undergoing　a　change；　is　converted　into　blood；　and　makes　its　way　to
  their　source　＇the　heart＇。　We　have　treated　of　all　this　when
  discussing　the　subject　of　nutrition；　but　must　here　recapitulate　what
  was　there　said；　in　order　that　we　may　obtain　a　scientific　view　of　the
  beginnings　of　the　process；　and　come　to　know　what　exactly　happens　to
  the　primary　organ　of　sense…perception　to　account　for　the　occurrence　of
  waking　and　sleep。　For　sleep；　as　has　been　shown；　is　not　any　given
  impotence　of　the　perceptive　faculty；　for　unconsciousness；　a　certain
  form　of　asphyxia；　and　swooning；　all　produce　such　impotence。　Moreover
  it　is　an　established　fact　that　some　persons　in　a　profound　trance
  have　still　had　the　imaginative　faculty　in　play。　This　last　point；
  indeed；　gives　rise　to　a　difficulty；　for　if　it　is　conceivable　that
  one　who　had　swooned　should　in　this　state　fall　asleep；　the　phantasm
  also　which　then　presented　itself　to　his　mind　might　be　regarded　as　a
  dream。　Persons；　too；　who　have　fallen　into　a　deep　trance；　and　have　come
  to　be　regarded　as　dead；　say　many　things　while　in　this　condition。　The
  same　view；　however；　is　to　be　taken　of　all　these　cases；　＇i。e。　that　they
  are　not　cases　of　sleeping　or　dreaming＇。
  As　we　observed　above；　sleep　is　not　co…extensive　with　any　and　every
  impotence　of　the　perceptive　faculty；　but　this　affection　is　one　which
  arises　from　the　evaporation　attendant　upon　the　process　of　nutrition。
  The　matter　evaporated　must　be　driven　onwards　to　a　certain　point；
  then　turn　back；　and　change　its　current　to　and　fro；　like　a　tide…race　in
  a　narrow　strait。　Now；　in　every　animal　the　hot　naturally　tends　to
  move　＇and　carry　other　things＇　upwards；　but　when　it　has　reached　the
  parts　above　＇becoming　cool＇；　it　turns　back　again；　and　moves
  downwards　in　a　mass。　This　explains　why　fits　of　drowsiness　are
  especially　apt　to　come　on　after　meals；　for　the　matter；　both　the　liquid
  and　the　corporeal；　which　is　borne　upwards　in　a　mass；　is　then　of
  considerable　quantity。　When；　therefore；　this　comes　to　a　stand　it
  weighs　a　person　down　and　causes　him　to　nod；　but　when　it　has　actually
  sunk　downwards；　and　by　its　return　has　repulsed　the　hot；　sleep　comes
  on；　and　the　animal　so　affected　is　presently　asleep。　A　confirmation
  of　this　appears　from　considering　the　things　which　induce　sleep；　they
  all；　whether　potable　or　edible；　for　instance　poppy；　mandragora；
  wine；　darnel；　produce　a　heaviness　in　the　head；　and　persons　borne
  down　＇by　sleepiness＇　and　nodding　＇drowsily＇　all　seem　affected　in
  this　way；　i。e。　they　are　unable　to　lift　up　the　head　or　the　eye…lids。
  And　it　is　after　meals　especially　that　sleep　comes　on　like　this；　for
  the　evaporation　from　the　foods　eaten　is　then　copious。　It　also
  follows　certain　forms　of　fatigue；　for　fatigue　operates　as　a　solvent；
  and　the　dissolved　matter　acts；　if　not　cold；　like　food　prior　to
  digestion。　Moreover；　some　kinds　of　illness　have　this　same　effect；
  those　arising　from　moist　and　hot　secretions；　as　happens　with
  fever…patients　and　in　cases　of　lethargy。　Extreme　youth　also　has　this
  effect；　infants；　for　example；　sleep　a　great　deal；　because　of　the
  food　being　all　borne　upwards…a　mark　whereof　appears　in　the
  disproportionately　large　size　of　the　upper　parts　compared　with　the
  lower　during　infancy；　which　is　due　to　the　fact　that　growth
  predominates　in　the　direction　of　the　former。　Hence　also　they　are
  subject　to　epileptic　seizures；　for　sleep　is　like　epilepsy；　and；　in　a
  sense；　actually　is　a　seizure　of　this　sort。　Accordingly；　the
  beginning　of　this　malady　takes　place　with　many　during　sleep；　and　their
  subsequent　habitual　seizures　occur　in　sleep；　not　in　waking　hours。
  For　when　the　spirit　＇evaporation＇　moves　upwards　in　a　volume；　on　its
  return　downwards　it　distends　the　veins；　and　forcibly　compresses　the
  passage　through　which　respiration　is　effected。　This　explains　why　wines
  are　not　good　for　infants　or　for　wet　nurses　（for　it　makes　no
  difference；　doubtless；　whether　the　infants　themselves；　or　their
  nurses；　drink　them）；　but　such　persons　should　drink　them　＇if　at　all＇
  diluted　with　water　and　in　small　quantity。　For　wine　is　spirituous；
  and　of　all　wines　the　dark　more　so　than　any　other。　The　upper　parts；
  in　infants；　are　so　filled　with　nutriment　that　within　five　months
  ＇after　birth＇　they　do　not　even　turn　the　neck　＇sc。　to　raise　the
