we　condense　the　observations　of　a　generation　in　a　single　axiom；　these　logical　analyses　by　which　we　fence　out　the　ignorance　we　cannot　reclaim；　and　fix　the　limits　of　our　knowledge；all　lead　us　up　to　the　inspiration　of　the　Almighty；　which　gives　understanding　to　the　world's　great　teachers。　　To　fear　science　or　knowledge；　lest　it　disturb　our　old　beliefs；　is　to　fear　the　influx　of　the　Divine　wisdom　into　the　souls　of　our　fellow…men；　for　what　is　science　but　the　piecemeal　revelation；uncovering；of　the　plan　of　creation；　by　the　agency　of　those　chosen　prophets　of　nature　whom　God　has　illuminated　from　the　central　light　of　truth　for　that　single　purpose？
  The　studies　which　we　have　glanced　at　are　preliminary　in　your　education　to　the　practical　arts　which　make　use　of　them；the　arts　of　healing；surgery　and　medicine。　　The　more　you　examine　the　structure　of　the　organs　and　the　laws　of　life；　the　more　you　will　find　how　resolutely　each　of　the　cell…republics　which　make　up　the　E　pluribus　unum　of　the　body　maintains　its　independence。　　Guard　it；　feed　it；　air　it；　warm　it；　exercise　or　rest　it　properly；　and　the　working　elements　will　do　their　best　to　keep　well　or　to　get　well。　　What　do　we　do　with　ailing　vegetables？　　Dr。　Warren；　my　honored　predecessor　in　this　chair；　bought　a　country…place；　including　half　of　an　old　orchard。　　A　few　years　afterwards　I　saw　the　trees　on　his　side　of　the　fence　looking　in　good　health；　while　those　on　the　other　side　were　scraggy　and　miserable。　　How　do　you　suppose　this　change　was　brought　about？　　By　watering　them　with　Fowler's　solution？　　By　digging　in　calomel　freely　about　their　roots？　　Not　at　all；　but　by　loosening　the　soil　round　them；　and　supplying　them　with　the　right　kind　of　food　in　fitting　quantities。
  Now　a　man　is　not　a　plant；　or；　at　least；　he　is　a　very　curious　one；　for　he　carries　his　soil　in　his　stomach；　which　is　a　kindof　portable　flower…pot；　and　he　grows　round　it；　instead　of　out　of　it。　　He　has；　besides；　a　singularly　complex　nutritive　apparatus　and　a　nervous　system。　　But　recollect　the　doctrine　already　enunciated　in　the　language　of　Virchow；　that　an　animal；　like　a　tree；　is　a　sum　of　vital　unities；　of　which　the　cell　is　the　ultimate　element。　　Every　healthy　cell；　whether　in　a　vegetable　or　an　animal；　necessarily　performs　its　function　properly　so　long　as　it　is　supplied　with　its　proper　materials　and　stimuli。　　A　cell　may；　it　is　true；　be　congenitally　defective；　in　which　case　disease　is；　so　to　speak；　its　normal　state。　　But　if　originally　sound　and　subsequently　diseased；　there　has　certainly　been　some　excess；　deficiency；　or　wrong　quality　in　the　materials　or　stimuli　applied　to　it。　　You　remove　this　injurious　influence　and　substitute　a　normal　one；　remove　the　baked　coal…ashes；　for　instance；　from　the　roots　of　a　tree；　and　replace　them　with　loam；　take　away　the　salt　meat　from　the　patient's　table；　and　replace　it　with　fresh　meat　and　vegetables；　and　the　cells　of　the　tree　or　the　man　return　to　their　duty。
  I　do　not　know　that　we　ever　apply　to　a　plant　any　element　which　is　not　a　natural　constituent　of　the　vegetable　structure；　except　perhaps　externally；　for　the　accidental　purpose　of　killing　parasites。　　The　whole　art　of　cultivation　consists　in　learning　the　proper　food　and　conditions　of　plants；　and　supplying　them。　　We　give　them　water；　earths；　salts　of　various　kinds　such　as　they　are　made　of；　with　a　chance　to　help　themselves　to　air　and　light。　　The　farmer　would　be　laughed　at　who　undertook　to　manure　his　fields　or　his　trees　with　a　salt　of　lead　or　of　arsenic。　　These　elements　are　not　constituents　of　healthy　plants。　　The　gardener　uses　the　waste　of　the　arsenic　furnaces　to　kill　the　weeds　in　his　walks。
  If　the　law　of　the　animal　cell；　and　of　the　animal　organism；　which　is　built　up　of　such　cells；　is　like　that　of　the　vegetable；　we　might　expect　that　we　should　treat　all　morbid　conditions　of　any　of　the　vital　unities　belonging　to　an　animal　in　the　same　way；　by　increasing；　diminishing；　or　changing　its　natural　food　or　stimuli。
  〃That　is　an　aliment　which　nourishes；　whatever　we　find　in　the　organism；　as　a　constant　and　integral　element；　either　forming　part　of　its　structure；　or　one　of　the　conditions　of　vital　processes；　that　and　that　only　deserves　the　name　of　aliment。〃　　〃　I　see　no　reason；　therefore；　why　iron；　phosphate　of　lime；　sulphur；　should　not　be　considered　food　for　man；　as　much　as　guano　or　poudrette　for　vegetables。　　Whether　one　or　another　of　them　is　best　in　any　given　case；whether　they　shall　be　taken　alone　or　in　combination；　in　large　or　small　quantities；　are　separate　questions。　　But　they　are　elements　belonging　to　the　body；　and　even　in　moderate　excess　will　produce　little　disturbance。　　There　is　no　presumption　against　any　of　this　class　of　substances；　any　more　than　against　water　or　salt；　provided　they　are　used　in　fitting　combinations；　proportions；　and　forms。
