The  cells  have  a  parallel  and  overlapping  arrangement,  held
together  by  a  scanty  cement-substance,  and  form  sheets,  rings,
or  tubes  of  muscle.  Indistinct  longitudinal  and  transverse
markings  are  sometimes  observed,  the  latter  being  probably
folds  in  the  cell-walls.  The  cells  have  an  envelope  but  no
separable  sarcolemma.  Some  authorities  describe  involuntary
muscle-cells  as  having  longitudinal  ridges,  or  "  intercellular
bridges,"  on  their  surface,  which  unite  with  the  ridges  of  the
adjacent  cells.
Nerve -tissue.
The  elements  of  the  nervous  structures  are  greatly  extended
cells,  called  neurons,  which  for  practical  convenience  may  be
divided  into  nerve-cells,  nerve-fibres,  and  nerve-terminals.
Their  description  will  be  deferred  to  that  of  the  nervous
system  in  general.
Free  Cells  and  Body-fluids.
The  tissues  considered  hitherto  are  all  fixed.  The  present
class  is  a  somewhat  miscellaneous  assemblage  of  those  body-
elements  that  are  not  fixed,  but  free  or  fluid.
They  may  be  grouped  as  follows  :  1 .  Unattached  cells,  free
in  the  solid  tissues.  2.  Cellular  fluids — that  is,  fluids  in
which  free  living  cells  are  suspended,  as  the  blood.  3.  Simple
homogeneous  fluids,  mainly  secretions.  4.  Emulsions.
