the expression of emotion in man and animals-第36章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


he　secretion　of　tears。　　This　holds　good　under　widely　different　emotions；　and　independently　of　any　emotion。　　It　is　not；　of　course；　meant　that　tears　cannot　be　secreted　without　the　contraction　of　these　muscles；　for　it　is　notorious　that　they　are　often　freely　shed　with　the　eyelids　not　closed；　and　with　the　brows　unwrinkled。　The　contraction　must　be　both　involuntary　and　prolonged；　as　during　a　choking　fit；　or　energetic；　as　during　a　sneeze。　The　mere　involuntary　winking　of　the　eyelids；　though　often　repeated；　does　not　bring　tears　into　the　eyes。　　Nor　does　the　voluntary　and　prolonged　contraction　of　the　several　surrounding　muscles　suffice。　As　the　lacrymal　glands　of　children　are　easily　excited；　I　persuaded　my　own　and　several　other　children　of　different　ages　to　contract　these　muscles　repeatedly　with　their　utmost　force；　and　to　continue　doing　so　as　long　as　they　possibly　could；　but　this　produced　hardly　any　effect。　　There　was　sometimes　a　little　moisture　in　the　eyes；　but　not　more　than　apparently　could　be　accounted　for　by　the　squeezing　out　of　the　already　secreted　tears　within　the　glands。

The　nature　of　the　relation　between　the　involuntary　and　energetic　contraction　of　the　muscles　round　the　eyes；　and　the　secretion　of　tears；　cannot　be　positively　ascertained；　but　a　probable　view　may　be　suggested。　The　primary　function　of　the　secretion　of　tears；　together　with　some　mucus；　is　to　lubricate　the　surface　of　the　eye；　and　a　secondary　one；　as　some　believe；　is　to　keep　the　nostrils　damp；　so　that　the　inhaled　air　may　be　moist；＇21＇　and　likewise　to　favour　the　power　of　smelling。　But　another；　and　at　least　equally　important　function　of　tears；　is　to　wash　out　particles　of　dust　or　other　minute　objects　which　may　get　into　the　eyes。　That　this　is　of　great　importance　is　clear　from　the　cases　in　which　the　cornea　has　been　rendered　opaque　through　inflammation；　caused　by　particles　of　dust　not　being　removed；　in　consequence　of　the　eye　and　eyelid　becoming　immovable。＇22＇　The　secretion　of　tears　from　the　irritation　of　any　foreign　body　in　the　eye　is　a　reflex　action；that　is；　the　body　irritates　a　peripheral　nerve　which　sends　an　impression　to　certain　sensory　nerve…cells；　these　transmit　an　influence　to　other　cells；　and　these　again　to　the　lacrymal　glands。　　The　influence　transmitted　to　these　glands　causes；　as　there　is　good　reason　to　believe；　the　relaxation　of　the　muscular　coats　of　the　smaller　arteries；　this　allows　more　blood　to　permeate　the　glandular　tissue；　and　this　induces　a　free　secretion　of　tears。　When　the　small　arteries　of　the　face；　including　those　of　the　retina；　are　relaxed　under　very　different　circumstances；　namely；　during　an　intense　blush；　the　lacrymal　glands　are　sometimes　affected　in　a　like　manner；　for　the　eyes　become　suffused　with　tears。

It　is　difficult　to　conjecture　how　many　reflex　actions　have　originated；　but；　in　relation　to　the　present　case　of　the　affection　of　the　lacrymal　glands　through　irritation　of　the　surface　of　the　eye；　it　may　be　worth　remarking　that；　as　soon　as　some　primordial　form　became　semi…terrestrial　in　its　habits；　and　was　liable　to　get　particles　of　dust　into　its　eyes；　if　these　were　not　washed　out　they　would　cause　much　irritation；　and　on　the　principle　of　the　radiation　of　nerve…force　to　adjoining　nerve…cells；　the　lacrymal　glands　would　be　stimulated　to　secretion。　As　this　would　often　recur；　and　as　nerve…force　readily　passes　along　accustomed　channels；　a　slight　irritation　would　ultimately　suffice　to　cause　a　free　secretion　of　tears。


＇21＇　Bergeon；　as　quoted　in　the　‘Journal　of　Anatomy　and　Physiology；'　Nov。　1871；　p。　　235。

＇22＇　See；　for　instance；　a　case　given　by　Sir　Charles　Bell；　‘Philosophical　Transactions；'　1823；　p。　　177。