  But　when　it　comes　to　substances　alien　to　the　healthy　system；　which　never　belong　to　it　as　normal　constituents；　the　case　is　very　different。　　There　is　a　presumption　against　putting　lead　or　arsenic　into　the　human　body；　as　against　putting　them　into　plants；　because　they　do　not　belong　there；　any　more　than　pounded　glass；　which；　it　is　said；　used　to　be　given　as　a　poison。　　The　same　thing　is　true　of　mercury　and　silver。　　What　becomes　of　these　alien　substances　after　they　get　into　the　system　we　cannot　always　tell。　　But　in　the　case　of　silver；　from　the　accident　of　its　changing　color　under　the　influence　of　light；　we　do　know　what　happens。　　It　is　thrown　out；　in　part　at　least；　under　the　epidermis；　and　there　it　remains　to　the　patient's　dying　day。　　This　is　a　striking　illustration　of　the　difficulty　which　the　system　finds　in　dealing　with　non…assimilable　elements；　and　justifies　in　some　measure　the　vulgar　prejudice　against　mineral　poisons。〃
  I　trust　the　youngest　student　on　these　benches　will　not　commit　the　childish　error　of　confounding　a　presumption　against　a　particular　class　of　agents　with　a　condemnation　of　them。　　Mercury；　for　instance；　is　alien　to　the　system；　and　eminently　disturbing　in　its　influence。　Yet　its　efficacy　in　certain　forms　of　specific　disease　is　acknowledged　by　all　but　the　most　sceptical　theorists。　　Even　the　esprit　moqueur　of　Ricord；　the　Voltaire　of　pelvic　literature；　submits　to　the　time…　honored　constitutional　authority　of　this　great　panacea　in　the　class　of　cases　to　which　he　has　devoted　his　brilliant　intelligence。　　Still；　there　is　no　telling　what　evils　have　arisen　from　the　abuse　of　this　mineral。　　Dr。　Armstrong　long　ago　pointed　out　some　of　them；　and　they　have　become　matters　of　common　notoriety。　　I　am　pleased；　therefore；　when　I　find　so　able　and　experienced　a　practitioner　as　Dr。　Williams　of　this　city　proving　that　iritis　is　best　treated　without　mercury；　and　Dr。　Vanderpoel　showing　the　same　thing　to　be　true　for　pericarditis。
  Whatever　elements　nature　does　not　introduce　into　vegetables；　the　natural　food　of　all　animal　life；directly　of　herbivorous；　indirectly　of　carnivorous　animals；are　to　be　regarded　with　suspicion。　　Arsenic…　eating　may　seem　to　improve　the　condition　of　horses　for　a　time；and　even　of　human　beings；　if　Tschudi's　stories　can　be　trusted；but　it　soon　appears　that　its　alien　qualities　are　at　war　with　the　animal　organization。　　So　of　copper；　antimony；　and　other　non…alimentary　simple　substances；　everyone　of　them　is　an　intruder　in　the　living　system；　as　much　as　a　constable　would　be；　quartered　in　our　household。　This　does　not　mean　that　they　may　not；　any　of　them；　be　called　in　for　a　special　need；　as　we　send　for　the　constable　when　we　have　good　reason　to　think　we　have　a　thief　under　our　roof；　but　a　man's　body　is　his　castle；　as　well　as　his　house；　and　the　presumption　is　that　we　are　to　keep　our　alimentary　doors　bolted　against　these　perturbing　agents。
  Now　the　feeling　is　very　apt　to　be　just　contrary　to　this。　　The　habit　has　been　very　general　with　well…taught　practitioners；　to　have　recourse　to　the　introduction　of　these　alien　elements　into　the　system　on　the　occasion　of　any　slight　disturbance。　　The　tongue　was　a　little　coated；　and　mercury　must　be　given；　the　skin　was　a　little　dry；　and　the　patient　must　take　antimony。　　It　was　like　sending　for　the　constable　and　the　posse　comitatus　when　there　is　only　a　carpet　to　shake　or　a　refuse…barrel　to　empty。　＇Dr。　James　Johnson　advises　persons　not　ailing　to　take　；five　grains　of　blue　pill　with　one　or　two　of　aloes　twice　a　week　for　three　or　four　months　in　the　year；　with　half　a　pint　of　compound　decoction　of　sarsaparilla　every　day　for　the　same　period；　to　preserve　health　and　prolong　life。　　Pract。　　Treatise　on　Dis。　　of　Liver；　etc。　　p。　　272。＇　The　constitution　bears　slow　poisoning　a　great　deal　better　than　might　be　expected；　yet　the　most　intelligent　men　in　the　profession　have　gradually　got　out　of　the　habit　of　prescribing　these　powerful　alien　substances　in　the　old　routine　way。　　Mr。　Metcalf　will　tell　you　how　much　more　sparingly　they　are　given　by　our　practitioners　at　the　present　time；　than　when　he　first　inaugurated　the　new　era　of　pharmacy　among　us。　　Still；　the　presumption　in　favor　of　poisoning　out　every　spontaneous　reaction　of　outraged　nature　is　not　extinct　in　those　who　are　trusted　with　the　lives　of　their　fellow…　citizens。　　〃On　examining　the　file　of　prescriptions　at　the　hospital；　I　discovered　that　they　were　rudely　written；　and　indicated　a　treatment；　as　they　consisted　chiefly　of　tartar　emetic；　ipecacuanha；　and　epsom　salts；　hardly　favorable　to　the　cure　of　the　p