752  *     Chemical  Elements  of  Living  Matter.  { Am^0°"r192h1arm- 
of  the  gland  (fresh  weight  of  gland  in  adults  29  to  231  gm.,  dry  sub- 
stance from  5  to  62  gm.)  Iodine  occurs  in  star  fishes  from 
0.007-0.24%,  in  sponges  1.5%  (as  iodospongin  C56H87IN10- 
S2O2),  *n  oysters,  sea  weed,  and  especially  brown  algae,  while  the 
corals,  Gorgonia  Carollinii,  may  contain  from  0.7-7.8%  of  I.  Be- 
sides in  the  thyroid  gland  iodine  is  reported  in  the  following  organs 
and  tissues  of  man : 
liver  1. 214  mg.  I  in  100  gm.  skin  and  hairs  0.88  mg.  I 
kidney  1.053  blood,  menstrual  0.09  " 
stomach  .909  "       normal  0.021  " 
14.  Silicon  is  always  present  in  small  amounts  in  vertebrates, 
mainly  as  a  constituent  of  the  teeth  and  bones : 
1000  grams  of  dry  substance  contain 
enamel  of  teeth  0.581  gm.  SiO,    tendons  0.064 
Wharton's  jelly  0.243  "     "    skin  0.044 
dura  mater  0.087  "     "    muscles  0.024 
It  is  also  present  in  radiolarians  and  siliceous  sponges  and  is  found 
in  all  plants,  predominant  in  the  stems  of  many  grasses,  in  diatoms 
and  in  marine  algae.  Cereal  straws  and  corn  stover  may  contain 
40-70%,  Si02  equisetaceae  70-80%  Si02  of  their  ash.  It  appears 
to  be  chiefly  in  the  cell  wall  and  to  act  as  support  and  protective 
substance. 
Content  of  SiO  2  in  the  Ash  of  Plants: 
100  parts  of  ash  contain  SiO 2 
in  root,  average 
in  stems,  average  6-70% 
in  leaves,  average  5~4°% 
in  seeds,  average  °-5"3% 
75.  Sodium  is  essential  for  animals  but  not  plants.  It  is  abund- 
ant in  blood  and  lymph  and  also  occurs  in  many  plant  saps  and 
juices.  In  muscles  (5-6%),  brain  (1^-2%),  and  liver  (2%)  there 
is  more  K  than  Xa,  while  the  proportion  of  K  :Na  is  equal  in  heart 
and  kidney,  but  in  the  pancreas  (1-4-1-3%),  spleen  (5%),  and 
bones  (6%)  there  is  more  Xa  than  K;  and  cartilage  contains  only 
sodium.   Like  K  it  has  an  osmotic  function. 
