1. People believe, the high or massive part of the head behind the orbits are used for storage of water to keep a reserve for periods with water limited or absent availability.
2. You can hear even stories like this (exact citation of an anonymous facebook member): "The way I see them put water up in their caskets through their eyes during watering when it bulges out and they suck water into it and then they pump it up into their cranial crest"...
3. Even some books and internet forums speculate about the function of high casques in the Yemen chameleons in a sense: "They collect dew which condenses on the casque and runs down the cheeks to enter the mouth at the mouth corners to be swallowed..."
All this you can find people stating totally seriously.
And, it is a total nonsense.
1. Casques are NOT used for any kind of water storage in any form and by any means.
2. Nothing like this happens of course. There is no way ANY animal would drink with eyes and there is NO WAY how to facilitate the described process, that is totally wrong. A cranial crest is a prominent bone-based formation on the head, it can not be pumped with water!
3. The water can condense on chameleon bodies at night and in the rainy season or at very high humidity and low temperatures reaching dew-point it happens, but it is not directed anywhere as the main hydration mode in chameleons besides of the normal drinking of water drops is the breathing in of fog.
What IS stored in casques?
Under normal circumstances in the wild: nothing. The content of the casque, which is often even concave, is just the flat and thin chewing muscle.
In captivity, in extremely overfed and obese chameleons, after the body cavity is filled with a huge fat body and liver is fat-degraded and tripple in size or bigger, the casques are often convex to bulged and heavily deformed. The content of this swelling is subcutaneous fat and heavily hypertrophied chewing muscle.
As this is never present in the wild, it is a solely captivity-induced pathology that has not yet been studied in detail.
What are casques actually good for and why they are often so prominent?
The function of casques is primarily in the process of interspecific identification of a potential mating partner thanks to the phenomenon called character displacement. Based on differently shaped casques and other cranial protuberances and skin derivates, the females can identify the right sexual partner of own species and refuse any reproduction attempts of a male belonging to another species.