It  is  located  to  the  left  and  dorsally  between  the  stomach  and  diaphragm,
has  an  irregular  oval  outline,  measures  about  five  inches  in  length  and
three  inches  in  thickness,  and  weighs  about  seven  ounces.  It  is  subject
to  great  variations  in  size  and  shape.  It  is  enveloped  by  a  thick  fibro-
elastic  capsule,,  or  tunica  albuginea,  containing  smooth  muscle  in  its
inner  portion.  External  to  this  is  also  a  peritoneal  investment,  or  tunica
serosa.  The  capsule  of  the  spleen  of  the  ox  is  especially  robust,  and
rich  in  smooth  muscle.  At  one  point,  the  liilum,  the  capsule  projects
into  the  spleen  as  a  large  mass  of  trabecular  tissue.  Over  the  entire
surface  also  other  trabeculse  project  from  the  capsule  into  the  paren-
chyma of  the  organ.  These  trabeculse  are  of  similar  structure  to  the
capsule.  The  supporting  tissue  of  the  parenchyma  is  a  delicate  reticu-


246  THE   LYMPHATIC   SYSTEM

lum.    This  is  continuous  with  the  fibro-elastic  terminal  processes  of  the
fibromuscular  trabeculae.
The  primary  trabeculae  divide  the  parenchyma  imperfectly  into
roughly  pyramidal  compartments  about  one  millimeter  in  diameter,  with
three  trabeculae  for  each  lobule.  This  lobulation  is  faintly  indicated  by
surface  markings.  According  to  Mall  this  unit  of  structure,  the  splenic
lobule,  is  further  subdivided  into  about  ten  smaller  compartments  by


FIG.  250. — PORTION  OF  SPLEEN  OF  CAT,  SHOWING  CAPSULE  (ABOVE  AND  AT

LEFT)  AND  FIVE  SPLENIC  NODULES.
Between  the  nodules  can  be  seen  vascular  trabeculae  continuous  with  the  capsule.
X60.
anastomosing  septa,  continuous  with  the  primary  trabeculaa.  The  divi-
sion of  the  spleen  into  lobules,  and  their  subdivision  into  lobular  com-
partments has  structural  significance  also  from  the  viewpoint  of  the
blood  supply.  A  knowledge  .of  the  microscopic  structure  of  the  spleen
is  dependent  upon  an  understanding  of  the  distribution  of  the  blood-
vessels.
