When you dissect a mouse it takes less than an hour (as compared to the horse that takes 4 people a month to dissect)! When you cut through their skin, fascia covers the abdomen and you can see into it. In fact not only is the fascia transparent but the organs themselves are so tiny that you can look into and through them and see the kidney behind the intestines too! Wow!
I was looking for the jugular vein and thought I found it, but turned out I was looking at a muscle (sternomastoideus m.). An entire muscle! That means you have to have a really good imagination to be able to see the jugular. That also means that lab people that can cannulate (find/hit) the vein every time are on my amaziac list. But its a skill I'm willing to forego.
As for rats...do you realize that their testes are a 1/3 the size of a horses? A fucking HORSE'S? Okay, think about that. how many of me would fit into a horse. probably 5-10, weight wise. what about a rat? At least a thou. But it would only take 3 testes of a rat to fit into the space a horse's testicle takes up. DAMN! They're Friggin' Huge!!!! On the same token, the guinea pig's vesicular glands are huge, causing one of the highest volume/ejaculation:body size ratios around. Boars are next in line. Funny, I wonder if that's why they're called guinea "pigs".