Merlin you are correct in a sense. The job of the colon (which is immediately INSIDE the vent if you dissect a bearded dragon) is to remove water, vitamins, minerals and electrolytes from waste material. This is the job of the colon in any species, with variance being how much is removed. diarrhea is caused by excessive water, whereas constipation is a lack of water. Now the colon connects to rectum which contains the sphincter muscles. Muscles can actually absorb liquid. If they could not you would never recieve an IM (Inter Muscular) injection. Each muscle within the body contains a series of proteins and fibers that interconnect. On basic chemical level - proteins are able to store, retain and release fluids. On the human body there are 2 sets of sphincter muscles that prevent liquid from "easily" entering the body. However it is possible. On a bearded dragon there is 1 set of sphincter muscles which is located inside the vent itself. It is theoretically possible for the Sarcomere cells within the sphincter to absorb water in small amounts.Even in the case of an enema - it is not the case the water "wets" the fecal matter - it re hydrates the colon allowing the natural process to restart. So simply put constipation is dehydration of the colon. This also leads to the point of why the argument is often made against sand and walnut shells. They are completely capable in sufficient portions to dehydrate the colon. From the standpoint of Anatomy is completely realistic to assume that a bearded dragon can absorb some water via the vent. I can gladly explain further with dissection pictures if needed. Although we are not identical to bearded dragons on some system (such as reproductive system) our GI tracts are essentially the same.