A jarhead is a man whose human head has been replaced with a plain glass jar. The jarhead operation was originally only used on US marines. President Teddy Roosevelt came up with the idea, just after his Rough Riders disbanded due to a plague of saddle sores. Roosevelt chose to surgically implant jars because of both the built-in storage and camouflage uses.
In the early years the jarheads would have to use seeing-eye dogs, due to lack of ocular facilities (eyes). Modern jarheads, however, have eyes transplanted into their hands. In 'Nam they had to ingest food through their anuses. Many complained that the rations always tasted like shit. Now, however, the United States Army employs Cuban slaves to cut holes in the backs of the marines, with skin flaps, so that food can be shoveled directly into the jarheads' stomachs.
Sometimes a Cuban falls in by accident, but these inept feeders are quickly digested and no harm is done to the jarhead.
The storage space created by the jars was used to carry tiny child soldiers into battle with the marines, doubling the "soldiers per set of feet" ratio. Currently in US war efforts around the world the jarheads carry mulch, MREs, IUDs, DDTs, PhDs, and other rations in their jars.