hy　one　cell　becomes　nerve　and　another　muscle；　why　one　selects　bile　and　another　fat；　we　can　no　more　pretend　to　tell；　than　why　one　grape　sucks　out　of　the　soil　the　generous　juice　which　princes　hoard　in　their　cellars；　and　another　the　wine　which　it　takes　three　men　to　drink；one　to　pour　it　down；　another　to　swallow　it；　and　a　third　to　hold　him　while　it　is　going　down。　Certain　analogies　between　this　selecting　power　and　the　phenomena　of　endosmosis　in　the　elective　affinities　of　chemistry　we　can　find；　but　the　problem　of　force　remains　here；　as　everywhere；　unsolved　and　insolvable。
  Do　we　gain　anything　by　attempting　to　get　rid　of　the　idea　of　a　special　vital　force　because　we　find　certain　mutually　convertible　relations　between　forces　in　the　body　and　out　of　it？　　I　think　not；　any　more　than　we　should　gain　by　getting　rid　of　the　idea　and　expression　Magnetism　because　of　its　correlation　with　electricity。　　We　may　concede　the　unity　of　all　forms　of　force；　but　we　cannot　overlook　the　fixed　differences　of　its　manifestations　according　to　the　conditions　under　which　it　acts。　　It　is　a　mistake；　however；　to　think　the　mystery　is　greater　in　an　organized　body　than　in　any　other。　　We　see　a　stone　fall　or　a　crystal　form；　and　there　is　nothing　stranger　left　to　wonder　at；　for　we　have　seen　the　Infinite　in　action。
  Just　so　far　as　we　can　recognize　the　ordinary　modes　of　operation　of　the　common　forces　of　nature；gravity；　cohesion；　elasticity；　transudation；　chemical　action；　and　the　rest；we　see　the　so…called　vital　acts　in　the　light　of　a　larger　range　of　known　facts　and　familiar　analogies。　　Matteuecci's　well…remembered　lectures　contain　many　and　striking　examples　of　the　working　of　physical　forces　in　physiological　processes。　　Wherever　rigid　experiment　carries　us；　we　are　safe　in　following　this　lead；　but　the　moment　we　begin　to　theorize　beyond　our　strict　observation；　we　are　in　danger　of　falling　into　those　mechanical　follies　which　true　science　has　long　outgrown。
  Recognizing　the　fact；　then；　that　we　have　learned　nothing　but　the　machinery　of　life；　and　are　no　nearer　to　its　essence；　what　is　it　that　we　have　gained　by　this　great　discovery　of　the　cell　formation　and　function？
  It　would　have　been　reward　enough　to　learn　the　method　Nature　pursues　for　its　own　sake。　　If　the　sovereign　Artificer　lets　us　into　his　own　laboratories　and　workshops；　we　need　not　ask　more　than　the　privilege　of　looking　on　at　his　work。　　We　do　not　know　where　we　now　stand　in　the　hierarchy　of　created　intelligences。　　We　were　made　a　little　lower　than　the　angels。　　I　speak　it　not　irreverently；　as　the　lower　animals　surpass　man　in　some　of　their　attributes；　so　it　may　be　that　not　every　angel's　eye　can　see　as　broadly　and　as　deeply　into　the　material　works　of　God　as　man　himself；　looking　at　the　firmament　through　an　equatorial　of　fifteen　inches'　aperture；　and　searching　into　the　tissues　with　a　twelfth　of　an　inch　objective。
  But　there　are　other　positive　gains　of　a　more　practical　character。　Thus　we　are　no　longer　permitted　to　place　the　seat　of　the　living　actions　in　the　extreme　vessels；　which　are　only　the　carriers　from　which　each　part　takes　what　it　wants　by　the　divine　right　of　the　omnipotent　nucleated　cell。　　The　organism　has　become；　in　the　words　already　borrowed　from　Virchow；　〃a　sum　of　vital　unities。〃　　The　strictum　and　laxum；　the　increased　and　diminished　action　of　the　vessels；　out　of　which　medical　theories　and　methods　of　treatment　have　grown　up；　have　yielded　to　the　doctrine　of　local　cell…communities；　belonging　to　this　or　that　vascular　district；　from　which　they　help　themselves；　as　contractors　are　wont　to　do　from　the　national　treasury。
  I　cannot　promise　to　do　more　than　to　select　a　few　of　the　points　of　contact　between　our　ignorance　and　our　knowledge　which　present　particular　interest　in　the　existing　state　of　our　physiological　acquisitions。　　Some　of　them　involve　the　microscopic　discoveries　of　which　I　have　been　speaking；　some　belong　to　the　domain　of　chemistry；　and　some　have　relations　with　other　departments　of　physical　science。
  If　we　should　begin　with　the　digestive　function；　we　should　find　that　the　long…agitated　question　of　the　nature　of　the　acid　of　the　gastric　juice　is　becoming　settled　in　favor　of　the　lactic。　　But　the　whole　solvent　agency　of　the　digestive　fluid　enters　into　the　category　of　that　exceptional　mode　of　action　already　familiar　to　us　in　chemistry　as　catalysis。　　It　is　therefore　doubly　difficult　of　explanation；　first；　as　being；　like　all　reactions；　a　fact　not　to　be　accounted　for　except　by　the　imaginative　appeal　to　〃affinity；〃　and　secondly；　as　being　one　of　those　peculiar　reactions　provoked　by　an　element　which　stands　outside　and　looks　on　without　compromising　itself。
