From what I remember, in the original comics, Spider-Man shot webs from his wrist because Peter Parker used his science background to design mechanical web shooters.
In the Sam Raimi movies, however, the webslinging is a side effect of the genetically-modified spider bite that gives him the rest of his powers.
My question is this: why would the spider DNA result in webs originating from his wrists? Natural spiders create webs with spinnerets located at the tip of their abdomen. Wouldn't it have been more believable if Spider-Man's webs came from, well, behind him?
Is there some explanation given as to why his web-producing organs appeared in his wrists, and not elsewhere?