Appendices and caeca

This morning I got an e-mail from the significant other of one of my students, informing me that said student would be missing class because he had just had his appendix removed. I sent back my standard reply "too bad, come to class or you fail" "Don't worry about missing class, take care of yourself, and we'll worry about this later," but since the comparative anatomy and physiology of the appendix is so cool, I thought I'd give my student something to enjoy while he was recovering. Thus, I sent his significant other this letter:

You should tell him [my ill student] that what the doctor removed was functionally a vestigial caecum.

He'll probably remember that we've talked about different methods herbivores use to digest cellulose (ruminants with symbiotic bacteria in their rumens, termites with symbiotic bacteria in their guts, leaf-cutter ants farming cellulose-digesting fungi), but one method used by animals to digest cellulose I left out of my lecture is a structure called the caecum.

A caecum is a blind-ended pouch that is attached to the digestive tract just between the small intestine and the large intestine. In herbivores that use caeca to digest cellulose, the caecum is quite large, and cellulose-rich food gets diverted into the caecum after it's passed through the small intestine. Inside the caecum, symbiotic bacteria digest the cellulose; once the cellulose is digested into glucose, it is moved out of the caecum and back into the large intestine.

Unfortunately, the large intestine doesn't do any absorption of nutrients (that's what the small intestine does), so all that digested cellulose just passes right through the large intestine, and then out of the body, without being absorbed. So, animals that have a caecum typically practice coprophagy, consumption of their own feces. They actually produce two types of fecal pellets: one that's just the standard waste (that they don't eat), and one that's primarily composed of nutrients from the caecum (which they do eat). After reingesting the digested cellulose, they absorb it as it passes through their small intestine. Rabbits digest cellulose using a caecum and coprophagy, as do a number of other non-ruminant herbivorous mammals.

Humans don't have an enlarged caecum, but we do have a very small pouch where a caecum would be, and we call it the appendix. It has very little function in us (since it's not big enough to house large populations of symbiotic bacteria), other than giving us appendicitis. It's a little "gift" from our evolutionary ancestors who did have a caecum ...

[Ed. note: yes, I'm using the British spelling of cecum. I think it looks cooler. And apparently the appendix does play some role in the immune system, so it's not completely without function.]