  The　doctrine　of　Mulder；　so　widely　diffused　in　popular　and　scientific　belief；　of　the　existence　of　a　common　base　of　all　albuminous　substances；　the　so…called　protein；　has　not　stood　the　test　of　rigorous　analysis。　　The　division　of　food　into　azotized　and　non…azotized　is　no　doubt　important；　but　the　attempt　to　show　that　the　first　only　is　plastic　or　nutritive；　while　the　second　is　simply　calorifacient；　or　heat…producing；　fails　entirely　in　the　face　of　the　facts　revealed　by　the　study　of　man　in　different　climates；　and　of　numerous　experiments　in　the　feeding　of　animals。　　I　must　return　to　this　subject　in　connection　with　the　respiratory　function。
  The　sugar…making　faculty　of　the　liver　is　another　〃catalytic〃　mystery；　as　great　as　the　rest　of　them；　and　no　greater。　　Liver…tissue　brings　sugar　out　of　the　blood；　or　out　of　its　own　substance；why？
  Quia　est　in　eo　　　　　　　　　　　Virtus　saccharitiva。
  Just　what　becomes　of　the　sugar　beyond　the　fact　of　its　disappearance　before　it　can　get　into　the　general　circulation　and　sweeten　our　tempers；　it　is　hard　to　say。
  The　pancreatic　fluid　makes　an　emulsion　of　the　fat　contained　in　our　food；　but　just　how　the　fatty　particles　get　into　the　villi　we　must　leave　Brucke　and　Kolliker　to　settle　if　they　can。
  No　one　has　shown　satisfactorily　the　process　by　which　the　blood…　corpuscles　are　formed　out　of　the　lymph…corpuscles；　nor　what　becomes　of　them。　　These　two　questions　are　like　those　famous　household　puzzles；Where　do　the　flies　come　from？　　and；　Where　do　the　pins　go　to？
  There　is　a　series　of　organs　in　the　body　which　has　long　puzzled　physiologists；organs　of　glandular　aspect；　but　having　no　ducts；the　spleen；　the　thyroid　and　thymus　bodies；　and　the　suprarenal　capsules。　We　call　them　vascular　glands；　and　we　believe　that　they　elaborate　colored　and　uncolored　blood…cells；　but　just　what　changes　they　effect；　and　just　how　they　effect　them；　it　has　proved　a　very　difficult　matter　to　determine。　　So　of　the　noted　glandules　which　form　Peyer's　patches；　their　precise　office；　though　seemingly　like　those　of　the　lymphatic　glands；　cannot　be　positively　assigned；　so　far　as　I　know；　at　the　present　time。　　It　is　of　obvious　interest　to　learn　it　with　reference　to　the　pathology　of　typhoid　fever。　　It　will　be　remarked　that　the　coincidence　of　their　changes　in　this　disease　with　enlargement　of　the　spleen　suggests　the　idea　of　a　similarity　of　function　in　these　two　organs。
  The　theories　of　the　production　of　animal　heat；　from　the　times　of　Black；　Lavoisier；　and　Crawford　to　those　of　Liebig；　are　familiar　to　all　who　have　paid　any　attention　to　physiological　studies。　　The　simplicity　of　Liebig's　views；　and　the　popular　form　in　which　they　have　been　presented；　have　given　them　wide　currency；　and　incorporated　them　in　the　common　belief　and　language　of　our　text…books。　　Direct　oxidation　or　combustion　of　the　carbon　and　hydrogen　contained　in　the　food；　or　in　the　tissues　themselves；　the　division　of　alimentary　substances　into　respiratory；　or　non…azotized；　and　azotized；these　doctrines　are　familiar　even　to　the　classes　in　our　high…schools。　　But　this　simple　statement　is　boldly　questioned。　　Nothing　proves　that　oxygen　combines　（in　the　system）　with　hydrogen　and　carbon　in　particular；　rather　than　with　sulphur　and　azote。　　Such　is　the　well…　grounded　statement　of　Robin　and　Verdeil。　　〃It　is　very　probable　that　animal　heat　is　entirely　produced　by　the　chemical　actions　which　take　place　in　the　organism；　but　the　phenomenon　is　too　complex　to　admit　of　our　calculating　it　according　to　the　quality　of　oxygen　consumed。〃　These　last　are　the　words　of　Regnault；　as　cited　by　Mr。　Lewes；　whose　intelligent　discussion　of　this　and　many　of　the　most　interesting　physiological　problems　I　strongly　recommend　to　your　attention。
  This　single　illustration　covers　a　wider　ground　than　the　special　function　to　which　it　belongs。　　We　are　learning　that　the　chemistry　of　the　body　must　be　studied；　not　simply　by　its　ingesta　and　egesta；　but　that　there　is　a　long　intermediate　series　of　changes　which　must　be　investigated　in　their　own　light；　under　their　own　special　conditions。　The　expression　〃sum　of　vital　unities〃　applies　to　the　chemical　actions；　as　well　as　to　other　actions　localized　in　special　parts；　and　when　the　distinguished　chemists　whom　I　have　just　cited　entitle　their　work　a　treatise　on　the　immediate　principles　of　the　body；　they　only　indicate　the　nature　of　that　profound　and　subtile　analysis　which　must　take　the　place　of　all　hasty　generalizations　founded　on　a　comparison　of　the　food　with　residual　products。
  I　will　only　call　your　attention　to　the　fact；　that　the　exceptional　phenomenon　of　the　laboratory　is　the　prevailing　law　of　the　organism。　Nutrition　itself　is　but　one　great　catalytic　proces