Editing Gizzard (section)

=== Koilin lining ===
In order to protect the muscles of the gizzard, the organ has a multi-layered membrane - or gastric cuticle - made of koilin, a carbohydrate-protein complex (and not keratin as once believed) to protect the muscles.<ref>{{cite book |last1=King |first1=Anthony Stuart |last2=McLelland |first2=John |title=Birds: Their Structure and Function |date=1984 |publisher=Baillière Tindall |location=London Philadelphia Toronto [etc.] |isbn=0702008729 |edition=2nd |url=https://avian-vet.com/blog/resources-for-veterinarians-5/birds-structure-and-function-free-pdf-4 |access-date=30 March 2025}}</ref> The thickness of this membrane varies with the types of food the animal eats, with diets heavier in grains, seeds, and insects creating thicker membranes than those consisting of fruits, worms, meat, and other softer edibles. In some animals the membrane is slowly worn and replaced over time, while others will discard the worn lining in its entirety periodically.<ref name=":0">{{cite journal |last1=Akester |first1=A. R. |title=Structure of the glandular layer and koilin membrane in the gizzard of the adult domestic fowl (Gallus gallus domesticus) |journal=Journal of Anatomy |date=August 1986 |volume=147 |pages=1–25 |pmc=1261543 |pmid=3693065 }}</ref>

The lining is critical to the proper functioning of the gizzard, but in some animals it can play an additional role as well.  The male [[hornbill]], for example, will fill its gizzard with fruit and then slough off the entire membrane to present it like a 'bag of fruit' to its mate during the nesting season.<ref name=":0" />