_ is undefined _.extend (Model.prototype, Events, {but it is included in the file

I have dowloaded backbone.js from http://documentcloud.github.com/backbone/ including this file inside the script tag and create a view in the onReady function like http://documentcloud.github.com/backbone/#View but I always get the message ** _ is undefined _.extend (Model.prototype, Events, {** but when I look in the _extend file, there is a 100% normal path. Has anyone had the same behavior?