As　soon　as　by　this；　or　by　some　other　means；　a　reflex　action　of　this　nature　had　been　established　and　rendered　easy；　other　stimulants　applied　to　the　surface　of　the　eyesuch　as　a　cold　wind；　slow　inflammatory　action；　or　a　blow　on　the　eyelids　would　cause　a　copious　secretion　of　tears；　as　we　know　to　be　the　case。　The　glands　are　also　excited　into　action　through　the　irritation　of　adjoining　parts。　　Thus　when　the　nostrils　are　irritated　by　pungent　vapours；　though　the　eyelids　may　be　kept　firmly　closed；　tears　are　copiously　secreted；　and　this　likewise　follows　from　a　blow　on　the　nose；　for　instance　from　a　boxing…glove。　A　stinging　switch　on　the　face　produces；　as　I　have　seen；　the　same　effect。　In　these　latter　cases　the　secretion　of　tears　is　an　incidental　result；　and　of　no　direct　service。　　As　all　these　parts　of　the　face；　including　the　lacrymal　glands；　are　supplied　with　branches　of　the　same　nerve；　namely；　the　fifth；　it　is　in　some　degree　intelligible　that　the　effects　of　the　excitement　of　any　one　branch　should　spread　to　the　nerve…cells　or　roots　of　the　other　branches。

The　internal　parts　of　the　eye　likewise　act；　under　certain　conditions；　in　a　reflex　manner　on　the　lacrymal　glands。　　The　following　statements　have　been　kindly　communicated　to　me　by　Mr。　Bowman；　but　the　subject　is　a　very　intricate　one；　as　all　the　parts　of　the　eye　are　so　intimately　related　together；　and　are　so　sensitive　to　various　stimulants。　A　strong　light　acting　on　the　retina；　when　in　a　normal　condition；　has　very　little　tendency　to　cause　lacrymation；　but　with　unhealthy　children　having　small；　old…standing　ulcers　on　the　cornea；　the　retina　becomes　excessively　sensitive　to　light；　and　exposure　even　to　common　daylight　causes　forcible　and　sustained　closure　of　the　lids；　and　a　profuse　flow　of　tears。　　When　persons　who　ought　to　begin　the　use　of　convex　glasses　habitually　strain　the　waning　power　of　accommodation；　an　undue　secretion　of　tears　very　often　follows；　and　the　retina　is　liable　to　become　unduly　sensitive　to　light。　In　general；　morbid　affections　of　the　surface　of　the　eye；　and　of　the　ciliary　structures　concerned　in　the　accommodative　act；　are　prone　to　be　accompanied　with　excessive　secretion　of　tears。　Hardness　of　the　eyeball；　not　rising　to　inflammation；　but　implying　a　want　of　balance　between　the　fluids　poured　out　and　again　taken　up　by　the　intra…ocular　vessels；　is　not　usually　attended　with　any　lacrymation。　When　the　balance　is　on　the　other　side；　and　the　eye　becomes　too　soft；　there　is　a　greater　tendency　to　lacrymation。　　Finally；　there　are　numerous　morbid　states　and　structural　alterations　of　the　eyes；　and　even　terrible　inflammations；　which　may　be　attended　with　little　or　no　secretion　of　tears。

It　also　deserves　notice；　as　indirectly　bearing　on　our　subject；　that　the　eye　and　adjoining　parts　are　subject　to　an　extraordinary　number　of　reflex　and　associated　movements；　sensations；　and　actions；　besides　those　relating　to　the　lacrymal　glands。　　When　a　bright　light　strikes　the　retina　of　one　eye　alone；　the　iris　contracts；　but　the　iris　of　the　other　eye　moves　after　a　measurable　interval　of　time。　The　iris　likewise　moves　in　accommodation　to　near　or　distant　vision；　and　when　the　two　eyes　are　made　to　converge。＇23＇　Every　one　knows　how　irresistibly　the　eyebrows　are　drawn　down　under　an　intensely　bright　light。　The　eyelids　also　involuntarily　wink　when　an　object　is　moved　near　the　eyes；　or　a　sound　is　suddenly　heard。　　The　well…known　case　of　a　bright　light　causing　some　persons　to　sneeze　is　even　more　curious；　for　nerve…force　here　radiates　from　certain　nerve…cells　in　connection　with　the　retina；　to　the　sensory　nerve…cells　of　the　nose；　causing　it　to　tickle；　and　from　these；　to　the　cells　which　command　the　various　respiratory　muscles　（the　orbiculars　included）　which　expel　the　air　in　so　peculiar　a　manner　that　it　rushes　through　the　nostrils　alone。

To　return　to　our　point：　　why　are　tears　secreted　during　a　screaming…fit　or　other　violent　expiratory　efforts？　As　a　slight　blow　on　the　eyelids　causes　a　copious　secretion　of　tears；　it　is　at　least　possible　that　the　spasmodic　contraction　of　the　eyelids；　by　pressing　strongly　on　the　eyeball；　should　in　a　similar　manner　cause　some　secretion。　　This　seems　possible；　although　the　voluntary　contraction　of　the　same　muscles　does　not　produce　any　such　effect。　　We　know　that　a　man　cannot　voluntarily　sneeze　or　cough　with　nearly　the　same　force　as　he　does　automatically；　and　so　it　is　with　the　contraction　of　the　orbicular　muscles：　Sir　C。　Bell　experimented　on　them；　and　found　that　by　suddenly　and　forcibly　closing　the　eyelids　in　the　dark；　sparks　of　light　are　seen；　like　those　caused　by　tapping　the　eyelids　with　the　fingers；　〃but　in　sneezing　the　compression