）　in　speaking　of　the　nerves；　says；　〃any　sudden　change　of　condition　of　whatever　kind　sets　the　nervous　principle　into　action。〃　　See　Virchow　and　Bernard　on　th