AmNo0vr'i92iarm'  \  Chemical  Elements  of  Living  Matter.  755 
33.  Cerium  occurs  in  traces  in  some  plants.   As  oxalate  it  has 
been  found  in  bones  (0.03  gm.  Cerium  oxalate  in  1  kg). 
34.  Tin  and  35.  Lead  have  been  found  in  some  organs.  Human 
kidney,  liver,  hairs,  and  nails  contain  traces  of  lead;  some  corals 
contain  small  amounts  of  lead. 
3<5.  Arsenic  occurs  in  some  animals  and  many  human  organs. 
In  chronic  arsenic  poisoning  the  amount  of  As  in  the  skin  is  in- 
creased considerably,  often  causing  a  brown  pigmentation  of  the 
skin,  probably  due  to  the  formation  of  sulfides.  (Keratin  contains' 
S.)    The  minute  traces  of  arsenic  in  human  organs  are: 
skin  .0026  mg.  As  in  100  gm.  liver     .0019  mg.  As  in^ioo  gm. 
pancreas    .0029  mg.  kidney  .001 5  mg. 
hairs         .0049  mg.  brain  .0013  mg. 
Elements  found  in  isolated  cases  in  animals .  and  plants  are : 
Geranium,  thallium,  mercury,  solenium,  silver. 
No  other  elements  have  been  reported  to  enter  into  cell 
organism. 
Summarizing  the  occurrence  of  elements  in  living  organisms, 
vegetable  or  animal,  only  four  elements  form  its  bulk,  namely 
97-99%  of  which  O  —  52-63%,  C  ==  20-38%,  H  =  7-10%  and 
N  ==  0.03-3%.  The  lowest  plants  have  in  addition;  S,  K,  and  Ca, — 
while  the  chlorophyll-containing  plants  also  contain  P,  Mg,  and 
Fe, — in  animals  CI,  Na,  Si,  and  I  are  necessary.  It  appears  that  with 
complexity  of  functions  the  number  of  elements  taking  part  in  cell 
metabolism  increases ;  hence  the  more  highly  developed  an  organism, 
the  greater  the  number  of  elements  which  enter  into  its  composition 
and  which  are  thus  usually  present  in  some  highly  specialized  organ ; 
as  e.  g.,  iodine  in  the  thyroid  glands. 
Considering  the  living  organism  as  a  chemical  machine  con- 
sisting of  protoplasmic  units  or  cells,  it  appears  that  the  essential 
elements  function  in  two  ways:  (a)  As  component  parts  of  the  cell 
structure  ;  e.  g.,  S  in  the  cell  wall  and  the  epithelial  tissue,  P  in  the 
nucleus,  Si  in  the  stem  of  certain  plants,  Ca  in  the  skeleton  of 
animals  and  the  stems  of  plants;  and  (b)  as  agents  causing  physical 
or  chemical  reactions;  e.  g.,  the  respiratory  metals  Fe  (vertebrate 
blood),  Mn,  Cu  (invertebrate  blood),  K  in  the  root  of  plants,  Mg 
in  seeds  and  leaves. 
