In addition to Gayle's suggestions, you might want to physically
hold the intestine straight while the formalin fixes and hardens it.  After
injecting formalin as Gayle said, I pin one end of it in a wax-lined
dissection pan, then stretch it gently, just enough to straighten it, and
pin the other end to keep it straight, then cover it with formalin. People
sometimes send me intestine specimens which they have simply removed from
the animal and dropped into formalin, resulting in a hardened gordian knot,
all parts of which are curved to some degree, so that pieces cut from the
intestine are shaped like parentheses.  It's difficult to get a good
perpendicular section of a curved object.  If you don't have a dissecting
pan you can easily make one; or you can just pin the specimen onto a strip
of heavy corrugated cardbord or soft wood and drop it specimen side down
into a dish of formalin.

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