  head＇；　for　in　them；　as　in　persons　deeply　intoxicated；　there　is　ever
  a　large　quantity　of　moisture　ascending。　It　is　reasonable；　too；　to
  think　that　this　affection　is　the　cause　of　the　embryo's　remaining　at
  rest　in　the　womb　at　first。　Also；　as　a　general　rule；　persons　whose
  veins　are　inconspicuous；　as　well　as　those　who　are　dwarf…like；　or
  have　abnormally　large　heads；　are　addicted　to　sleep。　For　in　the
  former　the　veins　are　narrow；　so　that　it　is　not　easy　for　the　moisture
  to　flow　down　through　them；　while　in　the　case　of　dwarfs　and　those　whose
  heads　are　abnormally　large；　the　impetus　of　the　evaporation　upwards
  is　excessive。　Those　＇on　the　contrary＇　whose　veins　are　large　are；
  thanks　to　the　easy　flow　through　the　veins；　not　addicted　to　sleep；
  unless；　indeed；　they　labour　under　some　other　affection　which
  counteracts　＇this　easy　flow＇。　Nor　are　the　'atrabilious'　addicted　to
  sleep；　for　in　them　the　inward　region　is　cooled　so　that　the　quantity　of
  evaporation　in　their　case　is　not　great。　For　this　reason　they　have
  large　appetites；　though　spare　and　lean；　for　their　bodily　condition
  is　as　if　they　derived　no　benefit　from　what　they　eat。　The　dark　bile；
  too；　being　itself　naturally　cold；　cools　also　the　nutrient　tract；　and
  the　other　parts　wheresoever　such　secretion　is　potentially　present
  ＇i。e。　tends　to　be　formed＇。
  Hence　it　is　plain　from　what　has　been　said　that　sleep　is　a　sort　of
  concentration；　or　natural　recoil；　of　the　hot　matter　inwards　＇towards
  its　centre＇；　due　to　the　cause　above　mentioned。　Hence　restless　movement
  is　a　marked　feature　in　the　case　of　a　person　when　drowsy。　But　where
  it　＇the　heat　in　the　upper　and　outer　parts＇　begins　to　fail；　he　grows
  cool；　and　owing　to　this　cooling　process　his　eye…lids　droop。
  Accordingly　＇in　sleep＇　the　upper　and　outward　parts　are　cool；　but　the
  inward　and　lower；　i。e。　the　parts　at　the　feet　and　in　the　interior　of
  the　body；　are　hot。
  Yet　one　might　found　a　difficulty　on　the　facts　that　sleep　is　most
  oppressive　in　its　onset　after　meals；　and　that　wine；　and　other　such
  things；　though　they　possess　heating　properties；　are　productive　of
  sleep；　for　it　is　not　probable　that　sleep　should　be　a　process　of
  cooling　while　the　things　that　cause　sleeping　are　themselves　hot。　Is
  the　explanation　of　this；　then；　to　be　found　in　the　fact　that；　as　the
  stomach　when　empty　is　hot；　while　replenishment　cools　it　by　the
  movement　it　occasions；　so　the　passages　and　tracts　in　the　head　are
  cooled　as　the　'evaporation'　ascends　thither？　Or；　as　those　who　have　hot
  water　poured　on　them　feel　a　sudden　shiver　of　cold；　just　so　in　the　case
  before　us；　may　it　be　that；　when　the　hot　substance　ascends；　the　cold
  rallying　to　meet　it　cools　＇the　aforesaid　parts＇　deprives　their
  native　heat　of　all　its　power；　and　compels　it　to　retire？　Moreover；　when
  much　food　is　taken；　which　＇i。e。　the　nutrient　evaporation　from　which＇
  the　hot　substance　carries　upwards；　this　latter；　like　a　fire　when　fresh
  logs　are　laid　upon　it；　is　itself　cooled；　until　the　food　has　been
  digested。
  For；　as　has　been　observed　elsewhere；　sleep　comes　on　when　the
  corporeal　element　＇in　the　'evaporation'＇　conveyed　upwards　by　the
  hot；　along　the　veins；　to　the　head。　But　when　that　which　has　been　thus
  carried　up　can　no　longer　ascend；　but　is　too　great　in　quantity　＇to　do
  so＇；　it　forces　the　hot　back　again　and　flows　downwards。　Hence　it　is
  that　men　sink　down　＇as　they　do　in　sleep＇　when　the　heat　which　tends
  to　keep　them　erect　（man　alone；　among　animals；　being　naturally　erect）
  is　withdrawn；　and　this；　when　it　befalls　them；　causes
  unconsciousness；　and　afterwards　phantasy。
  Or　are　the　solutions　thus　proposed　barely　conceivable　accounts　of
  the　refrigeration　which　takes　place；　while；　as　a　matter　of　fact；　the
  region　of　the　brain　is；　as　stated　elsewhere；　the　main　determinant　of
  the　matter？　For　the　brain；　or　in　creatures　without　a　brain　that
  which　corresponds　to　it；　is　of　all　parts　of　the　body　the　coolest。
  Therefore；　as　moisture　turned　into　vapour　by　the　sun's　heat　is；　when
  it　has　ascended　to　the　upper　regions；　cooled　by　the　coldness　of　the
  latter；　and　becoming　condensed；　is　carried　downwards；　and　turned
  into　water　once　more；　just　so　the　excrementitious　evaporation；　when
  carried　up　by　the　heat　to　the　region　of　the　brain；　is　condensed　into　a
  'phl