materials  between  the  cell  and  its  environment  by physical  and  chemical  means.  Solids  and  gases, such   as   oxygen,   proteins,   carbohydrates,   and mineral  salts,  pass  through  the  plasma  membrane by  a  process  known  as  DIFFUSION. The  NUCLEUS  is  a  small,  dense,  usually spherical  body  that  controls  the  chemical  reactions occurring in the cell. The substance contained in the nucleus is called NUCLEOPLASM. It is also important in the cell’s reproduction, since genetic information  for  the  cell  is  stored  there.  Every human  cell  contains  46  chromosomes,  and  each chromosome  has  thousands  of  genes  that  deter- mine  the  cell’s  function. The  CYTOPLASM  is  a  water-to-gelatinous substance  surrounding  the  nucleus  and  is  con- tained by the plasma membrane. The cytoplasm is  all  of  the  cell  protoplasm  except  the  nucleus. The  simplest  living  organism  consists  of  a single  cell.  The  amoeba  is  a  unicellular  animal. The  single  cell  of  such  a  one-celled  organism  must be  able  to  carry  on  all  processes  necessary  for  life. This cell is called a SIMPLE or UNDIFFEREN- TIATED   CELL. In multicellular organisms, cells vary in size, shape, and number of nuclei. When stained, the various   cell   structures   can   be   more   readily recognized under a microscope. Other differences such as the number and type of cells can be seen with the aid of a microscope. Many cells are highly specialized.    SPECIALIZED   CELLS   perform special  functions,  such  as  muscle,  which  contracts, or  epithelial  cells  of  the  skin,  which  protect. TISSUES Tissues are groups of specialized cells similar in structure and function. They are classified into five  main  groups:  epithelial,  connective,  muscular, liquid,  and  nervous. 1.   EPITHELIAL.   The   lining   tissue   of   the body  is  called  epitheliums.  It  forms  the outer  covering  of  the  body  known  as  the free surface of the skin. It also forms the lining  of  the  digestive,  respiratory,  and urinary  tracts;  blood  and  lymph  vessels; serous   cavities;   and   tubules   of   certain secretory  glands,  such  as  the  liver  and kidneys. This tissue has little intercellular fluid and may be further subdivided into three types: a.   Columnar.  The  chief  functions  of  this tissue  are  to  secrete  digestive  fluids  and absorb   digested   foods   and   fluids.   It 3-3 b. consists  of  long  narrow  cells  set  close together,   resembling   a   palisade-type fence  (fig.  3-3).  In  certain  areas,  such as the nostrils, bronchial tubes, and tra- chea, this tissue has a crown of micro- scopic  hairlike  processes  known  as  cilia. These   cilia   provide   motion   to   move secretions  and  other  matter  along  the surfaces from which they extend. They also act as a barrier, preventing foreign matter  from  entering  these  cavities. Squamous. This is the main protective tissue  of  the  body.  It  is  composed  of thin platelike or scalelike cells forming a mosaic pattern (fig. 3-4). This tissue Figure 3-3.—Columnar epithelial tissue Figure 3-4.—Squamous epithelial tissue.