is the fat. Cat intestines are surrounded by this flimsy translucent sack of mesothelium and connective tissue called a mesentery, and this one is full of hundreds of little nuggets of yellow-white fat. That's not counting the huge clumps of fat that are on the inside of the abdominal wall, or the even larger clumps that surround the kidneys. During the quarter, the fat steadily deteriorates into a slimy liquid which gets into your notebook, clothes, backpack, hair, and anything else it comes into contact with.
Sharks have a lot of oil too, in fact their livers are famous for it, but shark-liver oil, though overtly slipperier, is somehow less offensive. You expect a fish to be a little slimy, and would almost be disappointed if it did not oblige. Besides, a shark's gut cavity is nice and uncluttered. Not like the cat.
That mesentery that surrounds the intestines and keeps them from falling out your cat after you've sliced through his abdominal muscles is a part of the greater omentum called the omental bursa. It's one of several mesenteries. Your mesenteries keep your innards in place and prevent them from getting overly tangled, but they're as much a side effect of the way in which the embryo develops as anything else. Back when you still looked something like a cross between a tadpole and an earthworm, the different contents of your gut cavity all lined up neatly from back to belly. The whole arrangement was suspended between two vertical (well, dorsal to ventral) curtains of skin-like material called serosa, looking a bit like that vacuum-seal plastic.
Where the two layers came together without guts in between you got mesenteries. Somewhere between tadpole-worm and drooling baby, your guts twisted around a few times, and all those neatly-arranged mesenteries wound up in the strangest places. Just to confuse matters, anatomists decided to call the mesentery that holds your intestines together the mesentery. In the proper sense.
Along with the digestive system we covered the respiratory system, and there's not really much there to cover. Both were blessedly easy. Nature pretty much got it right the first time, and didn't make too many changes (outside of that whole "crawling out of the water" bit). Mind you, this was right after the midterm, and everyone was a crispy critter, but for once, I didn't feel lost. That was good, because I sure as hell was once we started on the blood vessels.