Most organs come in three tiers: rotten, animal, and human. Rotten organs (and small animal organs) are half as effective as human organs, and animal organs three-fourths a human organ. You start with a chest cavity full of human organs, but there is a little space to fit another organ in. If that's not enough, you can always make room…

Organs are usually bound to whatever entity was born with them. Entity's bodies painfully reject any organ owned by another, but organs owned by no-one are safe to use. Any organ that drops as loot will be safe to use, as will the occasional organ within an entity's chest cavity. Owned organs are best used for short periods of time to limit damage from Organ Rejection, or as raw material for something else.

Hearts affect your health. Lacking a heart will kill you, and having a bad heart makes you frail. Better and more hearts will result in more health.

Lungs affect your breath. Lacking lungs will kill you, and having fewer or worse lungs will cause shortness of breath. More and better lungs let you hold your breath longer.

Ribs affect your hardiness. Lacking ribs can cause your death, for having fewer or worse ribs will make you vulnerable to harm. More and better ribs let you resist damage better.

Muscles affect your strength and movement. Lacking muscles can cause your death, for having fewer or worse muscles will make you slow and weak. More and better muscles make you strong and fast.

Spleens affect your healing. Lacking a spleen can cause your death, for having a bad spleen will slow your healing from food. More and better spleens make you heal quickly.

Kidneys maintain your blood. Lacking kidneys can cause your death, for having fewer or worse kidneys can poison you. More and better kidneys are redundant.

Spines let you move. Lacking a spine will kill you, because without it you can't move. More and better spines let you resist damage, but ribs are better.

Livers remove toxins. Lacking a liver can cause your death, for having a bad liver will make toxins linger. More and better livers remove status ailments quicker.

Intestines affect your nutrition. Lacking intestines can kill you, for having fewer or worse intestines makes food less saturating. More and better intestines increase saturation from food.

Stomachs affect your fullness. Lacking stomachs can kill you, for having a bad stomach means food isn't filling. More and better stomachs increase fullness from food.

Appendixes affect… well, it's your appendix! Lacking an appendix will kill you emotionally, because who would throw out their lucky appendix? More and better appendixes lead to good things, collect them all!

Many creatures have unique organs that have effects other than or in addition to the above, and some objects have special functions within the chest cavity. For example, a couple items you can craft before you even make your first pickaxe will negate the damage dealt by the chest opener once installed.

NOTE: These are for the fabric version NOT the forge version, so I am not sure which mods are compatible (if the mods listed have a forge version, they could possibly work).

The following mods have cross-mod support. If there are mods that you would like to see support for, please let me know in the comments or on discord! Assigning chest cavities to entity types is nearly effortless, I just need to know that the entity exists.

-Age of Exile: Chest cavities are assigned to its various slime, spider, zombie, witch, chicken, and skeleton variants.

-Anthropophagy: A chest cavity is assigned to the pigglution.

-Backrooms: Organs can spawn in chests in the 0th, 1st, and 3rd layers, freshness improves with depth.

-Bewitchment: Chest cavities are assigned to all mobs.

-Biome Makeover: Chest cavities are assigned to all mobs.

-Creeper Spores: A chest cavity is assigned to creeperlings.

-Direbats: A chest cavity is assigned to the direbats.

-Rat's Mischief: A chest cavity is assigned to the rats.

-Requiem: When possessing a mob, you open and are effected by its chest cavity instead of yours.

-Small Endermen: Chest cavities have been assigned to the small and tiny endermen.

-Snow Mercy: Chest cavities have been assigned to the hoards of snow goons.

-Origins: None, but I am well aware of the desire for origin-specific chest cavities. It is a work in progress.